1. For every force there is an equal and opposite force
Newton’s 3rd Law
For every force there is an equal and opposite force

4. If your feet push down on the floor, what does the floor do?
Nothing
Absorbs the push
Pushes up on your feet, but with less force
Pushes up on your feet with an equal force
Pushes up on your feet with greater force

5. Can the floor push up on your feet with a force greater than your weight?
Yes, but nothing will happen
Yes, you will accelerate up into the air
No, it always equals your weight
No, it can only equal or be less than your weight
Yes, but only on a planet with stronger gravity

6. What is the force from that causes a car to accelerate down the road?
The engine
The transmission
The pistons
The road
The tires

7. What is the force from that causes a swimmer to accelerate across a pool?
The swimmer’s legs
The swimmers arms
The swimmer’s arms and legs
Gravity
The water

8. Swimming
Due to the 3rd Law, when you swim you push the water (blue), and it pushes you back just as hard (red) in the forward direction. The water around your body also produces a drag force (green) on you, pushing you in the backward direction. If the green and red cancel out, you don’t accelerate (2nd Law) and maintain a constant velocity.
Note: The blue vector is a force on the water, not the on swimmer! Only the green and red vectors act on the swimmer.

9. The Earth pulls on the Moon with gravity, causing it to orbit
The Earth pulls on the Moon with gravity, causing it to orbit. What does the Moon do to the Earth?
Pulls on the Earth with equal gravity, causing it to orbit
Nothing, it is too small
Pulls on the Earth with less gravity, having no effect
Pulls on the Earth with less gravity, causing tides

10. The Earth pulls on you with the force of gravity, what is the reaction to this force?
Normal force
You pull on the Earth with an equal force of gravity
You pull on the Earth with a smaller force of gravity
Nothing, only the Earth has gravity

11. The Forces are the same but the Masses and Accelerations are different
MEarth F
= aEarth
= aPerson F
MPerson

12. A gun shoots a bullet, which received the greatest force?
The bullet
The gun
Each received an equal force
Depends on the size of the bullet

13. The Forces are the same but the Masses and Accelerations are different
MGun F
= aGun
= aBullet F
MBullet

14. A bug splatters on a windshield, which received the greatest force?
The windshield
The bug
Each received an equal force
Depends on the size of the bug

15. The Forces are the same but the Masses and Accelerations are different
MCar F
= aCar F
MBug
= aBug

16. In the tug-of-war, what is the reading on the scale?
3000
2000
1000
0 N
1000 N
2000 N
3000 N

17. The tree pulls just like the tug-of-war team.

18. What will the scale read that is pulled from both sides by 10 N weights?

19. If a Mack truck and a Volkswagen have a head-on collision, which vehicle will experience the greater impact force? a) the Mack truck b) the Volkswagen c) both the same d) ... it depends on other factors

20. Does the scale read 100N, 200N, or zero?

21. For every force there exists an equal and opposite force
For every force there exists an equal and opposite force. Consider action and reaction forces in the case of a rock falling under the influence of gravity. If action is considered to be that of the earth pulling down on the rock, can you clearly identify the reaction force?

23. End of Chapter exercise questions
For each of the following interactions, identify action and reaction forces. (a) A hammer hits a nail. (b) Earth gravity pulls down on you. (c) A helicopter blade pushes air downward.
Identify the action-reaction pairs of forces for the following situations: (a) You step off a curb. (b) You pat your tutor on the back. (c) A wave hits a rocky shore.
When you drop a rubber ball on the floor it bounces almost to its original height. What causes the ball to bounce?
Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. How fast does the heavier cart roll compared with the lighter cart?

24. If a massive truck and small sports car have a head-on collision, upon which vehicle is the impact force greater? Which vehicle experiences the greater acceleration? Explain your answers.
Which team wins in a tug-of-war: the team that pulls harder on the rope, or the team that pushes harder against the ground? Explain.
A stone is shown at rest on the ground. (a) The vector shows the weight of the stone. Complete the vector diagram showing another vector that results in zero net force on the stone. (b) What is the conventional name of the vector you have drawn?

25. Here a stone is suspended at rest by a string
Here a stone is suspended at rest by a string. (a) Draw force vectors for all the forces that act on the stone. (b) Should your vectors have a zero resultant? (c) Why, or why not?
Here the same stone is being accelerated vertically upward. (a) Draw force vectors to some suitable scale showing relative forces acting on the stone. (b) Which is the longer vector, and why?

26. Problems
If you apply a net force of 5 N on a cart with a mass 5 kg, what is the acceleration?
If you increase the speed of a 2.0-kg air puck by 3.0 m/s in 4.0 s, what force do you exert on it?
A boxer punches a sheet of paper in midair, and brings it from rest up to a speed of 25 m/s in 0.05 s. If the mass of the paper is kg, what force does the boxer exert on it?
If you stand next to a wall on a frictionless skateboard and push the wall with a force of 30 N, how hard does the wall push on you? If your mass is 60 kg, what's your acceleration?