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Speed when a system is jerked into motion

  • #1

Homework Statement


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 they 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

Homework Equations


Kinematics and dynamics


The Attempt at a Solution


(i) I am able to do this

(ii) because the particles never reach the pulley, the highest point is 10 m, which is when the 8 kg-mass reaches the ground. Correct?

(iii) I do not understand this one. What is the meaning "speed with which the system is jerked into motion"? I interpreted it as the initial speed of the system and the answer should be zero? If not, maybe the speed when the particles start to move. But how to find it? I have acceleration and the initial speed is zero (because the particles are at rest and then released), but the speed depends on the time. What is the time that I should take to put to the equation of motion?

Thank you very much
 

Answers and Replies

  • #2
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(ii) Both are initially 5m above ground. what would be height of one if other is moved to ground. keep in mind that string is inextensible.

(iii) impulse is change in momentum i.e. dP
but even i dont know what "speed with which the system is jerked into motion" means
 
  • #3
(ii) Both are initially 5m above ground. what would be height of one if other is moved to ground. keep in mind that string is inextensible.
10 m?
(iii) impulse is change in momentum i.e. dP
but even i dont know what "speed with which the system is jerked into motion" means

This question confused me. Can anyone please interpret waht the question means?

Thank you very much
 

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