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: Two momentum questions

  1. Apr 6, 2007 #1
    1. The problem statement, all variables and given/known data
    A 72 kg man jumps
    a) If the jump results in an upward speed of 2.1 m/s, what additional upward force does the floor exert, if the person pushes the ground for 0.36 s?

    2. Relevant equations
    F= mv/t

    3. The attempt at a solution
    F_net = Impule/t

    F_byFloor - W = F_net

    f_byFloor = F_net + W
    = (mv_f / t) + mg
    = m ((v_f / t)+g)
    = 1126.32 N

    But, the answer in the book is 420 N, and it just ignored Weight of the person.
    If my Force equation (F_byFloor - W = F_net) is right?

    1. The problem statement, all variables and given/known data

    A bee lands on one end of a floating 4.75 g Popsicle stick. After sitting at rest for a moment, it runs towards the other end with a speed of 3.80 cm/s relative to still water. The stick moves in the opposite direction at 0.12 cm/s relative to the still water. What is the mass of bee?

    2. Relevant equations

    mv_b + Mv_popsicle = 0

    3. The attempt at a solution

    I am confused about the velocities. Like, How the results would be changed if I use bee velocity relative to Popsicle. And, do I need to have velocities of both objects relative to some stationary thing in order to solve this problem?
  2. jcsd
  3. Apr 6, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    Question 1) asks for 'additional upward force', not 'total upward force' implying you should ignore the weight. For 2) a consequence of the conservation of momentum is that the center of mass of SYSTEM doesn't accelerate unless acted upon by a force from OUTSIDE of the system. Consider the bee and the popsicle stick as a system.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook