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: Ive a problem? It so easy but I can't get it

  1. Jan 26, 2015 #1
    PLEASE HELP A 68 kg acrobat jumps from a 2 meter ledge on to a trampoline. If the acrobat bounces off the trampoline with a velocity of 10.6 m/s over a contact time of 1.02 seconds, what force did the trampoline exert on the acrobat?

    Well I ve tried to do it multiples times, using the formula Vf=Vi+a*t and solving for a. Then I substitute in the Force equation but I get the wrong number and my teacher told me is the wrong equation but i don't know what else to use.
  2. jcsd
  3. Jan 26, 2015 #2
    What you want to look up is the definition of impulse...
  4. Jan 26, 2015 #3
    I'm not sure we've been using this definition. But eventually, how can I use it?
    Shouldn't I find force?
  5. Jan 26, 2015 #4
    Sure, if you haven't heard of impulse you can start with Newton's 2nd law dp/dt = F, which can also be more loosely written:
    Δp = F Δt
  6. Jan 26, 2015 #5
    So in this case do I use that formula and calculate: Δp= F (found with acceleration) * 1.02s?
    So is Δp the force that the problem is asking me?
  7. Jan 26, 2015 #6
    Δp/Δt is the force, yes.
    And yes, Δt is the value you are given, but the "tricky" part is determining Δp because you are only given the value of p when the acrobat is bouncing off but have to determine p when he's coming on the trampoline...
  8. Jan 26, 2015 #7
    Is it different because the velocity changes right?
    For the first p I got p=68*10.6=720.8. So then how am I supposed to find the second one that you were talking about?
  9. Jan 26, 2015 #8
    Well, what's the acceleration of a falling body near the surface of the earth?
  10. Jan 26, 2015 #9
    9.8 m/s^2 ?
  11. Jan 26, 2015 #10
    correct, so what is v(t) for a falling object?
    how long will it take the object to fall 2 meters if it starts from rest?
  12. Jan 26, 2015 #11
    If he jumps off a ledge 2 m above the trampoline, what do you think his velocity was immediately before he made contact with the trampoline?

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted