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: Kinematics and impulse

  1. Feb 2, 2010 #1
    1. The problem statement, all variables and given/known data
    Two particles of masses 8 kg and 5 kg are connected to the two ends of a light inextensible string which passes over a fixed smooth pulley. Initially each of the two particles are held at a position which is 5 m above a horizontal ground. The objects are then released from rest. Assuming that the particles never reach the pulley, and also that the particles do not rebound when thhey strike the ground.

    (i) find the tension in the string and the acceleration of each particle
    (ii) find the distance above the ground of the point H, the highest point reached by the 5 kg mass
    (iii) find the speed with which the system is jerked into motion and the impulse experienced by the string during the jerk.

    2. Relevant equations
    F = ma
    Impulse = change in momentum
    p = mv

    3. The attempt at a solution
    I'm able to do part (i) and (ii). For part (i), I got T = 800/13 N and a = 30/13 ms-2

    I don't understand how to do the part (iii). What is the meaning of "which the system is jerked into motion"? I think maybe I have to find the speed when it moves for the first time, but no information about time given...

  2. jcsd
  3. Feb 2, 2010 #2


    User Avatar
    Homework Helper

    Find the velocities of masses when 8 kg mass hits the ground.
    At that instant T becomes zero. 5 kg mass stops accelerating. That may be the point of jerk. After that 5 kg mass retards with g. You can find distance traveled by it before coming to rest.
  4. Feb 2, 2010 #3
    Hi rl.bhat

    Oh, that's the point of jerk. Then to find the impulse on the string, take the initlal velocity as the velocity of point of jerk and final velocity = 0?

  5. Mar 19, 2011 #4
    Can u solve the question ii/ and iii/ a bit clearly...THx
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook