Homework Help: Internal and external forces in momentum

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1. Mar 6, 2016

Biker

1. The problem statement, all variables and given/known data
A question came to my miind:
Wouldn't everything be conserved?
I know that the law of momentum conservation only applies when the system doesn't have any external forces or the net of them is equal to 0.

Lets just say that someone hits the ball with a bat, He applies a force to the ball and the same happens to the bat ( Newton third law). So the momentum is conserved.

An object is moving on a surface that has friction, The surface tries to stop the object and the same happens to the surface. So the momentum is conserved.

An object is flying at a speed of v. The air tries to resist its motion with a force and the object also pushes air molecules with a force f. So Is the momentum conserved? ( I guess)
When do we actually say that the momentum changed? what am I missing here? Should I exclude the air and friction from my system and just consider the object is my system then it is appropriate to say that the momentum changed? but if I take as a whole then I should say that the momentum is conserved?

2. Relevant equations
P = mv
F * t = m dv
F = -f

It would be great if you can provide some examples so I can figure it out. Also point out stuff don't directly go the the final point.

2. Mar 6, 2016

Staff: Mentor

Momentum of your system can change if there are external forces. If you make the system large enough to include all sources of (previously) external forces, then momentum is always conserved, sure. You rarely want to care about the motion of Earth if you study cars moving on a street, however.

3. Mar 6, 2016

Biker

It is stunning to think that we actually move the earth :D.

Another thing if you don't mind, When two object collides is friction the contact force between them? Or it can be some other form of forces?
Like if a person with a speed jumps on top of a object then the object would move because of friction. but what about two balls colliding with each other.

4. Mar 6, 2016

Staff: Mentor

The forces that lead to deformation (balls, cars, ... doesn't matter) are not called friction, but internally there can still be some friction during this deformation process.

5. Mar 6, 2016

CWatters

If you draw the right system boundaries both momentum and energy are conserved. It's not always easy to account for all of the energy transformations so frequently you find you can only use conservation of momentum to solve problems.