Newton's second law - jumping from a height

  • #1

Homework Statement


When jumping from a height on to a hard surface, it is advisable to bend one's knees on landing. How does bending the knees affect the time one takes to come to rest? (1 mark)

With reference to Newton's second law, explain why it is a good idea to bend one's knees. (2 marks)


Homework Equations


F=ma


The Attempt at a Solution



I'm not quite sure how to answer this question at all...
 

Answers and Replies

  • #2
haruspex
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When landing, you need to change your momentum. What is the relationship between force, time, and change in momentum?
What is the difference between landing with legs straight and landing with them bent (and not held too rigidly) in terms of how forces change over time? Try to sketch a graph. What must the two graphs have in common?
 
  • #3
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With regards to the second part, you should make sure that you understand Newton's Second Law. More force acting upon a falling body is worse, whereas less force would be better, but how would bending your legs affect that force? Knowing the definitions of each component of F=ma should hopefully make the answer apparent.
 
  • #4
When landing, you need to change your momentum. What is the relationship between force, time, and change in momentum?
What is the difference between landing with legs straight and landing with them bent (and not held too rigidly) in terms of how forces change over time? Try to sketch a graph. What must the two graphs have in common?

Force is equal to rate of change in momentum? f=dp/dt I think. Well I don't know, do bent knees absorb force more? I'll try and draw graphs, thanks

With regards to the second part, you should make sure that you understand Newton's Second Law. More force acting upon a falling body is worse, whereas less force would be better, but how would bending your legs affect that force? Knowing the definitions of each component of F=ma should hopefully make the answer apparent.

I'm not sure how bending your knees would change the force. Would it decrease it? And since mass stays constant, acceleration would decrease?
 
  • #5
haruspex
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Force is equal to rate of change in momentum? f=dp/dt I think. Well I don't know, do bent knees absorb force more? I'll try and draw graphs, thanks

I'm not sure how bending your knees would change the force. Would it decrease it? And since mass stays constant, acceleration would decrease?

If you don't bend your knees it could hurt - why do you think that is?
 
  • #6
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Here is a fun way to learn:

Try to jump at least 10 times without bending your knees . (Do u experience any agony?)

Repeat the same experiment but this time bending your knees on every jump. (Feel Relaxed ? )
 
  • #7
HallsofIvy
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That is, if after jumping fro a height 10 times without bending your knees, you can do the same bending your knees!
 
  • #8
If you don't bend your knees it could hurt - why do you think that is?

I'm not sure where I was going with the 'acceleration would decrease thing' since it's always going to be 9.8, I think

I have an idea, is it because when the legs are bent the angle is different, so the component of the force going downwards is going to be smaller?

And also, when knees are bent it takes a longer time to decelerate, and there would be more displacement as the body is still moving lower. And since it takes a longer time to decelerate to 0, f=ma so the force would be lower?
 

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