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posted by  Pritzy on 6/4/2008 2:16:15 PM  |  status: Live  

Physical Science Question. Please Help me!

Course Textbook Chapter Problem
N/A The Physical Universe, 12th Edition Konrad B. Krauskopf Arthur Beiser 3 24,28,42
Question Details:
24.) Does every moving body possess kinetic energy? Does every stationary body possess potential energy?
 
28.) A ball is dropped from a height of 100m. At what height will half of its energy be potential and half kinetic?
 
42.) Is it possible for an object to have more kinetic energy but  less momentum than another object? Less kinetic energy but more momentum?
 
 
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posted by Steve (retired) on 6/4/2008 7:01:54 PM  |  status: Live
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Response Details:
24.

The answer to your question is yes.  Every moving body possess kinetic energy since the formula for kinetic energy is:

KE=(1/2)mv^2

If the body is moving, than v does not equal zero, and thus KE does not equal zero.

You have to be careful of which reference frame you are talking about though.  For example, if I am in a moving car, and I look at the seat belt, it appears to be stationary to me (and thus I calculate no kinetic energy), but to an observer on the road, it is moving with a velocity equal to the car's velocity and (and thus the observer calculates a non-zero kinetic energy).  

Thus, when you are using energy considerations to solve for a particular quantity (such as height, speed, distance, whatever), you must be aware of which reference frame you are working in.  This is a matter of definition though, for example you can define the road as being "stationary" and work within that reference frame.  Similarly, you can define the road as being at a height of zero, so that anything above the road has potential energy.

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posted by Steve (retired) on 6/4/2008 7:03:36 PM  |  status: Live
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42.

Momentum is m*v where m is mass and v is velocity.
Kinetic energy is 1/2mv^2

Comparing when they are equal, we see that m*v = m*v^2/2 yields v = v^2/2 or 2v = v^2 so only the value of 2 works. If velocity is less than 2 momentum will be greater and if velocity is greater than 2, kinetic energy will be greater.

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posted by brownkid on 6/4/2008 10:12:20 PM  |  status: Live
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24. both of these are true
28. 50meters since this is half the distance
42. yes because momentum depends on mass as well.
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