Elastic Collision of a Ball and Block on a Wire

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Homework Help Overview

The problem involves a ball attached to a wire that swings down to collide with a block on a frictionless surface. It is set in the context of elastic collisions and energy conservation, with specific masses and dimensions provided.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the need to find the velocity of the ball before the collision, with some suggesting the use of conservation of energy. Questions arise about the relevance of time in the calculations and the definitions of radial and tangential motion.

Discussion Status

Some participants have offered guidance on using conservation of energy for the first part of the problem, while others are exploring different interpretations of the motion involved. There is a mix of understanding regarding the relationship between time, energy conservation, and the mechanics of the collision.

Contextual Notes

Participants are navigating concepts related to energy conservation and the specifics of elastic collisions, with some expressing uncertainty about the terminology and approach to the problem.

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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