Advanced Higher Physics 2009

For your last paragraph yes, although it wouldn't compensate to a position where the available force from tension in the springs was more than the required centripetal force, because then there would be an unbalanced force and it would just move in again.

There are different torques acting on the table. You need to equate them as the table is not moving (balanced torques).

I'm just posting this while I work at the answer

25.6N is what I got.... but I didn't realise Torques could just be balanced

All right, thanks, that's pretty much what my calculation was. Although I'm unsure about the torque acting upwards and downwards bit

Remember that since force is a vector, torque must also be a vector as it is force times a scalar. For torques to balance there must be equal amounts acting in opposite directions.

Yeah I understand that... but I don't see how the torque is acting downwards or upwards? Just need to accept it I suppose. Thanks.

(In actual fact it's not according to wikipedia, it acts in a direction perpendicular to both the force and the postional vector of the object away from the axis. But let's not go there.... )

I was thinking clockwise or anti-clockwise or it's effect being that. Think I'll just let one go in a positive direction, the other a negative! Cheers

Your not going to get anything as difficult as that in an exam. (I take it thats from the scholar book..?)

By the way, what does everyone think for q 1c) iii) in 2001. Does the stone fly upwards or horizontal?

Personally, if the stone has just broken free this implies that the centrifugal force [inc. weight] and frictional force have cancelled out. Therefore, when it breaks free we only need consider the objects weight since friction and centrifugal force have buggered off so it goes parallel to the road?

I had this in my prelim, haven't actually got the 2001 paper though. As far as I remember I read the question wrong and got it wrong...

Okey dokes.

Torques could be thought as forces but with an "effect factor" of the distance away from the axis which it is acting. Since they are essentially forces, they could balance.

Working:

T=Fr
The weight of the table exert a torque of T=2.5*9.8*0.4=9.8Nm
The weight of the dish exert a torque of T=1*9.8*0.6=5.88Nm
Combined they give a torque of T=15.68Nm (acting down)

To balance this there must be a torque of 15.68Nm acting up. The only place where this could happen is for the force to be at 80cm where the chain is attached to the table.
The required upwards force is therefore F=T/r=15.68/0.8=19.6N

However this is the force acting straight up. We resolve for the tension in the chain by drawing a triangular force diagram (50 degrees and vertical 19.6N as opposite and Tension as hypotenuse). It can be seen that sin50=19.6/Te which solves for Te=25.6N

Hi, was wondering if you could help. you seem to know your stuff lol. I have a few queries i need to clear up.

2007 (SHM) - 4(b) Why is a = 0.10/2? And not simply 0.1?

- 4(e) If y=a, then shouldn't a=0.05^2? Hence a^2-y^2=0.
Then Ek=0?
But in the solutions, they have a, w and y. But what about the value for a?
Am i missing a key condition in the question?

BTW This question is open to everyone! Anyone with an explanation will be of use. Thanks!

For 4b, it's 0.1/2 because it starts 0.05m away from it's central position. The distance AB is twice the amplitude of the piston because it is the distance from the highest the piston can go to the lowest. The same way the vertical distance of a wave travelling horizontally from peak to trough is twice it's amplitude.

For e, it asks you to find the maximum kinetic energy of the piston. y doesn't equal a when KE is maximum.

Exactly. Think when it is going fastest.

For 4b, it's 0.1/2 because it starts 0.05m away from it's central position. The distance AB is twice the amplitude of the piston because it is the distance from the highest the piston can go to the lowest. The same way the vertical distance of a wave travelling horizontally from peak to trough is twice it's amplitude.

For e, it asks you to find the maximum kinetic energy of the piston. y doesn't equal a when KE is maximum.

Thank you! But if the piston covered the full distance of AB then a=0.10m right?