# Linear / Circular

1. Oct 21, 2009

### seb7

Forgive me for questioning basic science, but was wondering..

Two objects of the same mass, travelling towards each other in free space, collide in such a manner as to result in one of them to spin. Since some of the energy in one direction is now lost in the spin, ie. some of the linear force of one of the objects has been tranfered to a spinning motion, would it then be possible to then have a combined net force in the other direction?

Last edited: Oct 22, 2009
2. Oct 22, 2009

### Integral

Staff Emeritus
What other direction are you talking about?

Yes, it is possible for a body to have both translational and rotational motion. Look at a cars wheel, a baseball, a football, or nearly any ball for that matter.

3. Oct 22, 2009

### seb7

Integral, think you completely misunderstood. I'm talking about the action and opposite reaction, and the possibility of making something move in free space.

4. Oct 22, 2009

### Staff: Mentor

Translational KE has been transformed into rotational KE. I don't know what you mean by "in one direction".
I don't understand what you mean. In any case, the net force on the two objects due to their interaction will be zero, as per Newton's 3rd law.

5. Oct 22, 2009

### Staff: Mentor

1. As you have stated in your original post - two bodies collide. That means one of them was already moving, and momentum was conserved in the collision.

2. There is no problem with two bodies in free space starting to move - as long as both energy and momentum are conserved. Imagine two balls connected with compressed (not sure if that's the correct word) spring, hold together by the line. Cut the line - and bodies start to move in the opposite directions. Momentum is conserved, energy is conserved, bodies move in the 'free space'.

6. Oct 22, 2009

### seb7

Both bodies were moving towards each other, so has a combined movement of zero. They collide, object A bounces off in one direction, object B, (because of shape or something) bounces as well, but it is also put in to a spin.

Some of the linear motion must have transferred to circular motion. If we can do this without object A spinning, or even just spinning at a slower momentum than object B, wouldn’t we now have some combined movement. ie. a bending of Newton's 3rd law?

7. Oct 22, 2009

### Staff: Mentor

OK. Let's say that the total linear momentum of the two bodies is zero.
OK. An example might be a ball hitting a stick.

Not sure what you mean by this. Both linear and angular momentum are conserved.
Nope.

8. Oct 22, 2009

### mikeph

Linear kinetic energy can be transferred into rotational kinetic energy, but there is no conservation law for this transfer, only for linear and angular momentum. I think you may be thinking of 'force', 'momentum' and 'energy' as interchangeable, which is incorrect.