AlconH
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I'm struggling to wrap my head around the wording of a particular style of Mechnics question. It's from Chapter 3, Dynamics of a particle moving in a straight line.

Sometimes in the question, it will say:
"Find the magnitude of the force that Block B exerts on Block A"
or it will say
"Find the magnitude of the impulse of B on A during the impact"
or
"Find the magnitude of the force exerted by the lift on the man"

I don't understand what to work out, the solutions all seem to use different methods for all of them, and some involve the first particle mentioned, others involve the second. Can anyone help explain the quotes above in a way that I can understand?
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Kevin De Bruyne
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(Original post by AlconH)
I'm struggling to wrap my head around the wording of a particular style of Mechnics question. It's from Chapter 3, Dynamics of a particle moving in a straight line.

Sometimes in the question, it will say:
"Find the magnitude of the force that Block B exerts on Block A"
or it will say
"Find the magnitude of the impulse of B on A during the impact"
or
"Find the magnitude of the force exerted by the lift on the man"

I don't understand what to work out, the solutions all seem to use different methods for all of them, and some involve the first particle mentioned, others involve the second. Can anyone help explain the quotes above in a way that I can understand?
The first and third situations that you've described seem pretty similar to me - except one of the blocks has become a lift. Sometimes you end up having to do different methods for this because you have unknowns or you've worked out the unknowns in previous parts of the question. If you had A on top of B, which was on top of a moving lift, you might be asked about the first scenario. You can't directly work out B on A but you can use Newton's third law after working out A on B - which you can work out because that's not affected by the force that the lift exerts on B, as it's not on the bottom.

As for the second one there's a formula for the impulse, it shouldn't be too bad.

I'm not sure how many different ways there are of doing it, but as long as you understand each one you should be able to reproduce it with enough practice.
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AlconH
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(Original post by SeanFM)
The first and third situations that you've described seem pretty similar to me - except one of the blocks has become a lift. Sometimes you end up having to do different methods for this because you have unknowns or you've worked out the unknowns in previous parts of the question. If you had A on top of B, which was on top of a moving lift, you might be asked about the first scenario. You can't directly work out B on A but you can use Newton's third law after working out A on B - which you can work out because that's not affected by the force that the lift exerts on B, as it's not on the bottom.

As for the second one there's a formula for the impulse, it shouldn't be too bad.

I'm not sure how many different ways there are of doing it, but as long as you understand each one you should be able to reproduce it with enough practice.
I understand what you mean, but when it says "Find the magnitude of the force that Block B exerts on Block A" , I don't know what it's asking for? Do I work out the resultant for of A?
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Kevin De Bruyne
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(Original post by AlconH)
I understand what you mean, but when it says "Find the magnitude of the force that Block B exerts on Block A" , I don't know what it's asking for? Do I work out the resultant for of A?
When one object is on top of another they exert an equal and opposite force on eachother, so if A is on top of B there's an upward force from B acting on A, and a downward force from A acting on B. When you resolve forces you should be able to find that force, *but* for B, which is on something else like a lift, has a second unknown, which is from the support force that the lift exerts on B. But as A isn't touching the lift, it's not affected by the support force, so when resolving you only have one unknown, so you can find that, which is the force B exerts on A.

It it's changed so B is on top of A, with the same question that you've said, you find the resultant for B and use Netwon's Third Law because the forces are equal and opposite.

So I suppose the rule of thumb is to work out the resultant for the one on top.
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AlconH
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(Original post by SeanFM)
When one object is on top of another they exert an equal and opposite force on eachother, so if A is on top of B there's an upward force from B acting on A, and a downward force from A acting on B. When you resolve forces you should be able to find that force, *but* for B, which is on something else like a lift, has a second unknown, which is from the support force that the lift exerts on B. But as A isn't touching the lift, it's not affected by the support force, so when resolving you only have one unknown, so you can find that, which is the force B exerts on A.

It it's changed so B is on top of A, with the same question that you've said, you find the resultant for B and use Netwon's Third Law because the forces are equal and opposite.

So I suppose the rule of thumb is to work out the resultant for the one on top.
Got it, thanks for your help. It's much appreciated.
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