MAY THE FORCE OF PHYSICS

BE WITH YOU

What is the weight of a 46.2 gram bird?

m = 46.2 g = .0462 kgF = ma, F = (.0462 kg)(9.8 N/kg) = 0.453 N

W

Bell Ringer!

Net Force – Example 1 Finding acceleration

F = maMaking to the right +<+17.0 N – 9.0 N> = (5.0kg)a8.0 N = (5.0kg)aa = (8.0 N)/(5.0kg) = 1.6 m/s/s

5.0 kg17.0 N9.0 N

Net Force – Example 2 Finding an unknown force

F = maMaking to the right +<+450. N + F> = (35.0kg)(+9.0 m/s/s)450. N + F = 315 NF = 315 N - 450. N = -135 N (to the left)

35.0 kg450. N

F = ??

a = 9.0 m/s/s

Some other force is acting on the block

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Get your whiteboards and calculators ready!

W

5.0 kg7.0 N3.0 N

F = maMaking to the right +<7.0 N – 3.0 N> = (5.0kg)a4.0 N = (5.0kg)aa = .80 m/s/s

Find the acceleration:

W

23.0 kg5.0 N3.0 N

F = ma<5.0 N – 3.0 N – 6.0 N> = (23.0kg)a-4.0 N = (23.0kg)aa = -.1739 = -.17 m/s/s

Find the acceleration:

6.0 N

W

452 kg67.3 N

F = ??

F = ma<67.3 N + F> = (452 kg)(.12 m/s/s)<67.3 N + F> = 54.24 NF = 54.24 N - 67.3 N F = -13.06 = -13 N

Find the other force:

a = .12 m/s/s

A scalar is simply a number, a magnitude alone.

A force is usually shown as a vector, which includes both magnitude and a direction.

Force (or free-body) diagrams show the relative magnitude and direction of all forces acting upon an object. The object must be isolated and “free” of its surroundings.

Bell RingerCalculate your weight in Newtons.

HINT: You are going to need to convert your weight from pounds (lbs) first…

This is a free-body diagram of the Statue of Liberty. She is represented by a simple box. The forces acting on her are labeled with a magnitude and the arrow shows direction. Notice the surrounding objects are stripped away and the forces acting on the object are shown.

2207250 N

2207250 N

WW here represents the force of the weight of an object.

NN is the normal force, which represents the force the ground is pushing back up on the object.

2207250 N

2207250 N N =

W =

Think of the diagram on an XY plane.

If “up” is assumed to be the positive direction, then N is positive and W is negative.

496210 N496210 N

496210 N496210 N N =N =

W =W =

(Positive y-direction)+y

+x(Positive x-direction)

-2207250 N

(Positive y-direction)+y

+x(Positive x-direction)The sum of the forces in the y is zero.

The forces acting on the object cancel each other out.

•We know F = m * aa, where “a” is acceleration.

•If aa = 00, then F = m * 00 = 00.

•When F = 0, the object is not accelerating.

•We can then say that the forces acting on the

object cancel each other out and it is in a state of

static equilibrium.

Bell RingerWhat do each of the terms stand for?

• Fnorm

• Fgrav

• Ffriction

• Fair

• Ftens or T

(Normal force) = ground reaction force. always perpendicular to the ground. (N)

(Weight)

(Friction)

(Air resistance) – force of the air friction

(Tension)- pulling force exerted by a chain, cable, rope, or string on an object

Sitting Gorilla

Free Body Diagram of the Sitting Gorilla (The box represents the gorilla, W = weight of the gorilla,

N = Normal force)

W

N

Create a free body diagram (FBD) for each of the following situations. Draw a FBD of the gorilla:

This is also an acceptable diagram.

N

W

Sitting Gorilla

Create a free body diagram (FBD) for each of the following situations. Draw a FBD of the gorilla:

Parrot on wooden swing hung by ropes

Draw a FBD of the wooden swing:

Free Body Diagram of the wooden swing (The box represents the wooden swing, W = weight of the swing and the parrot, T represents the ropes that are in tension supporting the weight)

W

T2T1

Traffic Light supported by cables

Draw a FBD of the ring at point C:

A B

C

D

Free Body Diagram of the ring at point C (T represents the force of the cables that are in tension acting on the ring)

TCA

TCD

TCB

Draw a FBD of the traffic light:

Free Body Diagram of the traffic light (TCD represents the force of the cables acting on the light and W is the weight acting on the light)

W

TCD

Traffic Light supported by cables

A B

C

D

Class work. Draw free body diagrams for the following scenarios.

1. A book is at rest on a tabletop. A free-body diagram for this situation looks like this:

2. A girl is suspended motionless from the ceiling by two ropes. A free-body diagram for this situation looks like this:

4.

An egg is free-falling from a nest in a tree. A free-body diagram for this situation looks like this:

5. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. A free-body diagram for this situation looks like this:

6. A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:

7. A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. A free-body diagram for this situation looks like this:

8. A skydiver is descending with a constant velocity. Consider air resistance. A free-body diagram for this situation looks like this:

9. A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration. A free-body diagram for this situation looks like this:

10. A football is moving upwards towards its peak after having been booted by the punter. A free-body diagram for this situation looks like this:

11. A car is coasting to the right and slowing down. A free-body diagram for this situation looks like this: