Is the linear momentum conserved in the presence of friction?

AI Thread Summary
Linear momentum is conserved in collisions, even in the presence of friction, as friction only affects energy, not momentum. When an object collides with a stationary planet, the total momentum before and after the collision remains equal, regardless of whether the object comes to a stop or the planet gains speed. The conservation of momentum applies whether the object sticks to the planet or bounces off, as long as the system is closed. Friction does not convert linear momentum into other forms; it merely dissipates energy. Thus, momentum conservation holds true in both elastic and inelastic collisions involving friction.
paolo32
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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
 
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Welcome to PF!

Hi Paolo! Welcome to PF! :smile:

(btw, "still" is correct, but we usually say "stationary" or "at rest" instead, because "still" has more than one meaning!)
paolo32 said:
Hi,
Is the linear momentum conserved in the presence of friction if i consider certain systems?
For instance: supposing my system is made up of a planet still in space, and an object with a certain initial speed: the total momentum, the planet being initially still, will be the mass of the object times its initial speed. If then the object gets in contact with the planet giving rise 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 (planet's mass + the object's mass) times the planet's new speed = object's initial speed times the object's initial mass?


Momentum is always conserved in collisions.

Friction can only cause loss of total energy, not of total momentum. :wink:
 
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?
 
Bob S said:
… Is linear momentum conserved, or is some converted to angular momentum?

Momentum is always conserved in collisions.

Angular momentum also is always conserved in collisions. :smile:

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 :wink:)
 
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!
 

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