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: Solving for a specific #

  1. Dec 6, 2004 #1
    How do you slove for Mb from this equation (MaVa+MbVb)/Ma+Mb=Vab
  2. jcsd
  3. Dec 6, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    HINT: Multiply both sides by [itex]M_a + M_b[/itex]
  4. Dec 6, 2004 #3
    K got it, ty, got another one for you tho.

    This one is HARD.

    A 13 g bullet traveling 218 m/s penetrates a 2.0 kg block of wood and emerges going 161 m/s. If the block is stationary on a frictionless surface when hit, how fast does it move after the bullet emerges?
  5. Dec 6, 2004 #4
    Try Conservation of Momentum.
  6. Dec 6, 2004 #5
    Change the units into kg first...
    13g = 0.013kg


    0.013(218) = 0.013(161) + 2v


    v = \frac{0.013(218-161)}{2} = 0.3705 m/s
  7. Dec 6, 2004 #6
    Yea converting the g to kg was the missing step, thanks.
  8. Dec 6, 2004 #7
    No problems - I used to make careless mistakes like that all the time.
  9. Dec 6, 2004 #8
    A tennis ball may leave the racket of a top player on the serve with a speed of 62.0 m/s. If the ball's mass is 0.0600 kg and it is in contact with the racket for 0.0200 s, what is the average force on the ball?

    Would this force be large enough to lift a 60 kg person?
    large enough or
    not large enough

    I got .0600x62=3.72


    I dont think that is right because that seems way to small but I dont know what im missing...if it is incorrec that is.
  10. Dec 6, 2004 #9
    Try momentum-impluse theorem:
    [tex]F\Delta t = m\Delta v[/tex]
  11. Dec 6, 2004 #10


    User Avatar
    Science Advisor
    Homework Helper

    You're last calculation is wrong.As smb else said,apply the definition of force wrt to changes in momentum (the integrated form of Newton's second law) correctly.
    You should be getting round about 19 kgf which would not be enough to lift 60kg off the ground,since the force needed to do that is obviously 60kgf.
    Divagation ( culture useful):
    The greatest tennis player ever (to me ond not only,though he didn't win Rolland Garros),Pete Sampras (i don't believe the bull**** they say about Federer),used to have the strings of his racket streched to a tension of 34kgf.It was the most "tensioned" racket in the history of the sport.And even so,it broke frequently.He was famous for breaking an impressive number of rackets (strings) during serve,which meant that (at a maximum speed of 62m/s -he never served more than 220 kmph/137mph) for light balls (the ones used on grass) the time of impact between the racket and the ball was at the order of microseconds.Amazing,really!!That's the reason for those huge and perfect forehands crosscourt and down the line.Alongside genious,of course.
    I almost cried when he retired. :cry:
    Last edited: Dec 6, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook