Average Net force and high jumpers

  • #26
tms
644
17
Instead of arguing and asking me all these questions show me a equation that I can use and that will work and show me how it works, otherwise if I try to do the calculations using that equation and get something wrong and my answer turns out wrong then what am I supposed to do and you asking me questions ain't helping.
The purpose of the questions is to try to lead you to discover the solution on your own. You are more likely to understand and remember the solution when you get it yourself than when someone just hands it to you.
 
  • #27
It really would help if you wrote out the equations. That way you should see more clearly what your mistake is. Just start with
$$E_{initial} = E_{final} + \Delta E$$
and then substitute expressions for the initial and final energy, being careful not to reuse the same variable for different values.

SO I managed to ask my physics teacher and he told me this:
The high jumper’s energy at the top of his path is purely potential (as you have determined).

When he hits the mat he is doing work on the mat to compress it. To do this work he must give up energy. His final energy when he finally stops is also purely potential. Be careful when working out his final potential energy as you have to use both the height of the mat and how much it was compressed to find his final height.

Once you know his final energy, the energy he “lost” has gone into the mat. And since change in energy = work = force x displacement, you can determine the force by using: energy lost by high jumper = force applied to mat x compression of mat. Rearrange for force and Bob’s ya Uncle.

that is all I needed really. I was waiting for his reply and thought I would try out this website too, seems like teachers know what their doing. I only needed someone to explain to me simply what is going on and what equation to use. Thanks to him I got the answer
 
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  • #28
haruspex
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To do this work he must give up energy. His final energy when he finally stops is also purely potential. Be careful when working out his final potential energy as you have to use both the height of the mat and how much it was compressed to find his final height.
Precisely what I pointed out to you in post #18. You were using the full 2.13m for the lost height, not subtracting the remaining thickness of the mat.
Also, don't forget that the question is essentially wrong - the 'average force' cannot really be calculated this way. I'd be interested in your teacher's answer to that.
 

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