Question inelastic collisions and conservation of linear momentum

[MalibuPreMD]

If a moving truck and a stationary fire hydrant deform and then travel together upon colliding, is linear momentum conserved?

I would say yes (as does the answer explanation); however, the answer explanation also says: In inelastic collisions, kinetic energy is not conserved, because energy is lost due to friction in the deformation of the two objects.

I am confused because I always regarded friction as an external force, not an internal force. I understand that deformation by itself does not lead to a loss of conservation of linear momentum, but if we are including friction occurring DURING the deformation process why is it not considered an external force😕

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[Rabolisk]

It's an internal force because it is within the system, which includes the truck and the fire hydrant. Clearly during a collision, something happens between the truck and the fire hydrant, or within the system. The deformation process happens as a result of the collision within the system. Friction that happens after the collision, such as the friction between the surface and the system after the collision, would be an external force.

 

[RogueUnicorn]

i think the easier context with which to understand this is that elastic collision is the conservation of mechanical energy, i.e. kinetic and potential energies associated with motion/position. friction is not a mechanical energy, and thus any friction means energy is lost.

in a broad universal sense, energy is ALWAYS conserved, just changes form. however, this form change means we cannot recapture it into motion (this is a bit of a lie but true for MCAT-style collisions).

 

[dougkaye]

If a moving truck and a stationary fire hydrant deform and then travel together upon colliding, is linear momentum conserved?

I would say yes (as does the answer explanation); however, the answer explanation also says: In inelastic collisions, kinetic energy is not conserved, because energy is lost due to friction in the deformation of the two objects.

I am confused because I always regarded friction as an external force, not an internal force. I understand that deformation by itself does not lead to a loss of conservation of linear momentum, but if we are including friction occurring DURING the deformation process why is it not considered an external force😕

Momentum is always conserved if all the forces are internal to system. Friction is not an external force in the situation described because it is INSIDE the colliding objects as one/both of them deform. So KE not conserved (because it's an inelastic/partially inelastic collision) but momentum is. So in the perfectly inelastic (stick-together) collision, it should always work out, unless there are forces external to system, that the stuck-together final state (mf x vf) will have same momentum (mi x vi) as the initial state.