# Homework Help: Definition of work done by torque

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1. Nov 10, 2018 at 4:10 AM

### Cedric Chia

I' m trying to derive the work done by a torque from W = ∫ F ⋅ ds and I' ve looked up the internet, it said:

W = ∫ F ⋅ ds ( since ds = dθ × r ) ---------------------------------------- ( Line 1 )

it can be written as

W = ∫ F ⋅ dθ x r

this is a vector triple product , thus can also be written as

W = ∫ r × F ⋅ dθ

W = ∫ Torque ⋅ dθ ----------------------------------------------------- ( Line 2 )

My question is :
In what direction is dθ pointing so that when I cross-product dθ and r ( Line 1 ), it become ds ? And also, when I dot-product Torque and dθ ( Line 2 ) , what is the angle between them ?

2. Nov 10, 2018 at 4:18 AM

### haruspex

A rotation can be represented as a vector along the axis of rotation, so it is normal to the plane of rotation. If the force lies in that plane then it will be parallel to the rotation vector. But in general it need not be. E.g. consider a car skidding at an angle, brakes off. The frictional force wIll be at any angle to the rotation axis.

3. Nov 10, 2018 at 4:35 AM

### Cedric Chia

Thanks for the reply, so the dθ is not in the same direction with the unit vector θ hat ? ( which we introduced in the polar coordinates, θ hat is tangent to the circular motion ) . Instead, dθ is perpendicular to the plane of circular motion ( same direction as angular velocity ) ?

4. Nov 10, 2018 at 4:37 AM

### Orodruin

Staff Emeritus
Yes. In fact, $d\theta/dt$ is the angular velocity.

5. Nov 10, 2018 at 4:40 AM

### Cedric Chia

Thank you

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