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# 9.2: Unit 9 Practice and Assessment

## Outcome 1

1) 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 system to provide a force on it? Explain?

2) 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 the similarities or differences in the outcome.

## Outcome 3

2) Imagine you are being chased by a horde of zombies across the frozen wasteland of a post-apocalyptic Northwest United States. You attempt to lose the zombies by running across a frozen pond, but slip and fall. Provide some strategies for landing that will minimize the likelihood that you break through the ice. [Hint: Reduce the stress you apply to the ice. Be specific about how you will move your body and explain your strategy in the context of the impulse-momentum theorem, the relation between force and stress, and the definition of ultimate strength.]

## Outcome 2

3) You avoid breaking through the ice and get back to your feet, but the lead zombie is now only 1.5 m away and moving at 3 m/s…..

a)How long do you have until the zombie reaches you?

b) You are trying to start running again, and you lift one foot while you push off horizontally with the other foot. What is the normal force the ice must supply to your foot in order to hold you up?

c) Calculate 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.

d) If you take 3 steps, with each step putting your foot in contact with the ice for 0.30 s and apply the force you found above, what is the total impulse you apply to the ice?

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

f) How fast will you be moving after these three steps?

## Outcome 4

4) As you near the edge of the pond moving at 4.0 m/s on the ice and a zombie is behind you moving at 3.0 m/s when a helpful survivor like yourself throws you something to defend yourself with. It has a 11 kg mass and a pretty much horizontal velocity of 5.0 m/s in the direction opposite to your motion when it reaches you.

a) If you were to catch the weapon, what would your new speed be after catching it? Use your own mass in answering the question. Don’t forget to include the initial momentum of both objects in your analysis and be careful about directions.

b) Should you catch the weapon? (Would you still be moving faster than the 3.0 m/s zombie after catching it?)