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Homework Help: What equation to use

  1. Sep 20, 2016 #1
    1. The problem statement, all variables and given/known data
    You are observing a baseball in space, with mass m = 0.144 kg. It is moving past your spacecraft with speed v = 8 m/sec, to the north. Your Acme Tractor Beam can exert 1.2 Newtons of pull force on that baseball.
    CALCULATE: How long will it take your Acme Tractor Beam to slow down the baseball to a stop?

    2. Relevant equations
    I'm pretty sure I used the wrong equations. For the first time, I used the impulse equation FΔt=Δp
    So with this one, to calculate Δp I took mass (.144) x velocity (8m/s) and got 1.152
    So 1.152=(1.2N)Δt. I divided both sides by 1.2 to get Δt=.96
    This was incorrect.

    On the second attempt, I tried another equation. F(m/v) and got .0216. This was also wrong.

    3. The attempt at a solution
    See above!!

    Please help me figure out what equation I should be using!
  2. jcsd
  3. Sep 20, 2016 #2


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    Homework Helper
    Gold Member
    2017 Award

    Hello. Welcome to PF!

    Your first method looks correct as long as you can assume that the force on the ball due to the tractor beam is always toward the south. You should add units to your answer. (When you're in a spaceship in space, it's not clear what "north" means. But I guess we can just ignore that.)

    It is a good exercise to think about how the signs work out here. If you take north as the positive direction then the initial momentum is positive and the final momentum is zero. So, Δp is negative.

    For your second attempt, how do you justify the expression F(m/v)?
  4. Sep 20, 2016 #3
    How is 0.96s incorrect ?
    Last edited: Sep 20, 2016
  5. Sep 20, 2016 #4
    Thank you for helping! My prof just sent out an email that this question was not formatted correctly, so it looks like my first attempt was actually right. :)
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