Reading Quiz - Concordia Collegefaculty.cord.edu/luther/physics111/lectures/Chapter5_p111.pdf · A rod is suspended by a string as shown. ... A 50 kg student gets in a 1000 kg elevator

Physics 111, Concordia College

Reading Quiz

Chapter 5


1. The coefficient of static friction is A. smaller than the coefficient of kinetic friction. B. equal to the coefficient of kinetic friction. C. larger than the coefficient of kinetic friction. D. not discussed in this chapter.


1. The coefficient of static friction is A. smaller than the coefficient of kinetic friction. B. equal to the coefficient of kinetic friction. C. larger than the coefficient of kinetic friction. D. not discussed in this chapter.


2.  The drag force pushes opposite your motion as you ride a bicycle. If you double your speed, what happens to the magnitude of the drag force?

A.  The drag force increases. B.  The drag force stays the same. C.  The drag force decreases.


2.  The drag force pushes opposite your motion as you ride a bicycle. If you double your speed, what happens to the magnitude of the drag force?

A.  The drag force increases. B.  The drag force stays the same. C.  The drag force decreases.

3.  You are standing on a scale in an elevator that is accelerating upward. The reading on the scale is A.  Larger than your true weight. B.  The same as your true weight. C.  Smaller than your true weight.

3.  You are standing on a scale in an elevator that is accelerating upward. The reading on the scale is A.  Larger than your true weight. B.  The same as your true weight. C.  Smaller than your true weight.


•  Equilibrium •  Using Newton’s second

law •  Mass, weight, and

apparent weight •  Static and kinetic friction •  Applying Newton’s third

law

Chapter 5 Applying Newton’s Laws

Topics:


An object is in equilibrium when the net force acting on it is zero. In component form, this is

Equilibrium

If the net force is zero then the object does not accelerate. It must be at rest or have a constant velocity.


Equilibrium Problem Hints

•  If the acceleration is zero the net force must be zero.

•  Draw a Free-Body diagram of the object. •  Break the forces into x and y components. •  The sum of the x-components must be zero and

the sum of the y-components must be zero.


A 100 kg block with a weight of 980 N hangs on a rope. Find the tension in the rope if

a)  the block is stationary. b)  it’s moving upward at a steady speed of 5 m/s.


A wooden box, with a weight of 220 N, is pulled at a constant speed with a rope that makes an angle of 25° with the wooden floor. What is the tension in the rope?


A rod is suspended by a string as shown. The lower end of the rod slides on a frictionless surface. Which figure correctly shows the equilibrium position of the rod?


A rod is suspended by a string as shown. The lower end of the rod slides on a frictionless surface. Which figure correctly shows the equilibrium position of the rod?


A ball weighing 50 N is pulled back by a rope to an angle of 20°. What is the tension in the pulling rope?


Dynamical Systems


2nd Law Hints

•  Draw a free body diagram of the object. •  Find the components of the forces. •  Determine if any of the acceleration

components are known. •  Write out the 2nd law in component form.


A sled with a mass of 20 kg slides along frictionless ice at 4.5 m/s. It then crosses a rough patch of snow which exerts a friction force of 12 N. How far does it slide on the snow before coming to rest?


Macie pulls a 40 kg rolling trunk by a strap angled at 30° from the horizontal. She pulls with a force of 40 N, and there is a 30 N rolling friction force acting on trunk. What is the trunk’s acceleration?


Mass, Weight and Apparent Weight


Mass and Weight

–w = may = m(–g)

w = mg


Mass and Weight Units

•  An object with a mass of 1.0 kg has a weight of 9.8 Newtons on the surface of Earth.

•  9.8 Newtons is the equivalent of 2.2 lbs. •  1.0 kg is the equivalent of 6.9 x 10-2 slugs.

MKS English

Mass kilogram (kg) slug (sl)

Weight Newton (N) pound (lb)


Find the x- and y-components of w in each of these three coordinate systems.


A 75 kg skier starts down a 50-m-high, 10° slope on frictionless skis. What is his speed at the bottom?


The same burglars push the 1000 kg safe up a 20° frictionless slope with a horizontal force of 4000 N. What is the safe’s acceleration?


Burglars are trying to haul a 1000 kg safe up a frictionless ramp to their getaway truck. The ramp is tilted at angle θ. What is the tension in the rope if the safe is at rest? If the safe is moving up the ramp at a steady 1 m/s? If the safe is accelerating up the ramp at 1 m/s2? Do these answers have the expected behavior in the limit θ → 0° and θ → 90°?


Measuring Weight


Apparent Weight

•  When a person stands on a scale the scale reads the normal force that the scale exerts on the person.

•  If the person is not accelerating than the normal force equals the weight and the scale reads correctly.


Apparent Weight


Apparent Weight

•  If the person is acceleration the scale will not read the correct weight.

•  n is what the scale reads. This is not mg and is called the apparent weight.

mawnFy =−=∑mamgn +=


Elevator Rides

•  Consider a person standing in an elevator that is accelerating upward. The magnitude of the normal force on the person exerted by the floor of the elevator is

A.  larger than B.  the same as C.  smaller than

the magnitude of the weight force of the person.


Elevator

•  A 200 kg box is placed on a scale in a 2000 kg elevator. The elevator accelerates downward at 2.4 m/s2. What does the scale read while the elevator is accelerating?


A 50 kg student gets in a 1000 kg elevator at rest. As the elevator begins to move, she has an apparent weight of 600 N for the first 3 s. How far has the elevator moved, and in which direction, at the end of 3 s?


Friction Models


Types of Friction

•  Static - object at rest with respect to the surface.

•  Kinetic- object sliding on surface •  Rolling- object rolling along surface


Static Friction

fs max = µsn


Kinetic Friction

fk = µkn


Friction


Sliding Away

•  An object is held in place by friction on an inclined surface. The angle of inclination is increased until the object starts moving. If the surface is kept at this angle, the object

A.  slows down. B.  moves at uniform speed. C.  speeds up. D.  You can't tell.


A 75 kg skier starts down a 50-m-high, 10° slope on skis. The coefficient of kinetic friction between her skis and the snow is 0.06. What is her speed at the bottom?


Sliding on your seat

•  A car traveling at 20 m/s stops in a distance of 50 m. Assume that the acceleration is constant. The coefficients of friction between the seat and the passenger are µs = 0.5 and µk = 0.3. Will a 70 kg passenger slide off the seat if not wearing a seat belt?


Third Law Problems


Acceleration Constraints


Multiple Object Hints

•  Draw two (or more) free-body diagrams: one for each object.

•  Identify acceleration constraints. •  Identify action/reaction force pairs. •  Apply Newton’s 2nd Law to each object in

component form.


Block A has a mass of 1 kg; block B’s mass is 4 kg. They are pushed with a force of magnitude 10 N. a) What is the acceleration of the blocks? b) With what force does A push on B? B push on A?


What is the acceleration of block B?

40 N


Ropes and Pulleys


Pulleys

•  A pulley is used to change the direction of a string or rope. If we treat the pulley as massless and assume there is no friction in the bearings the tension in the string is unchanged in magnitude.


Bob the Window-washer •  Bob the window-washer

hangs from a pulley and rope arrangement attached to his window-washer basket as shown on the left. The rope will take a maximum tension of 1000 N before breaking. Bob and the basket together weigh 1500 N. One day Bob decides to tie one end of the rope off to a nearby flagpole as shown on the right.


Bob the Window-washer The rope will take a maximum tension of 1000 N before breaking. Bob and the basket together weigh 1500 N. One day Bob decides to tie one end of the rope off to a nearby flagpole as shown on the right. Which of the following is true for this new arrangement?

A.  The rope is less likely to break than before.

B.  The rope is just as likely to break as before.

C.  Bob will need a good health plan.


Multiple Pulleys Sally uses the pulley arrangement below to lift a 24 kg box. With what force must Sally pull on the rope to lift the box at constant velocity? (Assume the rope and pulley are massless and ignore friction. Use g = 10 m/s2)

A.  240 N B.  180 N C.  120 N D.  100N E.  60 N


Atwood’s Machine

Two blocks are connected by a string that passes over a pulley as shown at the right. Block A has a mass of 100 kg and block B has a mass of 115 kg. Find the acceleration of block A and the tension in the string. A

B

a = 0.68 m/s2

T = 1050 N


Crate Pulling

•  Bob pulls a pair of identical 10.0 kg crates across the floor with a rope that makes an angle of 30 degrees with respect to the horizontal. If the crates are accelerating at 2.0 m/s2, (a) what is the tension T1 and (b) the tension T2?


Block A, with mass 4.0 kg, sits on a frictionless table. Block B, with mass 2.0 kg, hangs from a rope connected through a pulley to block A. What is the acceleration of block A?

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Reading Quiz - Concordia Collegefaculty.cord.edu/luther/physics111/lectures/Chapter5_p111.pdf · A rod is suspended by a string as shown. ... A 50 kg student gets in a 1000 kg elevator