# Harmonic movement

#### transgalactic

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#### tiny-tim

Homework Helper
we got a rock which has a smaller rock inside, we tilt it by a small angle.

why the torques is negative
why they do
-torque=I*angular accelarations
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. #### 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

minus the torque) … that's the way θ is defined …

the rock inside pulls down, decreasing θ. can you explain this part..

#### tiny-tim

Homework Helper
but the this torque is going counter clock wise
No, the torque of the inside rock about the centre is clockwise. #### 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
?

#### tiny-tim

Homework Helper
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
?
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.

#### 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??

#### tiny-tim

Homework Helper
Patient: it hurts when I do that.
Doctor: then don't do that! 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.

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

#### transgalactic

can you explain this stuff??

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