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Circus performer

  1. Oct 17, 2009 #1
    1. The problem statement, all variables and given/known data

    A circus performer falls into a safety net from a height h above the net, coming to a halt after a further displacement within the safety net. In the following questions you will calculate various quantities regarding this event. In each question, derive a precise formula for the answer and only then substitute specific values into the variables of the formula. Do not use results from one question to solve another question as the round-off errors will accumulate. Choose downward as the positive y direction. Assume g = 9.81 m/s^2.


    1. Calculate his velocity as he makes contact with the safety net, if h = 4.0 m. Give your answer in m/s, to one decimal place.


    2. Find the change in the performer's momentum, to three significant figures, if he weighs 80.0 kg. Give your answer, including sign, in kg · m/s.


    3. Assume constant acceleration and calculate the stopping time of the performer to two decimal places, if Δy = 1m. Give your answer in seconds.


    4. Find the average force, including sign, acting on the performer while he is stopped by the net, using the impulse-momentum theorem. Give your answer in Newtons rounded to three significant figures.


    5. Assume that the safety net is not in place; the circus performer hits the concrete floor from a height of 4m, and stops almost instantly, in Δt = 0.02 sec. What average force, including sign, would act on him then? Give your answer in Newtons, rounded to three significant figures.


    2. Relevant equations



    3. The attempt at a solution

    I got 1 and 3, how should I do the rest?

    1. 8.9
    3. 0.45

    Are they right?
     
  2. jcsd
  3. Oct 17, 2009 #2

    ideasrule

    User Avatar
    Homework Helper

    3 isn't correct. How did you get 0.45?

    2. What's his initial momentum before hitting the net? What's his final momentum? Subtract.

    4. Write out the impulse-momentum theorem and you'll see.
    5. Again, write out the impulse-momentum theorem and you'll see.
     
  4. Oct 17, 2009 #3
    3.

    delta y = vot + 0.5 a t^2

    Vo = 0

    delta y = 0.5 g t^2

    t^2 = 2 delta y/g
    = 2 * 1/(9.81)
    = 0.20387 s
    t = 0.45 s?

    2. Initial momentum is 0 since vo=0. Final momentum, I can't use the value of the 1st question since it says not to.

    4. I = Fav delta t
    I don't know I, T..

    5. How you get I?
    Delta t = 0.02 s. How can I get I?
     
  5. Oct 17, 2009 #4
    Ok, I did it again. I managed to get 2. Is it +238? Since, vi=0
    And formula is mvf - mvi ?
     
  6. Oct 18, 2009 #5
    Why don't you reply?
    ???
     
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