Formal Homework Assignment 5

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10/26/14Formal Homework Assignment 5

Pahttp://session.masteringphysics.com/myct/assignmentPrintView?assignmentID=3179130

Formal Homework Assignment 5

Due: 11:15am on Tuesday, October 28, 2014

To understand how points are awarded, read the Grading Policyfor this assignment.

Problem 6.5

A construction worker with a weight of 860 stands on a roof that is sloped at 21 .

Part A

What is the magnitude of the normal force of the roof on the worker?

Express your answer to two significant figures and include the appropriate units.

ANSWER:

Correct

Problem 6.30

A 510 piano is being lowered into position by a crane while two people steady it with ropes pulling to the sides. Bob's rope pulls to the left, b

horizontal, with 510 of tension. Ellen's rope pulls toward the right, 26 below horizontal.

Part A

What tension must Ellen maintain in her rope to keep the piano descending at a steady speed?


ANSWER:

Correct

Part B

What is the tension in the main cable supporting the piano?


ANSWER:

Correct

A message from your instructor...

For "A Friction Experiment", be sure to advance the figures in the lower left hand corner of the problem page to see the choices you have for each p

A Friction Experiment

N

= 800n N

kg 15

N

= 550T N

= 5400T N


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During an experiment, a crate is pulled along a rough horizontal surface by a force and the magnitude of the acceleration along the xdirection,

measured. The vector has a component along the xdirection of magnitude . The experiment

is repeated several times, with different values of each time, while maintaining a constant

value for, , the vertical component of .

Part A

Create a plot of the force of static friction, , versus thexcomponent of the pulling force, , for the experiment. Let the point , along the

horizontal axis, represent the minimum force required to accelerate the crate. Choose the graph that most accurately depicts the relationship a

, and .

Hint 1. Characteristics of static friction

There are two important characteristics to keep in mind about the the force of static friction:

Only a stationary object can be acted upon by the force of static friction.

, where is the coefficient of static friction and is the magnitude of the normal force. This inequality means that the

actual force of static friction can have any magnitude between zero and a maximum value of .

Hint 2. Find the force of static friction

A horizontal force of magnitude is exerted on a stationary crate. The maximum force of static friction, , between the crate and theis 15 . Assume that is the only force, besides that of static friction, , acting horizontally on the crate.

What is when no horizontal force is applied to the crate, that is, when ? What is when ? What is the instant

crate starts to move?

Enter your answers numerically in newtons. Separate each answer with a comma. For example if the answers are 100, 200, and

enter 100,200,-50.

Hint 1.Applying Newton's 2nd law

F a

F FxFx

Fy F

fs Fx Fmin

Fx Fmin

nfs s s n

ns

FH fs, maxN FH fs

fs = 0 NFH fs = 10 NFH fs


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A horizontal force is applied to the crate. However, the force of static friction, , opposes this force and causes the crate to remain

stationary, meaning that . From Newton's 2nd law we know that

.

This yields

when applied to this specific problem.

ANSWER:

ANSWER:

Correct

Notice that until the pulling force exceeds , the force of static friction is exactly equal in magnitude to the pulling force.

Part B

Create a plot of the force of kinetic friction, , versus thexcomponent of the pulling force, , for the experiment. Let the point , along th

horizontal axis, represent the minimum force required to accelerate the crate. Choose the graph that most accurately depicts the relationship a

, and .

Hint 1. Characteristics of kinetic friction

There are three important characteristics to keep in mind about the force of kinetic friction:

Only an object that is sliding with respect to a surface can be acted upon by the force of kinetic friction.

points in a direction that is parallel to the surface of contact and opposes the motion of the object.

, where is the coefficient of kinetic friction and is the magnitude of the normal force.

s

= 0ax

=mFx ax

= =m = 0Fx FH fs ax

= 0,10,15fs N

A

B

C

D

E

Fx fs, max

fk Fx Fmin

Fx Fmin

fk= nfk k k N


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ANSWER:

Correct

The most important things to keep in mind when dealing with kinetic friction are the following:

Only an object that is sliding with respect to a surface can be acted upon by the force of kinetic friction.

points in a direction that is parallel to the surface of contact and opposes the motion of the object.

, where is the coefficient of kinetic friction and is the magnitude of the normal force.

Part C

After all the trials are completed, a graph of acceleration as a function of force is plotted. Assuming the presence of both static and kine

friction, which of the following graphs is most nearly correct?

ANSWER:

Correct

Problem 6.26

Part A

A 5.10 object initially at rest at the origin is subjected to the time-varying force shown in the figure.

What is the object's velocity at

Express your answer with the appropriate units.

A

B

C

D

fk= nfk k k N

ax Fx

A

B

C

D

kg

t= 6 s?


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ANSWER:

Correct

Kinetic Friction Ranking Task

Below are eight crates of different mass. The crates are attached to massless ropes, as indicated

in the picture, where the ropes are marked by letters. Each crate is being pulled to the right at the

same constant speed. The coefficient of kinetic friction between each crate and the surface on

which it slides is the same for all eight crates.

Part A

Rank the ropes on the basis of the force each exerts on the crate immediately to its left.

Rank from largest to smallest. To rank items as equivalent, overlap them.

Hint 1. General problem-solving strategy

To make reasonable comparisons among forces you should proceed as follows:

1. Isolate individual objects via free-body diagrams.

2. Identify the forces that act on each object.

3. Determine the acceleration of each object.

4. Finally, apply Newtons 2nd law to each object.

You may ultimately be able to do this in your head, but if you are not at that stage, you should draw the necessary free-body diagrams to an

this problem.

Hint 2. Evaluate the effect of friction

The coefficient of friction between each crate and the surface on which it slides is identical. Based on this fact, is the frictional force acting o

each crate identical?

ANSWER:

= 3.92v ms


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Hint 3. Examine the top chain of crates

Examine the entire chain of three masses pulled by rope A, and consider the total frictional force impeding this chains motion. To what valu

mass is this force proportional?

ANSWER:

ANSWER:

Correct

Normal and Frictional Forces Ranking Task

Six boxes held at rest against identical walls.

Part A

Rank the boxes on the basis of the magnitude of the normal force acting on them.


Hint 1. How to approach the problem

Since each box is at rest, Newtons 2nd law states that the forces acting on each box must sum to zero. Apply this fact to the horizontal forc

acting on each box.

yes

no

1

2

3

4

5

6

kg

kg

kg

kg

kg

kg


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ANSWER:

Correct

Part B

Rank the boxes on the basis of the frictional force acting on them.


Hint 1. Determine the direction of the frictional force

In which direction does the frictional force on each box act?

ANSWER:

Correct

Hint 2. Newton's 2nd law

The force of friction on each box acts in the vertical direction. Since each box is at rest, Newtons 2nd law states that the forces acting on ea

box must sum to zero. Apply this fact to the vertical forces acting on each box.

ANSWER:

horizontal only

vertical only

both horizontal and vertical


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Correct

Problem 5.44

Johnny jumps off a swing, lands sitting down on a grassy slope, and slides down the slope before stopping. The coefficient of kinetic fric

between grass and the seat of Johnny's pants is .

Part AWhat was his initial speed on the grass?

ANSWER:

Correct

Problem 6.21

A 4200 truck is parked on a 16 slope.

Part A

How big is the friction force on the truck? The coefficient of static friction between the tires and the road is 0.90.


ANSWER:

20 3.5m0.5

2.96 m/s

kg


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Correct


For "PSS 6.2 Dynamics Problems" , in the first two parts you will not use any of the numbers associated with the problem. When drawing your free b

diagram, be sure to monitor the angles at which you are drawing the forces.

PSS 6.2 Dynamics Problems

Learning Goal:

To practice Problem-Solving Strategy 6.2 for dynamics problems.

A box of mass 3.1 slides down a rough vertical wall. The gravitational force on the box is 30 . When the box reaches a speed of 2.5 , you

pushing on one edge of the box at a 45 angle (use degrees in your calculations throughout this problem) with a constant force of magnitude =

shown in figure. There is now a frictional force between the box and the wall of magnitude 13 .

How fast is the box sliding 3.0 after you started pushing on it?

PROBLEM-SOLVING STRATEGY 6.2Dynamics problems

MODEL:Make simplifying assumptions.

VISUALIZE:Draw a pictorial representation.

Show important points in the motion with a sketch, establish a coordinate system, define symbols, and identify what the problem is trying you

to find. This is the process of translating words into symbols.

Use a motion diagram to determine the objects acceleration vector .

Identify all forces acting on the object, and show them on a free-body diagram.

Its OK to go back and forth between these steps as you visualize the situation.

SOLVE:The mathematical representation is based on Newton's second law:

.

The vector sum of the forces is found directly from the free-body diagram. Depending on the problem, either

Solve for the acceleration, and then use kinematics to find velocities and positions; or

Use kinematics to determine the acceleration, and then solve for unknown forces.

ASSESS:Check that your result has the correct units, is reasonable, and answers the question.

Model

= 1.1"104f N

kg N m/s Fp

Ns

a

= =mFnet i F

i a


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Paghttp://session.masteringphysics.com/myct/assignmentPrintView?assignmentID=3179130

Start by making a simplifying assumption: We will assume all the forces acting on the box are constant, so now you can model the box as a particle

with a constant acceleration.

Visualize

Part A

Using our simplified model, in which we know that the forces are constant (but we don't know what their magnitudes are), which of the following

diagrams would be a reasonablerepresentation of the motion of the box?

Check all that apply.

ANSWER:

Correct

Before you start pushing on the box, the box is acted upon by only the gravitational force that pulls the box downward. The resulting acceleravector is directed downward. It should be noted that there is no frictional force at this point, since the normal force acting on the box from the

vertical wall is zero at this point.

The important point in the box's motion is the moment when you start pushing on it. At that moment, three possible scenarios may occur. The

may continue to slide down the wall at an increasing speed, but with a smaller acceleration than before because of the opposing effect of the

pushing force and frictional force. In this case, would point downward (diagram D). Alternatively, if the opposing effect of the pushing force

frictional force were such that the box's speed would decrease, the box would be slowing down as it moves downward and would point upw

(diagram B). Somewhere in between these two cases, a third scenario arises in which the net force on the box is zero and the box slides dow

wall at a constant speed (diagram A). Only the direction and magnitude of the net force can tell you which of these scenarios is the correct o

this problem. Keep in mind that in all cases the motion is always vertical, meaning that the horizontal component of the acceleration must be

Part B

Still using our simplified model (in which we do not know the magnitudes of the forces), draw a free-body diagram showing all the forces acting box afteryou start pushing on it. The positive yaxis is taken to be upward. The black dot represents the box. Since our model is about having c

forces of unknown magnitude, you do not need to draw the vectors to scale, but your final diagram should be physically reasonable.

Draw the vectors starting at the black dot. The location and orientation of the vectors will be graded. The relative lengths of the vecto

not be graded.

ANSWER:

A

B

C

D

a

a


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Correct

If all the vectors in your free-body diagram were drawn with their correct relative lengths, their vector sum would provide som

quantitative information about the box's acceleration. Since this is not the case here, you cannot obtain any further information about the box'

acceleration without first performing some calculations based on Newton's second law.

Note that in the diagram above, a coordinate system was established so that the positive xaxis points to the right and the positive yaxis poin

upward. This is the same coordinate system used throughout the rest of this problem. Keep in mind that you are trying to find the box's speed

after starting to push on it.

Solve

Part C

Find the box's speed at 3.0 after you first started pushing on it.

Express your answer in meters per second to three significant figures.


This is a one-dimensional kinematics problem. You are given the box's initial speed and need to calculate its final speed after a certain perio

time. You know that motion occurs only in the vertical direction, so there's no need to write down any equation in the xdirection. All you nee

, the ycomponent of the box's acceleration, which can be calculated by applying Newton's second law in the direction.

Hint 2. Set up Newton's second law in the ydirection

Newton's second law states that , the ycomponent of the net force acting on an object, is equal to the ycomponent of the object's

acceleration multiplied by its mass, that is, .

Using the coordinate system shown in Part B, enter an expression for in terms of the forces acting on the box. Use , , and fo

magnitudes of the friction force, the gravitational force, and the normal force, respectively; use for the magnitude of the pushing force.

Express your answer in terms of some or all of the variables , , , and .

=Fnet i F

i

vf s

ay y

(Fnet )y( =mFnet )y ay

(Fnet)y f FG n

Fp

f n FG Fp


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Hint 1. Find , the ycomponent of the pushing force

Enter an expression for the ycomponent of the pushing force, . Recall that you push on the box at a 45 angle. Use for the

magnitude of the pushing force.

Express your answer in terms of , the magnitude of the pushing force.

ANSWER:

ANSWER:

Hint 3. Determine which kinematic equation to use

Which of the following equations is the most appropriate one to calculate the box's speed 3.0 after you first started pushing on it?

ANSWER:

ANSWER:

Correct

Assess

Part D

Assuming that the angle at which you push on the edge of the box is again 45 , with what magnitude of force should you push if the box we

slide down the wall at a constant velocity? Note that, in general, the magnitude of the friction force will change if you change the magnitude of t

pushing force. Thus, for this part, assume that the magnitude of the friction force is .

Express your answer in newtons to three significant figures.

Hint 1. Dynamic equilibrium

If the box slides down the wall at a constant velocity, its acceleration must be zero. This is the condition for dynamical equilibrium, or

Set up again Newton's second law in the ydirection as you did in the previous part, keeping in mind that now the magnitude of the push

force is unknown. Use the equilibrium condition , and solve for .

ANSWER:

Fpy

Fpy Fp

Fp

=Fpy sin(45)Fp

=( = ( =Fnet)y i Fi)y f + sin(45)FG Fp =may

s

= + 2 ( )v2fy

v2iy ay yf yi

= + t + (tyf yi viy12

ay )2

= + tvfy viy ay

= 18.8 vf m/s

Fp

f= 0.566Fp

=FnetFp

( = 0Fnet )y Fp


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Correct

Your results make sense. If you push with a force of magnitude 23 , as described in Part C, the box will continue to speed up because the

component of the net force, and therefore the ycomponent of the box's acceleration, remains negative (i.e., it points downward). In this case

effect of the pushing force is simply to reduce the magnitude of the box's acceleration. Then, to reduce the box's acceleration further to zero,

need to push with a force of magnitude , as you have just calculated. Note that if you pushed even harder, the acceleration will be

positive, causing the box to slow down and possibly come to a stop. These are the three scenarios whose motion diagrams were identified inA.

Problem 6.39

A rifle with a barrel length of 61 fires a 11 bullet with a horizontal speed of 410 . The bullet strikes a block of wood and penetrates to a dep

.

Part A

What resistive force (assumed to be constant) does the wood exert on the bullet?


ANSWER:

Correct

Part B

How long does it take the bullet to come to rest after entering the wood?


ANSWER:

Correct

Problem 6.22

A 1500 car skids to a halt on a wet road where = 0.51.

Part A

How fast was the car traveling if it leaves 66- -long skid marks?


ANSWER:

= 51.8Fp N

N

> 23NFp

cm g m/scm

= 7100fk N

= 6.3"10#4t s

kg k

m

= 26v ms


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Correct


For Problem 6.49, the angle is measured with respect to the horizontal. The problem wants the answer to have three significant figures.

Problem 6.49

It's a snowy day and you're pulling a friend along a level road on a sled. You've both been taking physics, so she asks what you think the coefficient

friction between the sled and the snow is. You've been walking at a steady , and the rope pulls up on the sled at a 33.0 angle. You estima

the mass of the sled, with your friend on it, is 66.0 and that you're pulling with a force of 81.0 .

Part A

What answer will you give?

ANSWER:

Correct

Problem 6.56

A person with compromised pinch strength in his fingers can exert a force of only 6.2 to either side of a pinch-held object, such as the book show

following figure.

Part A

What is the heaviest book he can hold vertically before it slips out of his fingers? The coefficient of static friction between his fingers and the boo

is 0.82.


ANSWER:

1.5 m/s

kg N

0.113

N

= 10w N


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Correct


For Problem 6.42, treat the thrust force just like you would any other force. Even though a careful analysis of thrust can be complicated, here thrust

as a generic force, just as if the person was being pulled by a rope. You should also assume he starts from rest.

Problem 6.42

Sam, whose mass is 76 , takes off across level snow on his jet-powered skis. The skis have a thrust of 210 and a coefficient of kinetic friction o

of . Unfortunately, the skis run out of fuel after only 12 .

Part A

What is Sam's top speed?


ANSWER:

Correct

Part B

How far has Sam traveled when he finally coasts to a stop?


ANSWER:

Correct


For Problem 6.55, the coefficient of static friction is 0.5 while the coefficient of kinetic friction is 0.2.

Problem 6.55

A 1.0 kg wood block is pressed against a vertical wood wall by the 12 N force shown.

kg N0.1 s

= 21v ms

= 351s m


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Part A

If the block is initially at rest, will it move upward, move downward, or stay at rest?

ANSWER:

Correct

Pushing Too Hard

A baggage handler at an airport applies a constant horizontal force with magnitude to push a box, of mass , across a rough horizontal surface

very small constant acceleration .

Part A

The baggage handler now pushes a second box, identical to the first, so that it accelerates at a rate of . How does the magnitude of the forc

the handler applies to this box compare to the magnitude of the force applied to the first box?


Apply Newton's 2nd law to the first box to obtain an equation relating the force applied to the acceleration of the box. Then, do the same for

upward

downward

at rest

F1 ma

2a

F1


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second box. Compare these equations to determine the relationship between and .

Hint 2. Identify the forces that act on each box

To apply Newton's 2nd law, you must determine which forces contribute to the acceleration of each box. Of the following forces, which act a

the direction of the box's acceleration?


ANSWER:

Hint 3.Apply Newton's 2nd law to the first box

The first box has mass and acceleration . It is pushed with a force of magnitude . Applying Newton's 2nd law to this box yields which

the following equations?

Hint 1. Newton's 2nd law

Newton's 2nd law states that:

,

where is the magnitude of the net force on the object, is the mass of the object, and is the acceleration of the object along th

direction of the applied net force.

Hint 2. Equation for the force of kinetic friction

Recall that the force of kinetic friction on a given object moving relative to a surface is

,

where is the coefficient of friction and is the normal force.

ANSWER:

Hint 4.Apply Newton's 2nd law to the second box

The second box has mass and acceleration . It is pushed with a force of magnitude . Applying Newton's 2nd law to this box yields

of the following equations?ANSWER:

2 1

The normal force exerted by the floor on the box.

The weight of the box.

The force of static friction.

The force of kinetic friction.

The force exerted on the box by the baggage handler.

m a F1

F= ma

F m a

= nfk kk n

+ =maF1 fk =maF1 fk+ ma= F1 fk+ ma= fk F1

m 2a F2

=maF2 fk 2 = 2maF2 fk = 2maF2 fk 2 =maF2 fk


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Hint 5. Put it all together

In the previous two subparts, you determined that

and

.

Combine these two equations to obtain one equation that contains both and .

ANSWER:

ANSWER:

Correct

Now see if you can apply this problem-solving technique to answer the next question.

Part B

Now assume that the baggage handler pushes a third box of mass so that it

accelerates at a rate of . How does the magnitude of the force that the handler applies

to this box compare to the magnitude of the force applied to the first box?

Hint 1.Apply Newton's 2nd law to the first box

The first box has mass and acceleration . It is pushed with a force of magnitude . Applying Newton's 2nd law to this box yields which

the following equations?

ANSWER:

=maF1 fk

= 2maF2 fkF1 F2

=ma +F2 F1= 3maF2 F1= 2 + maF2 fk F1

0


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Hint 2.Apply Newton's 2nd law to the third box

The second box has mass and acceleration . It is pushed with a force of magnitude . Applying Newton's 2nd law to this box yiel

which of the following equations?

Hint 1. Find the force of kinetic friction

Recall that the force of kinetic friction on a given object moving relative to a surface is

,

where is the coefficient of friction and is the normal force.

If represents the force of kinetic friction that acts on the first box, what is the force of kinetic friction acting on the third box?

ANSWER:

ANSWER:

Hint 3. Put it all together

In the previous two subparts, you determined that

and

.

Combine these two equations to obtain one equation that contains both and .

ANSWER:

Hint 4. Determine the importance of a small acceleration

+ =maF1 fk =maF1 fk+ ma= F1 fk+ ma= fk F1

m/2 2a F3

= nfk kk n

fk

12

fkfk2fk

=maF3 fk = 2maF3 fk

= 2maF312

fk

= maF3 12 fk

=maF1 fk

= maF312

fkF1 F3

= maF312

fk

= F3 F112

fk

= + + 2maF3 F132

fk


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You were told in the problem introduction that is very small. Consider what a very small implies about the relative sizes of and . It

help you to consider the following expression:

.

Which of the following statements are correct?

ANSWER:

ANSWER:

Correct

PSS 7.1 Interacting-Objects Problems - Copy

Learning Goal:

To practice Problem-Solving Strategy 7.1 for interacting-objects problems.

A 1260 - car pushes a 2160 - truck that has a dead battery. When the driver steps on the accelerator, the drive wheels of the car push agains

ground with a force of 4510 . Take the friction between the truck's tires and the ground to be 465 . What is the magnitude of the force the car ap

the truck?

PROBLEM-SOLVING STRATEGY 7.1Interacting-objects problems

MODEL:Identify which objects are part of the system and which are part of the environment. Make simplifying assumptions.

VISUALIZE:Draw a pictorial representation.

Show important points in the motion with a sketch. You may want to give each object a separate coordinate system. Define symbols, and

identify what the problem is trying to find.

Identify acceleration constraints.

Draw an interaction diagram to identify the forces on each object and all action/reaction pairs.

Draw a separatefree-body diagram for each object.

Connect the force vectors of action/reaction pairs with dashed lines. Use subscript labels to distinguish forces that act independently on more

than one object.

SOLVE: Use Newton's second and third laws.

Write the equations of Newtons second law for eachobject, using the force information from the free-body diagrams.

Equate the magnitudes of action/reaction pairs.

Include the acceleration constraints, the friction model, and other quantitative information relevant to the problem.

Solve for the acceleration, and then use kinematics to find velocities and positions.

ASSESS:Check that your result has the correct units, is reasonable, and answers the question.

Model

The car and the truck are separate objects that form the system. Since only the straight-line motion of the car and truck is involved in this problem, m

them as particles. The earth and the road surface are part of the environment.

a a 1 k

=maF1 fk

is much larger than .

is slightly larger than .is larger than .

F1 fk

F1 fkfk F1

0


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Part A

Assuming that the car pushes the truck to the right, which of the following free-body diagrams is the correct diagram for the situation described

problem? In each case, the diagram on the left refers to the car, and that on the right to the truck. Dashed lines connect action/reaction pairs. T

following notation is used: , , and are, respectively, friction, normal force, and gravity; subscripts C and T stand for "acting on car" and "

truck"; is the force exerted on the truck by the car; and is the force exerted on the car by the truck. Note that the force vectors a

drawn in scale.

Hint 1. Draw the free-body diagram for the car

Complete the free body diagram of the car by drawing the forces that act on it. Assume the car pushes the truck to the right.

Draw the vectors with their tails at the black dot, which represents the car. The orientation of your vectors will be graded. The leng

your vectors will not be graded.

ANSWER:

Hint 2. Draw the free-body diagram for the truck

Complete the free-body diagram of the truck by drawing the forces that act on it. Assume the car pushes the truck to the right.

Draw the vectors with their tails at the black dot, which represents the truck. The orientation of your vectors will be graded. The le

of your vectors will not be graded.

ANSWER:

f n FG

FConT F

TonC


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ANSWER:


23/32




24/32



Correct

As the car pushes the truck with a force , the truck pushes back on the car with a reaction force . Since acts on the truc

must be shown on the truck's free-body diagram; similarly, acts on the car, so it is shown on the car's free-body diagram.

There is also a second action/reaction pair that is not shown in the diagram above. When the car pushes against the ground with a force of 4

, the ground pushes back on the car with a force of equal magnitude. To propel forward, the car must be pushing backward against the groun

the ground must respond by pushing the car forward. This forward-directed force exerted by the ground is , a static friction force acting on

car's tires. It is a static friction force because, even though the tires are rolling, the bottom of each tire, where it contacts the road, isinstantaneously at rest. Furthermore, note that it is this forward-directed static friction force that propels the system forward!

Now that you have identified all the forces acting on the system, don't forget to include the necessary acceleration constraints in your pictoria

representation. In this problem, there is only one acceleration constraint: the car and the truck are both accelerating along the xdirection and

same rate, so

.

A Book on a Table

A book weighing 5 N rests on top of a table.

Part A

A downward force of magnitude 5 N is exerted on the book by the force of

ANSWER:

Correct

Part B

An upward force of magnitude _____ is exerted on the _____ by the table.

ANSWER:

FConT F

TonC F

ConT

FTonC

fC

= =aC aT ax

the table

gravity

inertia

.


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Correct

Part C

Do the downward force in Part A and the upward force in Part B constitute a 3rd law pair?

Hint 1. The force of gravity

The force of gravity is another name for the force exerted by the earth (or any astronomical object) on objects near its surface.

Hint 2. Exploring Newton's 3rd law

Indicate whether the following statements about Newton's 3rd law are true, false, or indeterminate.

1. According to Newton's 3rd law, every real force has a unique pair force.

2. The pair force is called a "fictitious force."

3. The force and pair force must act on different point masses.

4. The force and the pair force must always have the same magnitude and must also act in exactly opposite directions.

Enter t for true, f for false, or i for indeterminate for each statement, separating the answers with commas (e.g., if all but the first

statement were true, you would enter f,t,t,t).

ANSWER:

ANSWER:

Correct

Part D

The reaction to the force in Part A is a force of magnitude _____, exerted on the _____ by the _____. Its direction is _____ .

Hint 1. The force of gravity

The force of gravity is another name for the force exerted by the earth (or any astronomical object) on objects near its surface.

ANSWER:

6 N / table

5 N / table

5 N / book

6 N / book

, , ,t f t t

yes

no


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Correct

Part E

The reaction to the force in Part B is a force of magnitude _____, exerted on the _____ by the _____. Its direction is _____.

ANSWER:

Correct

Part F

Which of Newton's laws could we have used to predict that the forces in Parts A and B are equal and opposite?


ANSWER:

Correct

Since the book is at rest, either Newton's 1st or 2nd law can tell us that the net force on it must be zero. This means that the force exerted on

the earth must be equal and opposite to the force exerted on it by the table.

Part G

Which of Newton's laws could we have used to predict that the forces in Parts B and E are equal and opposite?

Check all that apply.ANSWER:

Correct

5 N / earth / book / upward

5 N / book / table / upward

5 N / book / earth / upward

5 N / earth / book / downward

5 N / table / book / upward

5 N / table / earth / upward

5 N / book / table / upward

5 N / table / book / downward

5 N / earth / book / downward

Newton's 1st law

Newton's 2nd lawNewton's 3rd law

Newton's 1st law

Newton's 2nd law

Newton's 3rd law


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Problem 7.1

A weightlifter stands up at constant speed from a squatting position while holding a heavy barbell across his shoulders.

Part A

Draw a free-body diagram for the barbells.

Draw the force vectors with their tails at the dot. The orientation of your vectors will be graded. The exact length of your vectors will n

graded but the relative length of one to the other will be graded.

ANSWER:

Correct

Part B

Draw a free-body diagram for the weight lifter.

Draw the force vectors with their tails at the dot. The orientation of your vectors will be graded. The exact length of your vectors will n

graded but the relative length of one to the other will be graded.

ANSWER:


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Correct

Blocks in an Elevator Ranking Task

Three blocks are stacked on top of each other inside an elevator as shown in the figure.

Answer the following questions with reference to the eight forces defined as follows.

the force of the 3 block on the 2 block, ,






the force of the 1 block on the floor, , and

the force of the floor on the 1 block, .

Part A

Assume the elevator is at rest. Rank the magnitude of the forces.


Hint 1. Newton's 3rd law

Newtons 3rd law states that when two objects exert forces on each other, these forces are always equal in magnitude and opposite in direc

Hint 2. Contact forces

If two objects touch each other, they exert forces on each other according to Newton's 3rd law. If two objects do not touch each other, they

cannot exert forces on each other. (Here we are assuming that the forces of gravity and electromagnetism between the objects are negligib

kg kg F3 on 2kg kg F2 on 3kg kg F3 on 1kg kg F1 on 3kg kg F2 on 1kg kg F1 on 2kg F1 on floor

kg Ffloor on 1


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For instance, the blocks with masses 1 and 3 are not in contact, therefore, the force must be zero. While statements such as

"block 3 applies a force to block 1 that is transferred through block 2" might sound reasonable, in physics such statements don't make sens

ANSWER:

Correct

Part B

Now, assume the elevator is moving upward at increasing speed. Rank the magnitude of the forces.


Hint 1. Effects of acceleration

If the elevator is accelerating, the net force on each block cannot be zero. However, note that the forces you are asked to compare do not

comprise the net force on any of the blocks.

ANSWER:

g g 1 on 3


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Correct

Problem 7.5

Block A in the figure is heavier than block B and is sliding down the incline. All surfaces have

friction. The rope is massless, and the massless pulley turns on frictionless bearings. The rope

and the pulley are among the interacting objects, but you'll have to decide if they're part of the

system.

Part A

Draw a free-body diagram for the block A.

The orientation of your vectors will be graded. The exact length of your vectors will not be graded.

ANSWER:


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Correct

Part B

Draw a free-body diagram for the block B.

The orientation of your vectors will be graded. The exact length of your vectors will not be graded.

ANSWER:

Correct

Score Summary:


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Your score on this assignment is 98.1%.

You received 41.21 out of a possible total of 42 points.

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Formal Homework Assignment 5