1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Newton's second law - jumping from a height

  1. Jan 10, 2013 #1
    1. The problem statement, all variables and given/known data
    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)

    2. Relevant equations

    3. The attempt at a solution

    I'm not quite sure how to answer this question at all...
  2. jcsd
  3. Jan 10, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    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?
  4. Jan 10, 2013 #3
    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.
  5. Jan 10, 2013 #4
    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?
  6. Jan 11, 2013 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    If you don't bend your knees it could hurt - why do you think that is?
  7. Jan 11, 2013 #6
    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 ? )
  8. Jan 11, 2013 #7


    User Avatar
    Science Advisor

    That is, if after jumping fro a height 10 times without bending your knees, you can do the same bending your knees!
  9. Jan 21, 2013 #8
    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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook