You are Here: Home >< Physics

# What is 'torque'?

Announcements Posted on
Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
1. The title is intentionally simplistic so as to helpfully convey a dilemma I've come across during some electromagnetism reading (specifically when reading over the torque that results on a rectangular current loop in a uniform B field). I'm used to computing and thinking that I physically understood torque...

Whilst I'm happy with the nut and bolt calculation RE displacement vector crossed with force applied, I realised that I do not know what the physical justification is for the existence of torques.

For example taking this thought experiment:

There's a 3 dimensional rod in deep space (no force of any type acting on this extended body) with a rocket booster on one end. If the booster is fired, will it undergo translational or rotational motion?

Returning to the source of my doubt, the current loop, why should the loop rotate about the central axis of the loop - what determines that this will be the rotation axis?
2. (Original post by Sm0key)
There's a 3 dimensional rod in deep space (no force of any type acting on this extended body) with a rocket booster on one end. If the booster is fired, will it undergo translational or rotational motion?
Both.

Think about why objects orbiting each other 'wobble' when referenced to a third fixed point not associated with either body. Now apply a similar thought process to the rocket attached to the rod.
3. (Original post by uberteknik)
Both.

Think about why objects orbiting each other 'wobble' when referenced to a third fixed point not associated with either body. Now apply a similar thought process to the rocket attached to the rod.
Are you referring to the barycentric orbit for 2 bodies (like here: https://en.wikipedia.org/wiki/Barycenter)?

If so I always understood that both are undergoing all rotational motion? Still unsure about the concept I'm afraid.
4. (Original post by Sm0key)
The title is intentionally simplistic so as to helpfully convey a dilemma I've come across during some electromagnetism reading (specifically when reading over the torque that results on a rectangular current loop in a uniform B field). I'm used to computing and thinking that I physically understood torque...

Whilst I'm happy with the nut and bolt calculation RE displacement cross force applied, I realised atet I don not know what the physical justification is for the existence of torques.

For example taking this thought experiment:

There's a 3 dimensional rod in deep space (no force of any type acting on this extended body) with a rocket booster on one end. If the booster is fired, will it undergo translational or rotational motion?

Returning to the source of my doubt, the current loop, why should the loop rotate about the central axis of the loop - what determines that this will be the rotation axis?
For the rod example - you are applying an external force, so the centre of mass will accelerate. You are also applying an external torque, so the object will rotate. In this case, it's obvious where it will rotate around (presumably around it's centre of mass).

For the torque of a dipole question, you still have enough symmetry to 'see' where it will rotate around: There's two external forces that form a couple, and the loop is symmetric. (I can't work out how to actually describe what axis it is beyond a VERY unhelpful "it's the perpendicular bisector of the point of action of the two forces).

In general, you may not be able to use symmetry arguments to determine the axis of rotation. In these cases you are going to have to resort to fairly high-powered dynamics methods I suspect - either using Euler's Equations or some other method.

## Register

Thanks for posting! You just need to create an account in order to submit the post
1. this can't be left blank
2. this can't be left blank
3. this can't be left blank

6 characters or longer with both numbers and letters is safer

4. this can't be left empty
1. Oops, you need to agree to our Ts&Cs to register

Updated: May 2, 2016
TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Today on TSR

### Who is getting a uni offer this half term?

Find out which unis are hot off the mark here

### Brexit: Why you should study law

Poll
Study resources

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.