EK Physics Question

[Scarletblack]

Don't remember what number it is, but here it is anyway:

In a karate demonstration, a heavy anvil is placed on the chest of a karate master, who is lying on his back on the floor. A second master strikes the anvil with a sledgehammer making an inelastic collision. The first master is unharmed because the block protects the first master from the sledgehammer's:

A. Momentum
B. Impulse
C. Kinetic Energy
D. Kinetic Energy and Momentum

Also, the question right after:

In the karate demonstration of the previous question, during the collision between the sledgehammer and the anvil:

A. The sledgehammer experiences a greater change in momentum than the anvil

B. The sledgehammer experiences a smaller change in velocity than the anvil

C. The sledgehammer experiences a smaller change in impulse than the anvil

D. The center of mass of the sledgehammer experiences a greater displacement than the center of mass of the anvil.

Answers are C and D respectively

So yeah, I have no idea how to approach these questions. Help me plz >_<
So plz help

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[Cool Beans]

Hey Scarletback,
I'll admit these questions are fairly tricky and are not my favorite but I'll try to give a little help.
First of all, when you're thinking about collisions the following should be at the forefront of your mind:
1. For elastic collions, both momentum and kinetic energy are conserved.
2. For inelastic collisions, only momentum is conserved (but not kinetic energy).

Since momentum is conserved in an inelastic collision, the anvil can't be protecting the master from the sledgehammer's momentum (Rules out A and D). And since impulse (Ft) is equal to the change in momentum (delta mv), then the impulse of the collision will also be conserved and the master isn't protected from that either (Rules out B). This leaves C which makes sense as kinetic energy is always lost in an inelastic collision so the master is 'protected' from a portion of the sledgehammer's kinetic energy.

The 2nd question is tricky too. For momentum to be conserved, the sledgehammer and anvil must have equal changes in momentum (Rules out A). And if their changes in momentum are equal, then the anvil (having a larger mass) will have a smaller change in velocity (Rules out B). And finally since they have the same change in momentum, then they will have the same impulse (impulse momentum theorem mentioned above)(Rules out C). This leaves D. I don't necessarily agree that this is a correct answer, but the other choices can be ruled out as incorrect.

Hope this helps and let me emphasize that I don't think these are the greatest questions.

 

[RiPSTONE]

That was a great explanation. Thanks @Cool Beans!