3pointonefour
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A ball of mass 0.2kg, moving along a horizontal surface, hits a fixed vertical wall at right angles. The ball rebounds at right angles to the wall with speed 3.5m/s. Given that the magnitude of the impulse exerted by the ball on the wall is 2Ns, find the speed of the ball just before it hits the wall.

So I did this question and found 2 different answers, one with u = 13.5m/s and one with u = 6.5m/s.
The difference in answers came from which impulse I took: do I take the impulse exerted by the wall on the ball, or vice versa?

Thanks in advance
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3pointonefour
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Bump
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Muttley79
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(Original post by 3pointonefour)
Bump
Post your working .- I'm not sure what you are saying.
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3pointonefour
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Here's my working Muttley79
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3pointonefour
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Pmub
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buuuump
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Muttley79
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Stop bumping the thread .... the ball should be moving away from the wall after the impact.
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3pointonefour
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(Original post by Muttley79)
Stop bumping the thread .... the ball should be moving away from the wall after the impact.
So this means that I should use the impulse of the wall on the ball... The one exerting left?

And sure thing
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Muttley79
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(Original post by 3pointonefour)
So this means that I should use the impulse of the wall on the ball... The one exerting left?

And sure thing
When a ball hits a wall, the wall "forces" the ball away from the wall so the impulse on the ball is away from the wall.
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3pointonefour
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(Original post by Muttley79)
When a ball hits a wall, the wall "forces" the ball away from the wall so the impulse on the ball is away from the wall.
Ok thank you. So it would be 6.5 m/s then
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Muttley79
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(Original post by 3pointonefour)
Ok thank you. So it would be 6.5 m/s then
It should be - have you got answers to check?
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Sir Cumference
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(Original post by 3pointonefour)
Here's my working Muttley79
By Newton's 3rd Law, the impulse exerted on the wall by the ball is equal and opposite to the impulse exerted on the ball by the wall. Remember that impulse is proportional to the force.

If you were to consider the impulse on the wall by the ball (directed to the right since the ball is pushing the wall to the right), then you would need to consider the change of momentum of the wall. But there are other forces stopping the wall from moving and you'd have to include these in your calculations to give you a total impulse (which would actually be 0 since the wall is not moving).

Since you don't know anything about these forces on the wall, the only way to do this question is to consider the change of momentum of the ball. Thus you must consider the impulse that is directed to the left.
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3pointonefour
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(Original post by Notnek)
By Newton's 3rd Law, the impulse exerted on the wall by the ball is equal and opposite to the impulse exerted on the ball by the wall. Remember that impulse is proportional to the force.

If you were to consider the impulse on the wall by the ball (directed to the right since the ball is pushing the wall to the right), then you would need to consider the change of momentum of the wall. But there are other forces stopping the wall from moving and you'd have to include these in your calculations to give you a total impulse (which would actually be 0 since the wall is not moving).

Since you don't know anything about these forces on the wall, the only way to do this question is to consider the change of momentum of the ball. Thus you must consider the impulse that is directed to the left.
So the impulse in the Impulse-Momentum equation refers to the Impulse that is acting on that specific body (in this case, the ball)?

Thanks a bunch, I'm just wanting to clear up misconceptions on my understanding
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Sir Cumference
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(Original post by 3pointonefour)
So the impulse in the Impulse-Momentum equation refers to the Impulse that is acting on that specific body (in this case, the ball)?

Thanks a bunch, I'm just wanting to clear up misconceptions on my understanding
Yes, the impulse on an object is equal to the change of momentum of the object. From your working, it looks like you weren't clear on this.

You can take right as positive if you want but it would still mean that the impulse on the ball is directed to the left so it would be negative in this case.
Last edited by Sir Cumference; 1 year ago
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3pointonefour
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(Original post by Notnek)
Yes, the impulse on an object is equal to the change of momentum of the object. From your working, it looks like you weren't clear on this.

You can take right as positive if you want but it would still mean that the impulse on the ball is directed to the left so it would be negative in this case.
Okay, thanks very much, makes more sense now
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