Why Isn't Linear Momentum Conserved?

[student34]

Homework Statement

A thin metal bar, 2.00 m and a mass of 9.18 kg hangs vertically from a ceiling by a frictionless pivot. Suddenly it is struck 1.50 m below the ceiling by a small 3.00 kg ball, initially traveling horizontally at 10.0 m/s. The ball rebounds in the opposite direction with a speed of 6.00 m/s.

(a) Find the angular speed of the bar just after the collision. ***The answer in the textbook is 5.88 rad/s, and that makes sense to me.***

(b) During the collision, why is the angular momentum conserved but not the linear momentum?

Homework Equations

m*v(initial)*l = Iω + m*v(final)*l

The Attempt at a Solution

I have absolutely no idea how this is possible. I was always taught that momentum is always conserved.

 

[Borek]

What are forces at the pivot point?

 

[student34]

What are forces at the pivot point?

There are tension and gravity forces, but I don't understand how they would affect the linear momentum of the ball in the horizontal dimension.

 

[Doc Al]

There are tension and gravity forces, but I don't understand how they would affect the linear momentum of the ball in the horizontal dimension.

Hint: Does the pivot move? Why not?

 

[swordthrower]

It would be useful here to keep in mind what criteria need to be met for linear momentum to be conserved.

 

[PeroK]

Linear momentum is conserved (if you consider the whole system).

Linear momentum is not necessarily conserved (if you only consider part of the system).

E.g. bouncing ball: momentum conserved if you consider the Earth's momentum; momentum clearly not conserved if you consider the ball only.

 

[student34]

Yeah, I should have knew that, thanks everyone!