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Mag of impulse required to change speed question.

  1. Aug 28, 2008 #1
    1. The problem statement, all variables and given/known data

    A proton has mass 1.7x10-27 kg. What is the magnitude of the impulse required in the direction of motion to increase its speed from 0.995c to 0.998c? Where c is the speed of light.

    2. Relevant equations

    Im not quite sure of what equation is supposed to be used. I tried using (momentumfinal)=(momentuminitial) + Fnet*(changeintime)

    3. The attempt at a solution

    Ok my attempt at the problem is the only way i can think of it and the answer is still wrong. I first multiplied the speeds by the mass to get a momentum. Then used the two momentums and plugged them into the equation above and solved for Fnet*(changintime) which equals Impulse if im not mistaken. I got the number 1.53e-21 for my answer.

    The only problems i can think of is maybe those speeds are speeds(needs to be velocity).. and can't multiply by the mass to get a momentum but i don't see why that would cause a problem. Overall im stumped on this problem and not sure why what ive done doesn't work. I even have talked to others in the class and no one can figure it out.

    Thanks for all the help in advance guys.
  2. jcsd
  3. Aug 29, 2008 #2
    Considering how high the speed of the particle is, I wonder if you have to consider relativistic momentum?
  4. Aug 29, 2008 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Yes, 0.995c is definitely relativistic. mv will not work for calculating momentum.
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