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# Physics a level question help - moments?! Watch

1. this question IS about moments, right?
I need help with all of question 4 pleeassee. is the answer to part b 'the line the Q force acts'? (or something along those line? idk if i worded it correctly)
also, idk if what i wrote for the 1st question is correct?
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2. Yes P+Q=W is correct.
You may also see it expressed as P+Q-W=0

The magnitude of the force W is just the weight of the tray.
You have been told its mass.
So...

Finding P and Q is the moments bit.

To find Q take moments about the end where P is. This eliminates P

The anticlockwise moment of Q is that unknown force times the distance* from P
The clockwise moment is the weight W you calculated in the previous part times its distance* from P
What equation can you write if the moments balance? (This is the case if the tray is in equilibrium.)

* distance is always "perpendicular" distance. In this case it's the distance given as the angle is 90deg between the force and the measured distance along the tray.
3. (Original post by Stonebridge)
Yes P+Q=W is correct.
You may also see it expressed as P+Q-W=0

The magnitude of the force W is just the weight of the tray.
You have been told its mass.
So...

Finding P and Q is the moments bit.

To find Q take moments about the end where P is. This eliminates P

The anticlockwise moment of Q is that unknown force times the distance* from P
The clockwise moment is the weight W you calculated in the previous part times its distance* from P
What equation can you write if the moments balance? (This is the case if the tray is in equilibrium.)

* distance is always "perpendicular" distance. In this case it's the distance given as the angle is 90deg between the force and the measured distance along the tray.

thank you!

ahh, im finding the moments bit hard. i never know where the pivot is. is his thumb the pivot?
4. (Original post by hibaj)
thank you!

ahh, im finding the moments bit hard. i never know where the pivot is. is his thumb the pivot?
The pivot provides the upwards reaction force so no it isn't the thumb
5. (Original post by raiden95)
The pivot provides the upwards reaction force so not it isn't the thumb
are you sure?

i think i got the right answer
6. (Original post by raiden95)
The pivot provides the upwards reaction force so not it isn't the thumb
This guy and his smartness.
Screw you and your A's!
7. (Original post by hibaj)
are you sure?

i think i got the right answer
Yes I'm sure, the diagram shows where the reaction force is, so you probably did get it right. But for questions where you have to draw the diagram yourself the reaction force provides the upwards force and in this case it isn't the thumb it's the fingers. Hope that helps
8. (Original post by Amanbabbar./)
This guy and his smartness.
Screw you and your A's!
I'm not 'this guy'
9. (Original post by raiden95)
I'm not 'this guy'
That's what they all say.
10. (Original post by Amanbabbar./)
That's what they all say.
Urm, you don't do physics, you don't belong here
11. (Original post by raiden95)
Urm, you don't do physics, you don't belong here
Aaaw, you'll cry when I'm gone.
See yaaa!
12. (Original post by Amanbabbar./)
Aaaw, you'll cry when I'm gone.
See yaaa!
Goodbye
13. (Original post by raiden95)
Yes I'm sure, the diagram shows where the reaction force is, so you probably did get it right. But for questions where you have to draw the diagram yourself the reaction force provides the upwards force and in this case it isn't the thumb it's the fingers. Hope that helps
ohh, i think i understand, kind of ;p thanks
14. (Original post by hibaj)
ohh, i think i understand, kind of ;p thanks
Welcome Are you doing unit 2 aqa?
15. (Original post by hibaj)
thank you!

ahh, im finding the moments bit hard. i never know where the pivot is. is his thumb the pivot?
In this question it doesn't really matter.
If you take moments about P then Q is the pivot.
If you take moments about Q then P is the pivot.
In this case it can be either as neither point is "fixed".
16. (Original post by Stonebridge)
In this question it doesn't really matter.
If you take moments about P then Q is the pivot.
If you take moments about Q then P is the pivot.
In this case it can be either as neither point is "fixed".
ok, thank you!

when they give you weight, is that always the weight from the centre of mass?
17. (Original post by raiden95)
Welcome Are you doing unit 2 aqa?
Yep. You?
18. (Original post by hibaj)
Yep. You?
Yes, good luck
19. (Original post by hibaj)
ok, thank you!

when they give you weight, is that always the weight from the centre of mass?
Yes. In most cases the object will be described as "uniform" and the C of M is then at the mid point.
In this example they just give you the position of the W force so you don't have to worry about it.

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