Physics

posted by Bob

A 55kg person pushes on the floor with 800 N of force for 0.25 seconds. How high do they jump off the ground?
I know how to find acceleration but I can't find distance.

  1. Damon

    change in momentum = force * time

    force up = 800 -55(9.81)
    = 260 N net up force

    so
    momentum = m Vi = 260*.25 = 65
    so
    Vi = 65/55 = 1.18 m/s up

    (1/2)mVi^2 = m g h
    h = Vi^2/[2(9.81)] = .071 meters
    = 7.1 cm

  2. Bob

    I haven't learned momentum yet, is there any way to do it with only Forces and F=ma as well as kinematics?
    Thanks

  3. Damon

    Not really.
    I bet you have heard the word "Impulse"
    that is force * time
    which is change of momentum

    F = m a = m dv/dt
    so
    F dt = m dv = "impulse"

    but m dv is change in m v
    and m v is "momentum
    therefore
    F * time = change in momentum = impulse

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