x Turn on thread page Beta
 You are Here: Home >< Maths

# m3- springs watch

1. 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
Attached Files
2. m3.doc (459.5 KB, 36 views)
3. (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
4. (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
5. 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
Attached Images
6. Scan0003.pdf (445.7 KB, 25 views)
7. (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

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 27, 2016
Today on TSR

### Been caught plagiarising...

...for the 2nd time this year

### Mum says she'll curse me if I don't go to uni

Discussions on TSR

• Latest
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE