# Is momentum conserved?

1. Mar 23, 2012

### azizlwl

In a ballistic pendulum test, the mv is added to the system and that total remains constant over time. This what conservation of Momentum states. But at the end of the swing velocity is zero. Thus no momentum. What is conserved here?

As in energy total energy always remains the same. In momentum its nor true even in ordinary motion with resistance, the velocity decreases thus momentum.
To me it is just a Law of Union/Separation. After Union/Separation its not use/applicable.

Last edited: Mar 23, 2012
2. Mar 23, 2012

### Khashishi

The momentum is transferred into the stand and then into the earth.

3. Mar 23, 2012

### savu

Thank you aziz and khashishi ,I agree with both of your answers with what everyone may know that momentum is conserved.With a rule that states that "momentum before is equal to momentum after".
If anyone is inquiring about energy then the rules that YES energy is also conserved but in a different form.

4. Mar 23, 2012

### savu

YES it is.The rules states that momentum before is equal to momentum after.√

5. Mar 23, 2012

### azizlwl

Thank you. I mean here i see the Conservation of Momentum only applies to union and separation of items. In other instances other laws apply. When a body moves, it follows the Newton's first Law. When a body moves and changes direction or/and magnitude, Newton 2nd law is used and sometimes we use KE and PE, conservation of energy.

Thus COM only applies only JUST before the union/separation and JUST after that.
Conservation law should be at any instances/places.

6. Mar 23, 2012

### HallsofIvy

Staff Emeritus
Conservation of momentum holds only as long as there is no "exernal" force. The force of gravity is an external force.

7. Mar 24, 2012

### Rap

Yes. As the pendulum rocks back and forth, the Earth rocks back and forth in the opposite direction. Momentum is always conserved, at all times.

8. Mar 24, 2012

### azizlwl

Thank you
I agree that with no external force, means there's no change in momentum. Total sum of momentum remains constant.
I still cannot figure out why we need conservation of momentum in solving eg. ballistic pendulum.
Initial mv=final mv then we get velocity. Then KE to PE.
Why not just KE incoming bullet to PE without resorting to COM if we assume no energy expended in the process(I've been making this error frequently). Conservation of energy is intuitively easy for accept.

9. Mar 24, 2012

### Rap

But a lot of energy is expended in the process. When the bullet collides with the pendulum, it sticks to the pendulum, and that's an inelastic collision. Kinetic energy is not conserved. Most of the kinetic energy of the bullet is turned into heat and a little of it is turned into sound.

10. Mar 24, 2012

### azizlwl

ok now i understand why we have to resort to COM.

In a collision,

$mv_{in}=mv_{out}$ where no external forces involve.

$KE_{in}\geq KE_{out}$

So applying COM is the most ideal transformation for a collision.