Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about acceleration

  1. Apr 16, 2008 #1
    if someone was standing on a skateboard at rest and then jumped off, how would you calculate the person's horizontal jumping speed (if the skateboard rolled in the opposite direction). what information would i need? (the masses of the person and the skateboard and the speed which the skateboard went?)
    thanks for any help!
  2. jcsd
  3. Apr 16, 2008 #2


    User Avatar

    Conservation of momentum - (total mass of skateboarder + board) * velocity = velocity of skateboarder * his mass + velocity of board + its mass.

    For instance, 65kg boarder, 5kg board, initially at 10m/s. Initial momentum = (65+5)kg*10m/s = 700kgm/s. Then assume the board stopped completely - 65kg boarder now takes on all the momentum, his speed will be 700kgm/s divided by 65kg - approx 10.76m/s.

    On the other hand, unseatbelted dude and car - momentum = 60kg + 1000kg * 30m/s = 82400kgm/s. The car barely has to slow down to kill said dude.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook