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More quick work and energy questions

  • Thread starter future_vet
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  • #1
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An object hits a wall and bounces back with half if its original speed. What is the ratio of the final kinetic energy to the initial kinetic energy?
I am not sure I understand the question, is it 1:2?

A ball drops some distance and gains 30 J of kinetic energy. Do not ignore air resistance. How much gravitational potential did the ball lose?
I would say more than 30J since we have to take air resistance into consideration.

Thanks!
 

Answers and Replies

  • #2
radou
Homework Helper
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An object hits a wall and bounces back with half if its original speed. What is the ratio of the final kinetic energy to the initial kinetic energy?
I am not sure I understand the question, is it 1:2?
Not so fast. Write down the kinetic energies and look at the ratio.
 
  • #3
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First is 1 and second is 0.5 => 1/0.5 =2. Is this correct?
 
  • #4
radou
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First is 1 and second is 0.5 => 1/0.5 =2. Is this correct?
No, it isn't. The first kinetic energy is 1/2 m v^2, and the second is 1/2 m (1/2 v)^2. What is their ratio?
 
  • #5
AlephZero
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First is 1 and second is 0.5 => 1/0.5 =2. Is this correct?
No - the kinetic energy of the object is (1/2)mv^2.
 
  • #6
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I am not sure I get it, but it looks like 1:0.5 to me...
 
  • #7
radou
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I am not sure I get it, but it looks like 1:0.5 to me...

Do you know how to take the ratio of 1/2 m (1/2 v)^2 (final kinetic energy) and 1/2 m v^2 (initial kinetic energy)?
 
  • #8
169
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I guess not... It's ok, I'll figure it out.

Thanks!
 

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