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

# M1 moments watch

1. I managed to do parts a) and b) but for part c) I'm stuck on what to do next,
So far I've taken moments about C to get Wx = 5000. I resolved vertically to get 1000 + Rc = W (Rc is reaction force of C). I then tried moments about W and got Rc(x) = 5000 - 1000x this didn't help either.

I can't seem to find the weight from this, is there something I missed out? thank you
Attached Images

2. (Original post by .raiden.)
I managed to do parts a) and b) but for part c) I'm stuck on what to do next,
So far I've taken moments about C to get Wx = 5000. I resolved vertically to get 1000 + Rc = W (Rc is reaction force of C). I then tried moments about W and got Rc(x) = 5000 - 1000x this didn't help either.

I can't seem to find the weight from this, is there something I missed out? thank you
First of all, taking moments about C was a good idea. That gives you an equation with two variables, which immediately suggests that you need another equation with the same two variables.

You should realise that taking moments about C was a good idea precisely because it got rid of Rc. You therefore do not want an equation with Rc in it at all, so although you resolved forces correctly, the equation is not useful in this question.

Therefore you should look back to the setup from part (a). You were given an assumption that the log was uniform, but you are now told that it is unlikely to be so. So you revisit the case where the force is being applied at A and this time take moments about D which will give you an equation in terms of W and x. You then have a simple case of simultaneous equations, from which W and x should drop out pretty quickly.
3. (Original post by Brister)
First of all, taking moments about C was a good idea. That gives you an equation with two variables, which immediately suggests that you need another equation with the same two variables.

You should realise that taking moments about C was a good idea precisely because it got rid of Rc. You therefore do not want an equation with Rc in it at all, so although you resolved forces correctly, the equation is not useful in this question.

Therefore you should look back to the setup from part (a). You were given an assumption that the log was uniform, but you are now told that it is unlikely to be so. So you revisit the case where the force is being applied at A and this time take moments about D which will give you an equation in terms of W and x. You then have a simple case of simultaneous equations, from which W and x should drop out pretty quickly.
Thank you i will try this now
4. (Original post by Brister)
First of all, taking moments about C was a good idea. That gives you an equation with two variables, which immediately suggests that you need another equation with the same two variables.

You should realise that taking moments about C was a good idea precisely because it got rid of Rc. You therefore do not want an equation with Rc in it at all, so although you resolved forces correctly, the equation is not useful in this question.

Therefore you should look back to the setup from part (a). You were given an assumption that the log was uniform, but you are now told that it is unlikely to be so. So you revisit the case where the force is being applied at A and this time take moments about D which will give you an equation in terms of W and x. You then have a simple case of simultaneous equations, from which W and x should drop out pretty quickly.
Yes i got the answer, i really need to always look at previous parts as this never occured to me

5. (Original post by .raiden.)
Yes i got the answer, i really need to always look at previous parts as this never occured to me

Hey, if you can improve a little after every question you do, then you are studying effectively

This is quite a neat question, really. You should always try to understand why the questions ask you to do certain things. The reason they apply a force until the log is about to tip is that you get rid of one of the reaction forces. You then basically ignore the other reaction force by taking moments about the axis through the contact. You cannot find W or x from one equation, so you just repeat the procedure at the other end.

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: April 8, 2013
Today on TSR

### How much will your degree earn you?

Find out where yours ranks...

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