79 Unit 8 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 2

3) 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.

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

b)  You are afraid of breaking through the ice, so you don’t want to apply any more normal force than 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 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?

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

e) How fast will you be moving after these three steps? [Hint: Use your previous answer and the definition of momentum].

Outcome 3

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?)

Outcome 4

5) Catching or avoiding the weapon that was thrown to you causes you to 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.