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: Impulse question.

  1. Dec 15, 2007 #1
    1. The problem statement, all variables and given/known data

    Hail stones of mass 0.0650 kg are falling straight down with a speed of 15 m/s when they strike a car roof. If the hailstones bounce to a height of 12 cm above the car, what is the impulse that the car roof imparts to a single hailstone?

    2. Relevant equations
    p=mv J=Ft

    3. The attempt at a solution
    I tried finding the downward momentum first and then equalling it to the upward momentum of the hailstones but I am stuck on where to plug in the height of the bounce.
    Do I need to find velocity or time of the hailstone on the way up first? If I found time I think I could multiply it by Force to get J (Impulse).
  2. jcsd
  3. Dec 15, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    Find the velocity up and then use p=mv, J=delta(mv) where delta means the difference between down and up (don't forget one is negative relative to the other). Velocity up isn't equal to velocity down. The lost energy goes into denting the car roof.
  4. Dec 15, 2007 #3
    Thank you,
    I am still not sure how to find v, we have not done any of these type of questions in class.
    I have tried making both sides equal to the minus of the other, but come up with the same (15m/s) because the mass is the same. Time is also unknown so I'm not sure.
    I tried to find the original height by equalling momentum on either side and came up with 0.76 meters. I used this to determine the time to fall as 0.05 s. From this I just now found the velocity up as 2.4 m/s. Now, using the info you gave me, initial p=.975 and final p=-.156.
    so delta p=1.13 kg*m/s. The answer is 1.07 so I'm not sure if this is the way to do this.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook