1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Why is my momentum equation giving an incorrect force?

  1. Nov 24, 2016 #1
    1. The problem statement, all variables and given/known data
    There are two boxes, sitting right next to each other. Call the box on the left "box A" and the box on the right "Box B". A 50 pound force starts pushing A into B. Find their speed at 5 seconds, and how much force Box B puts on Box A. Box A weighs 100 pounds, Box B is 50 pounds and the coefficient of kinetic friction is 0.25.

    2. Relevant equations
    (mv)_1 + (integral of force with repsect to time) = (mv)_2

    3. The attempt at a solution
    First, I solved for their speed and acceleration using basic equations of motion (It is in fact 13.4 ft/s. This implies an acceleration of 2.68 ft/s.) Then, I solved for the force that Box B places on box A. My attempt goes like this:

    F_(b on a) * 5 = (50/32.2)*13.4. This yields and answer of 4.1 lbs. However, by doing F_(b on a)-(50(0.25))=(50/32.2)(2.68) I get the correct answer, (because of Newton's third law.)

    Why does my first equation not yield the correct answer? Thanks for any help!
  2. jcsd
  3. Nov 24, 2016 #2


    User Avatar

    Staff: Mentor

    Is your F_(b on a) the only force acting on block B? The change in momentum (or impulse) will be due to the net force acting on the block.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted