Is the linear momentum conserved in the presence of friction?

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Discussion Overview

The discussion centers around the conservation of linear momentum in systems where friction is present, particularly in the context of collisions involving a planet and an object. Participants explore various scenarios, including the effects of friction and the implications for momentum conservation during different types of interactions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Paolo questions whether linear momentum is conserved when an object with initial speed comes into contact with a stationary planet, introducing friction into the system.
  • One participant asserts that momentum is always conserved in collisions, suggesting that friction affects energy but not momentum.
  • Another participant discusses a scenario where an object strikes a planet off-center, raising the question of whether linear momentum is conserved or if some is converted to angular momentum.
  • A later reply reiterates that momentum is conserved in collisions and distinguishes between linear and angular momentum, noting that they do not interchange.
  • Another example is provided where a bullet passes through a block, emphasizing that momentum remains conserved despite the presence of friction.
  • It is mentioned that momentum is conserved even when a moving car brakes to rest, challenging the perception that momentum is lost in such cases.

Areas of Agreement / Disagreement

Participants express differing views on the effects of friction on momentum conservation, with some asserting that momentum is always conserved while others raise specific scenarios that complicate this assertion. The discussion remains unresolved regarding the nuances of momentum conservation in the presence of friction and angular momentum considerations.

Contextual Notes

Participants do not fully explore the assumptions underlying their claims, such as the specific conditions of the collisions or the definitions of momentum and friction in various contexts.

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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