# Is the linear momentum conserved in the presence of friction?

1. Mar 11, 2009

### paolo32

Hi,
Is the linear momentum conserved in the presence of friction if i consider certain systems?
For instance: supposing my system is made up by a planet still in space, and an object with a certain initial speed: the total momentum, being the planet initially still, will be the mass of the object times it's initial speed.. If then the object gets in contact with the planet giving life to friction, will the momentum be conserved? if after the friction the object stops moving, will the planet obtain a certain speed such that the (planets mass + the objects mass) times the planets new speed = objects initial speed times the object initial mass?
thanks a lot in advance and sorry for the language inaccuracy (i'm italian infact)
Paolo

Last edited: Mar 11, 2009
2. Mar 11, 2009

### tiny-tim

Welcome to PF!

Hi Paolo! Welcome to PF!

(btw, "still" is correct, but we usually say "stationary" or "at rest" instead, because "still" has more than one meaning!)

Momentum is always conserved in collisions.

Friction can only cause loss of total energy, not of total momentum.

3. Mar 12, 2009

### Bob S

If an object strikes a planet and recoils backwards with a coefficient of restitution (COR) = 1,then the momentum transfer is twice what it would be if the object stuck (COR = 0). In both these cases momentum is conserved. Now consider the following case: The object strikes the planet off center (tangently) and sticks, causing the planet to spin. Is linear momentum conserved, or is some converted to angular momentum?

4. Mar 13, 2009

### tiny-tim

Momentum is always conserved in collisions.

Angular momentum also is always conserved in collisions.

They don't interchange.

(when the planet's angular momentum changes, that's because an off-centre object does have some angular momentum about the planet's axis )

5. Mar 19, 2009

### Deathless

A bullet that hits and passes through a wooden block - Lots of friction here and momentum is still conserved.

As far as momentum is concerned, it is conserved even when a moving car brakes to rest.
It may seem that momentum is not conserved in this case but it actually still is!