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m3- springs

Hi

I've posted the question and the solution from the book.

I don't understand why it's 2g - T and not T - 2g

Thanks
m3.doc470.5KB
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Avatar for TeeEm
TeeEm
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Original post by maggiehodgson
Hi

I've posted the question and the solution from the book.

I don't understand why it's 2g - T and not T - 2g

Thanks


the direction of x increasing is "downwards"
hence the direction of x double dot must also be downwards
So this is the "positive direction", so 2g - T
Original post by maggiehodgson
Hi

I've posted the question and the solution from the book.

I don't understand why it's 2g - T and not T - 2g

Thanks


By Newton II, the nett force vector causes an acceleration vector that points in the same direction as the nett force vector. Here acceleration is +ve downwards, so we have to add up the force vectors taking that convention into account i.e. 2g is a +ve force, since it points downwards, T is a -ve force, as it points upward.

So we have 2g + (-T) = ma

To make this clear to yourself when solving 1D mechanics problems, you should make sure to:

1) draw a picture labelling the all of the force arrows
2) draw an arrow, labelled "+ve" pointing in the direction you choose as +ve
3) Add up the force vectors according to that convention
(edited 8 years ago)
Sorry for the delay in reply. I've been working on it trying to make sense of what I was doing (I'm a bit slow on the uptake with mechanics).

I've done something that seems to turn out OK but I would appreciate it if you would check my logic.

Thanks
Scan0003.pdf456.4KB
Original post by TeeEm
the direction of x increasing is "downwards"
hence the direction of x double dot must also be downwards
So this is the "positive direction", so 2g - T


Yes. I think I might have got it now.

Thanks

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