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

Finding momentum, friction, and forces

  1. Jan 22, 2012 #1
    Hello forum, Vaironl here.

    I believe this will be harsh thing to ask for but I will post about 3 different long problems (Easy problems) in this thread.
    I don't understand a couple of things in my physics class since I arrived from a trip about 2weeks ago and missed a lot of import information.

    I just need help finding things I don't know.

    Thanks in advance.

    Note, for some reason my book list things in kgm/s and I must convert to newtons but I thought newtons are kgm/s^2, you guys will understand what am talking about when you read the problems

    Problem 1:

    (a) What is the momentum of an 8kg bowling ball rolling at 2m/s?
    8kg * 2m/s = 16kgm/s

    (b) If the bowling ball rolls into a pillow and stops in 0.5s, calculate the average force it exerts on the pillow?
    Ft= MΔV
    F(0.5s) = 8kg * -2m/s
    F(0.5s) = -1kgm/s
    F = -32kgm/s or -32N ? how can I correctly do this calculation

    (c) What average force does the pillow exert on the ball?
    32N ?

    (a)What is the momentum of a 50kg carton that slides at 4m/s across an icy surface?
    50kg*4m/s = 200 kgm/s

    (b)The sliding carton skids onto a rough surface and stops in 3s. Calculate the force of friction it encounters
    F(3s) = 50kg*-4m/s
    F=67N? Again I don't know how can taking seconds from kgm/s converts this to Newtons
    if possible can someone please provide me with instructions on how kgm/s transfer into Newtons.

    (a)What impulse occurs when an average force of 10N is exerted on cart for 2.5s?
    Impulse = FΔt
    = 10N(-2.5s)
    = 25N/s

    (b)What change in momentum does the cart undergo?

    (c)If the mass of the cart is 2kg and the cart is initially at rest, calculate it's final speed.
    M=2kg V=0m/s

    I can't even think about what solution its for this one.
    Please if possible don't give me the answers but guide me closely to them (Sounds weird)
  2. jcsd
  3. Jan 22, 2012 #2


    User Avatar
    Homework Helper

    That is the unit for momentum, mass x velocity.

    The ball is rolling so you need to include angular momentum as well. Assume it's a solid uniform sphere.

    You didn't divide both sides by s: F(...) = ... kg m / s2

    N = kg m / s2, so N s = kg m / s (momentum).
  4. Jan 22, 2012 #3
    Ft = mv is, in terms of units, N*s = kg[itex]\frac{m}{s}[/itex]

    so to solve for F would you divide both sides by t

    F = [itex]\frac{mv}{t}[/itex]

    in terms of units, you are also dividing both sides by seconds (s), but another way to think about dividing by something is to say that you are multiplying by the reciprocal


    N = kg[itex]\frac{m}{s}[/itex][itex]\frac{1}{s}[/itex]

    which results in N = kg[itex]\frac{m}{s^{2}}[/itex]

    I hope that helps clear up the confusion about units
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook