# Conservation of momentum?

• JerryF
In summary: Therefore, the object's velocity does not increase to conserve momentum.In summary, the velocity of a body does not increase when it travels through a continuously less dense medium in order to conserve momentum. Momentum is still conserved, but the velocity of the object does not increase as the resistance from the less dense medium is always acting to slow it down. The medium exerts a force on the object to slow it down, but this force is offset by an equal and opposite force on the particles of the medium, which carry the lost momentum.

#### JerryF

Hi all,

If a body has a given initial momentum and then travels through a continuously less dense medium would it's velocity increase to conserve momentum?

Thanks
Jerry

No

JerryF said:
Hi all,

If a body has a given initial momentum and then travels through a continuously less dense medium would it's velocity increase to conserve momentum?

Thanks
Jerry
Momentum is typically not conserved for a body moving through a medium, as it will be subject to a resisting force. As it moves to a less dense medium it may decelerate less, but it cannot speed up without an external force to accelerate it.

JerryF said:
If a body has a given initial momentum and then travels through a continuously less dense medium would it's velocity increase to conserve momentum?
No.
The resistance from the medium may become less as the medium comes less dense, but that resistance is always opposed to the direction of motion so always acts to slow the object and reduce its momentum.

Momentum is still conserved however. The medium exerts a force on the object to slow it, but by Newton's third law there is an equal and opposite force acting on the particles of the medium. At least some of these end up moving in the same direction that the object was moving, and these carry the momentum that the object lost.

Last edited:
anorlunda