# Homework Help: Harmonic movement

1. Mar 1, 2009

### transgalactic

Last edited by a moderator: May 4, 2017
2. Mar 1, 2009

### tiny-tim

Hi transgalactic!

No, the equation given is -∆mgR/2 sinθ = (I1 + I2)θ''

and the left hand side is the torque (not minus the torque) … that's the way θ is defined …

the rock inside pulls down, decreasing θ.

3. Mar 2, 2009

### transgalactic

but the this torque is going counter clock wise
it should be taken as positive.
i cant understand what it has to do with

can you explain this part..

4. Mar 2, 2009

### tiny-tim

No, the torque of the inside rock about the centre is clockwise.

5. Mar 2, 2009

### transgalactic

you are correct
but the problem is if the rock tilts to the left side
the solution still gives a minus sign
despite the fact that the torque is positive
i was told that its because of the sinus graph
?

6. Mar 2, 2009

### tiny-tim

I'm not following you

if the inside rock tilts to the left side, then sinθ is negative, so the equation means the angular acceleration must be positive … which it is!

yes, the torque is the same sign as the angular acceleration (anti-clockwise and positive) …

it will be in any apparatus …

the equation doesn't say "negative torque = acceleration", it says "torque = acceleration", but the formula for the torque just happens to have a minus in it.

7. Mar 2, 2009

### transgalactic

i dont use negative sinx
i dont have such thing
i just take the hypotenuse multiply it by sinus theta *Mg
and then look if its goes clock wise or counter clockwise.

if the rock leans to the left and we loo from the point the rock touches the ground
Mg*sin theta=Ma^2

but in another solution it states
-Mg*sin theta=Ma^2

why?

how to get the minus using the method i use??

8. Mar 2, 2009

### tiny-tim

erm … don't use your method!

don't "look if its goes clock wise or counter clockwise"!

physics is equations …

if x is negative, then sinx is negative

you can't write "just look at the diagram and you can see it's clockwise" in the middle of an exam proof!

9. Mar 2, 2009

### transgalactic

but the angle is a parameter
and if x is negative
then sinx will give us a negative value

we cant say
-sinx

its just adding a minus to the sinus

10. Mar 3, 2009

### transgalactic

can you explain this stuff??