Year 12s: what will be the first way you research your future options? >>

question about moments

MEPS1996

if a rigid object is floating in space and a single force is applied to it, will it rotate or just move translationally?

Reply 1

ghostwalker

Original post by MEPS1996

if a rigid object is floating in space and a single force is applied to it, will it rotate or just move translationally?

Depends. Does the line of action of the force pass through the centre of mass of the object or not?

Reply 2

MEPS1996

Original post by ghostwalker

Depends. Does the line of action of the force pass through the centre of mass of the object or not?

what happens in both cases?

Reply 3

ghostwalker

Original post by MEPS1996

what happens in both cases?

In both bases it will move translationally, and if the line of action of the force does not pass through the centre of mass, it will rotate as well.

Reply 4

MEPS1996

Original post by ghostwalker

In both bases it will move translationally, and if the line of action of the force does not pass through the centre of mass, it will rotate as well.

why is this? I understand objects in equilibrium have equal clockwise and anticlockwise moments at all points, but an object with a force through its centre of mass still has some resultant moment at some points...

Reply 5

ghostwalker

Original post by MEPS1996

why is this?

I wouldn't attempt to answer that.

It may be covered in later mechanics modules, specifically dealing with moments of inertia, and angular momentum. Or it may be left to uni.

Reply 6

MEPS1996

Original post by ghostwalker

I wouldn't attempt to answer that.

It may be covered in later mechanics modules, specifically dealing with moments of inertia, and angular momentum. Or it may be left to uni.

ok thanks ghostwalker,
I know there is a rule that an object which is in equilbrium has moments balanced about all points. However, for the object with a force through its centre of mass this is not the case. Therefore for moments to be balanced around every point the object must be in translational and rotational equilbrium. Correct? So at my level, Sixth form, usually we know an object is in equilibrium so we can deduce things about forces as moments are balanced. My question is, what happens if an object is not in equilbrium. What then can the moments equations tell us. Or indeed what can we deduce from moments equations about an objects motion- rotation and translation?

Reply 7

ghostwalker

Original post by MEPS1996

...

Knowing the centre of mass and moments of inertia, then for any applied force we can ascertain the change in translational and rotational motion.

The translational part is straight forward, we just apply F=ma in its vector form, where the acceleration refers to the acceleration of the centre of mass.

The rotational part isn't that difficult for simple shapes, from what I remember - perhaps restricting it to two dimensions.

The translational part and the rotation parts are treated separately, IF I recall correctly, but it really is a long time since I looked at this.