Elastic Collision of a Ball and Block on a Wire

AI Thread Summary
The discussion revolves around a physics problem involving an elastic collision between a ball and a block. The ball, attached to a wire, swings down and strikes a block on a frictionless surface. Participants suggest using conservation of energy to determine the ball's velocity just before the collision, emphasizing that the time taken for the fall is irrelevant. The conversation clarifies that understanding radial and tangential motion is not necessary for solving the problem. The key takeaway is to apply conservation of energy principles rather than focusing on time or specific motion types.
emmi1987
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Homework Statement


A ball is attached to one end of a wire, the other end being fastened to the ceiling. The wire is held horizontal and the ball is released from rest. It swings downward and strikes a block initially at rest on a horizontal frictionless surface. Air resistance is negligible and the collision is elastic. The masses of the ball and block are, respectively, 1.6 kg and 2.4 kg and the length of the wire is 1.2 m. Find the velocity (magnitude and direction) of the ball just before the collision and just after the collision.

... I don't even know where to start with this one, I can't find anything about radial? (not sure if that's the appropriate term) motion in regards to velocity. Any help would really be appreciated.


Homework Equations





The Attempt at a Solution

 
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emmi1987 said:
... I don't even know where to start with this one, I can't find anything about radial? (not sure if that's the appropriate term) motion in regards to velocity. Any help would really be appreciated.

Hi emmi1987! :smile:

Radial is towards/away from the centre.

Tangential is round the circle (and tangential velocity = angular velocity times radius). :wink:

But you don't need either … just use conservation of energy for the first part, and the usual elastic collision equations for the second part. :smile:
 
Thanks! I'm golden on the second part.. but for the first part, do i need to find the time it takes for the ball to fall in order to find v? and then use conservation of energy?
 
emmi1987 said:
… for the first part, do i need to find the time it takes for the ball to fall in order to find v? and then use conservation of energy?

Hi emmi1987! :smile:

No, the time doesn't matter …

conservation of energy doesn't depend on how you get from state A to state B (state = position + speed), or how long it takes …

KE(A) + PE(A) = KE(B) +PE(B). :smile:
 
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