Conceptual Physics Chapters 6 and 7: Newton's 3rd Law and Momentum

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PART I Problems (60)

Solve these problems on your own paper. Solve each problem on its own page, do not put more than one problem on a page! You must use the Three-Column-Method to receive any credit for your work.

1. Consider a 0.345 kg baseball pitched to a batter at 35 m/s (About 80 mph!).

a) What is the momentum of the baseball?

b) If the pitcher applied a force of 5.5 N to the baseball for 2.2 s, what is the change in momentum of the baseball?

c) What is the acceleration of the baseball?

2. A 60 kg student on his skateboard has a momentum of 210 kgm/s.

a) What is the velocity of the student?

b) At the bottom of the hill the student slows to a stop taking 10.5 s to do so. What is the magnitude of the force of friction that brought the student to a stop?

c) What is the magnitude of the impulse that brought the student to a stop?

PART II: Short Answer (90)

Answer the questions below on your own paper. Put each question on its own page, do not put more than one question on a page. Answer each question in ‘a’, ‘b’, ‘c’ bullet format.

3. A student claims that he can jump high enough to touch the ceiling of this classroom.

a) What provides the force that propels him upwards?

b) Is this force less than, equal to, or greater than his weight?

c) Explain how you know your answer to part ‘b’ is correct.

d) In order for the student to jump he had to apply a force to the floor; was this force less than, equal to or greater than his weight?

e) Using one of Newton’s Laws, explain how you know your answer to part ‘d’ is correct.

4. If you throw an egg at a wall going 10 m/s the egg will break on impact. But, if you throw the same egg (before you throw it at the wall!) at a bed sheet going 10 m/s it will not break. Explain the following:

a) Why is the change in momentum the same in both cases?

b) Why is the impulse the same in both cases?

c) In terms of impulse, why did the egg break when it hit the wall?

d) In terms of impulse, why did the egg not break when it hit the bed sheet?

PART III True/False (30)

Indicate whether the statement is true or false. Mark your answer document ‘A’ for true and ‘B’ for false.

PART IV Rewrite (60)

On your own paper rewrite the false statements so that they will be true. Be sure to address all of the areas of the false statement that are false. You will receive ten (10, 5+5, X) points for each correctly rewritten statement. Do NOT copy the false statements nor is it necessary to rewrite the true statements.

5. According to Newton’s third law, whenever one object exerts a force on another object that second object always exerts an equal an opposite force back on the first object.

6. In order to make a cart move forward a horse must pull harder on the cart than the cart pulls on the horse.

7. A bicycle and a parked car have a head-on collision; since the bike is less massive than the car it receives a greater force of impact than the parked car.

8. During a tug-o-war the side that wins is, of course, the side the pulls harder on the rope.

9. In order for a person to jump from a standing position the floor must push up on that person with a force greater than his weight.

10. Impulse equals momentum; the equation for this is Ft = p.

11. Cheerleaders perform on padded mats because the padding in the mats increases the time it takes to come to a stop when the land or fall; the longer the time it takes to stop the less force is needed to stop so there is less chance of injury.

12. Momentum is defined as the product of an object’s velocity and impulse.

13. When a batter ‘follows through’ when hitting a ball the contact time between the ball and the bat is longer, this means the change in the ball’s velocity is greater too since Ft = mDv.

14. It takes more force to stop something than it takes to bounce something, this is because when you stop something the change in velocity is greater than when you bounce it.

PART V: Multiple Choice (48)

Identify the choice that best completes the statement or answers the question.Carefully place your answer choice on your answer document.

15. Which of these is an example of a Newton’s third law action-reaction pair?

a.	A person standing on the floor has the force of gravity pulling her down while the floor exerts a normal force pushing her up.	c.	A catcher exerts a force on a ball while Earth exerts a force on the catcher.
b.	A ball being accelerated downward by the force of Earth’s gravity while Earth is being accelerated upward by the ball’s force of gravity.	d.	An object falling through the air has the force of gravity pulling it down while the force of air resistance pushes it up.

16. A student kicks a football. The action force is the foot hitting the ball. The reaction force is

a.	the force of friction between the student and the ground.	c.	the force of the football against the student’s foot.
b.	the weight of the football.	d.	the air resistance on the football.

17. As an object falls the action force is the force of gravity pulling the ball downward; the reaction force is

a.	the pull of the object’s gravity on Earth.	c.	acceleration of the ball at 9.8 m/s^2.
b.	the air resistance on the object.	d.	the object’s weight.

18. A force is exerted in the tires of a car to accelerate the car along the road. The source of the force that causes this accelerated motion is

a.	the car’s engine.	c.	the normal force of the road.
b.	the car’s tires.	d.	the friction force of the road.

19. According to Newton’s Third Law it is impossible to hit a hanging sheet of tissue paper with a large force because

a.	the tissue paper will easily tear.	c.	the tissue paper cannot hit back with a large force.
b.	the tissue paper is largely affected by air resistance.	d.	the tissue paper is nearly weightless and massless.

20. A rocket is able to accelerate the vacuum of space. The force that accelerates the rocket is the force of

a.	the exhaust gases on the rocket.	c.	of the rocket on the exhaust gases.
b.	of the vacuum of space pulling on the rocket.	d.	the exhaust gases pushing on the vacuum of space.

21. Two teams of ten students are involved in a tug-o-war. Each team pulls with a force of 4000 N. What is the tension in the rope?

a.	zero newtons	c.	4000 N
b.	2000 N	d.	8000 N

22. When you walk across a floor

a.	you push on the floor in the same direction as your motion.	c.	you must exert a force on the floor greater than your weight.
b.	the floor pushes on you in the direction of your motion.	d.	the floor must push on you in the direction opposite your motion.

23. Compared to a sports car moving at 10 m/s, the same sports car moving at 30 m/s has

a.	the same momentum.	c.	six times the momentum
b.	three times the momentum.	d.	nine times the momentum

24. A cannon recoils after shooting a cannon ball. The velocity of the cannon’s recoil is small because the

a.	impulse on the cannon is small.	c.	mass of the cannon is large.
b.	force on the cannon is small.	d.	mass of the cannon ball is large.

25. The change in the momentum of an object is equal to the

a.	force acting on it.	c.	velocity of the object.
b.	impulse acting on it.	d.	acceleration of the object.

26. A cannon ball shot from a long barrel cannon will travel a lot farther than a cannon ball shot from a short barrel cannon. This is because the long barrel cannon delivers more

a.	force.	c.	momentum
b.	inertia	d.	impulse

27. In order to catch a ball a ball player moves his hand backward in the direction of the ball’s motion. Doing this reduces the force of impact on the player’s hand because

a.	the time of the impact force is increased.	c.	a force is required to accelerate the hand.
b.	the velocity of the hand is negative.	d.	the ball comes to a stop suddenly.

28. When a student jumps from a table they always bend their knees when they land. By doing this the time it takes the student to stop is increased by at least ten times what it would be with a stiff legged stance. This means the force needed to stop is

a.	reduced to zero.	c.	increased by a factor of ten times.
b.	increased to ten times the student’s weight.	d.	decreased by a factor of ten times.

29. A moving train car coasting along a frictionless track gently collides with an stationary identical train car. As a result the cars couple together. Compared to the velocity of the first car before the collision the velocity of the two cars together is

a.	zero.	c.	the same as the single car alone.
b.	half as large as the single car alone.	d.	twice as large as the single car alone.

30. Before a gun is fired the momentum of the gun-bullet system is zero. When the gun is shot the momentum of the gun-bullet system IS

a.	still zero.	c.	the product of the mass and velocity of the gun.
b.	the product of the mass and velocity of the bullet.	d.	the product of the momenta of the gun and the bullet.

Conceptual Physics Chapters 6 and 7: Newton's 3rd Law and Momentum

Answer Section

PROBLEM

1. ANS:

m = 0.345 kg a) p = mv a) p = 0.345 kg x 35 m/s

v = 3.5 m/s b) Imp = Dp p = 12.1 kgm/s

a) p = ? kgm/s Imp = Ft b) Dp = 5.5 N x 2.2 s

b) F = 5.5 N Dp = Ft Dp = 12.1 kgm/s

t = 2.2 s c) F = ma or kgm/s)

Dp = ? kgm/s c)

c) a = ? m/s²

2. ANS:

m = 60 Kg a) p = mv a)

p = 210kgm/s v = 3.5 kgm/s

a) v = ? m/s b) Ft = Dp b) Dp = 60 kg (0 m/s - 3.5 m/s)

b) v_f = 0 m/s Dp = 60 kg x 3.5 m/s (Ignore the (-))

t = 10.5 s Dp = m(v_f - v_i) Dp = 210 kgm/s

F = ? N c) Imp = Ft

Dp = ? kgm/s F = 20 N

c) Imp = ? Ns c) Imp = 20 N x 10.5 s

Imp = 210 Ns

SHORT ANSWER

3. ANS:

A student claims that he can jump high enough to touch the ceiling of this classroom.

a) The floor actually provides the force that propels him upwards.

b) This force, provided by the floor, is greater than the student’s weight.

c) I know that this force must be greater than the student’s weight because the only way the student can accelerate upwards is if a force compels him to do so and that force must be greater than his weight if his feet leave the floor.

d) In order for the student to jump he had to exert a force greater than his weight.

e) I know the student had to exert a force greater than his weight because, according to Newton’s Third Law that is how the floor was able to produce an equal force on the student.

4. ANS:

If you throw an egg at a wall going 10 m/s the egg will break on impact. But, if you throw the same egg (before you throw it at the wall!) at a bed sheet going 10 m/s it will not break. Explain the following:

a) The change in momentum is the same because in both cases the egg went from 10 m/s to zero m/s.

b) The impulse is the same in both cases because impulse is equal to the change in momentum. If the change in momentum is the same, so is the impulse. Put another way, the impulse of the wall = Ft; while the impulse of the bed sheet = Ft. While the force and time are different for the wall and bed sheet the product of the force and time is the same for both; it is like 20 N x 2 s = 2 N x 20 s.

c) In terms of impulse, or in terms of force and time, the egg broke when it hit the wall because the time it took toe egg to stop was so small the force had to be very large in order to stop the egg.

d) In terms of impulse, or in terms of force and time, the egg did not break when it hit the bed sheet because the time it took the egg to stop was so great that only a small force was need to stop the egg.

TRUE/FALSE

5. ANS: T

6. ANS: F

In order for a cart to move forward a horse must push harder on the ground than the cart pulls on the horse.

7. ANS: F

A bicycle and a parked car have a head-on collision; according to Newton’s Third Law, both the bike and the car receive the same impact force.

8. ANS: F

During a tug-o-war the side that wins is, of course, the side that pushes harder on the ground.

9. ANS: T

10. ANS: F

Impulse equals change in momentum, the equation for this is Ft = mDv.

11. ANS: T

12. ANS: F

Momentum is defined as the product of an object’s mass and velocity.

13. ANS: T

14. ANS: F

It takes more force to bounce something than to stop something, this is because when you bounce something there are two impulses, two ‘Ft’s, one to stop the object and another to make it go again.

MULTIPLE CHOICE

15. ANS: B

16. ANS: C

17. ANS: A

18. ANS: D

19. ANS: C

20. ANS: A

21. ANS: C

22. ANS: B

23. ANS: B

24. ANS: C

25. ANS: B

26. ANS: D

27. ANS: A

28. ANS: D

29. ANS: B

30. ANS: A