# 55 Unit 6 Review, Practice and Assessment

### Learning Objectives

- Apply the Law of Conservation of Momentum to explain how locomotion is achieved.[2]
- Apply the Law of Conservation of Momentum to analyze collisions.[3]
- Apply the Impulse-Momentum Theorem to analyze collision forces.[2]
- Apply Newton’s Laws of motion to analyze predict the motion of systems subjected to external forces.[3]

**Outcome 1**

1) An asteroid speeds through deep space, distant enough from anything else to be essentially unaffected by gravity. Will the asteroid slow down because it has no power supply? Will it speed up? Explain?

2) Explain how a rocket can change speed when it is passing through deep space.

3) Explain how a person changes horizontal speed when walking.

**Outcome 2**

4) A 25 **kg **child jumps from the couch onto arms of their 80 **kg **parent. The child was moving at 2 **m/s**, what is the final speed of the child + parent combination?

**Outcome 3**

5) Imagine you are standing on a frozen pond.

a) What is the normal force the ice must supply to your foot in order to hold you up?

b) You want to start moving, but you are afraid of breaking through the ice, so you don’t want to push down on the ice any more than what you calculated above. What is the maximum possible force your foot can apply to the ice horizontally without slipping? Cite your source for the necessary friction coefficient between ice and shoes.

c) If you take a steps lasting 0.30 **s **while applying the force you found above, what is the total impulse you apply to the ice?

d) What is the total impulse you received from the ice? How do you know?

e) How fast will you be moving after this step?

6) Imagine that you slip and fall. Provide some strategies for landing that will minimize the likelihood that you break through the ice. Be specific about how you will move your body and explain your strategy using in terms of Newton’s Laws of Motion or the Impulse-Momentum Theorem, elastic and inelastic collisions, the relation between force and stress, and the definition of ultimate strength.

7) A 50 **kg **person is running 5 **m/s** toward a giant exercise ball with mass 2 **kg**, which is then kicked at 15 **m/s**, hits them square in the chest, and bounces directly back at 212 **m/s**.

a) What is the change in momentum of the ball?

b) What is the change in momentum of the person?

c) If the collision lasts 0.2**s**, what is the force on the person from the ball?

**Outcome 4**

8) An asteroid speeds through space, distant enough from anything else to be essentially unaffected by gravity. Will the asteroid slow down because it has no power supply? Explain?

9) Does your car slow down when you take your foot off the gas? Compare and contrast this situation to the previous one involving the asteroid and *explain* why the outcome is the same or different.

10) A person with mass of 65 **kg **is out walking two dogs when suddenly the dogs pull in opposite directions. Dog 1 pulls with a force of 500 **N** to the right. Dog 2 pulls with 300 **N** to the left. The person plants their feet on the wet grass (outside their center of mass so they don’t fall down). The static friction coefficient between their shoes and the wet grass is 0.3 and the kinetic friction coefficient is 0.2.

a) Draw a free body diagram of the dog walker. Don’t forget to include directions with forces, accelerations, and velocities when answering the following questions.

b) What is the maximum possible static friction force on the dog walker?

c) Does the dog walker begin to slide?

c) What is the net force on the dog walker?

d) What is the acceleration of the dog walker?

e) What is the velocity of the dog walker after 1** s**?

f) What distance will the dog walker have moved in 1 **s**?

the outward force supplied by an object in response to being compressed from opposite directions, typically in reference to solid objects.

any interaction that causes objects with mass to change speed and/or direction of motion, except when balanced by other forces. We experience forces as pushes and pulls.

a physical quantity that expresses the internal forces that neighboring particles of material exert on each other

a graphical illustration used to visualize the forces applied to an object

the total amount of remaining unbalanced force on an object

the change in velocity per unit time, the slope of a velocity vs. time graph