# Force acting on system

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Hey. I need some help on this question (It's attached along with the diagram). So I first took to drawing free body diagrams of each of the masses but I don't think I've shown all the forces; I'm not particularly sure of my diagram for M2. Also, I'm not sure on the strategy to solve the question. I've assumed there is no friction (because of smooth surfaces). Am I meant to use Newton's second for each of the masses? I don't think I am because it talks of motion relative to the bodies. Any ideas please?

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#2

I think you will be using N2 - the condition referring to relative motion just means that all three bodies will have the same acceleration

There are a couple of issues with your force diagrams: F acts on the box, not the mass m2: you do however need horizontal normal contact forces in two of the diagrams.

I think you are better off considering the box and the pulley as a single object: however if you treat them separately you must not neglect the forces they exert on each other.

There are a couple of issues with your force diagrams: F acts on the box, not the mass m2: you do however need horizontal normal contact forces in two of the diagrams.

I think you are better off considering the box and the pulley as a single object: however if you treat them separately you must not neglect the forces they exert on each other.

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(Original post by

I think you will be using N2 - the condition referring to relative motion just means that all three bodies will have the same acceleration

There are a couple of issues with your force diagrams: F acts on the box, not the mass m2: you do however need horizontal normal contact forces in two of the diagrams.

I think you are better off considering the box and the pulley as a single object: however if you treat them separately you must not neglect the forces they exert on each other.

**Mr Gum**)I think you will be using N2 - the condition referring to relative motion just means that all three bodies will have the same acceleration

There are a couple of issues with your force diagrams: F acts on the box, not the mass m2: you do however need horizontal normal contact forces in two of the diagrams.

I think you are better off considering the box and the pulley as a single object: however if you treat them separately you must not neglect the forces they exert on each other.

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#4

Normal reactions good - but you still need the forces on the box at the pulley, and the external force F on the box.

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#5

(Original post by

Hey. I need some help on this question (It's attached along with the diagram). So I first took to drawing free body diagrams of each of the masses but I don't think I've shown all the forces; I'm not particularly sure of my diagram for M2. Also, I'm not sure on the strategy to solve the question. I've assumed there is no friction (because of smooth surfaces). Am I meant to use Newton's second for each of the masses? I don't think I am because it talks of motion relative to the bodies. Any ideas please?

**samjohnny**)Hey. I need some help on this question (It's attached along with the diagram). So I first took to drawing free body diagrams of each of the masses but I don't think I've shown all the forces; I'm not particularly sure of my diagram for M2. Also, I'm not sure on the strategy to solve the question. I've assumed there is no friction (because of smooth surfaces). Am I meant to use Newton's second for each of the masses? I don't think I am because it talks of motion relative to the bodies. Any ideas please?

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(Original post by

Normal reactions good - but you still need the forces on the box at the pulley, and the external force F on the box.

**Mr Gum**)Normal reactions good - but you still need the forces on the box at the pulley, and the external force F on the box.

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Only one tension in string.

**brianeverit**)Only one tension in string.

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#8

(Original post by

Here's what I've got for the free body diagram of the box M. I've assumed - as the question states - that the pulley is massless and frictionless but, because it's attached to the box, it is a part of it and hence is included in the box's diagram. Is that right?

**samjohnny**)Here's what I've got for the free body diagram of the box M. I've assumed - as the question states - that the pulley is massless and frictionless but, because it's attached to the box, it is a part of it and hence is included in the box's diagram. Is that right?

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Yes, that's how I'd do it. But don't forget the external force F.

**Mr Gum**)Yes, that's how I'd do it. But don't forget the external force F.

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#10

(Original post by

Thanks. Here's what I'm doing so far: for each of the 3 systems I'm adding together all their force vectors and equating them to m*a. How do I proceed on from there?

**samjohnny**)Thanks. Here's what I'm doing so far: for each of the 3 systems I'm adding together all their force vectors and equating them to m*a. How do I proceed on from there?

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(Original post by

For one of the objects, both the horizontal and vertical directions are interesting: for the other two, only the horizontal equations are interesting. This gives you four equations which between them involve a tension, a normal reaction, the acceleration, M, m1, m2, g and F. Eliminate the first three of these and you can get F in terms of M, m1, m2 and g.

**Mr Gum**)For one of the objects, both the horizontal and vertical directions are interesting: for the other two, only the horizontal equations are interesting. This gives you four equations which between them involve a tension, a normal reaction, the acceleration, M, m1, m2, g and F. Eliminate the first three of these and you can get F in terms of M, m1, m2 and g.

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#13

(Original post by

I hope you don't mind, would it be possible for you to quickly check over my work. I think I might have missed a step or something because I've not quite got what you said I should.

**samjohnny**)I hope you don't mind, would it be possible for you to quickly check over my work. I think I might have missed a step or something because I've not quite got what you said I should.

Your last line, that F= a(M-m2-m1) reflects these errors in that applying N2 to the whole system would give you F= a(M+m2+m1).

I think your answer is not expected to include a. From your equations 1,3,5,6, you should seek to eliminate a, t and N_2

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