# Momentum and Impulse

1. Dec 18, 2011

### c77793

Why does the ''P2=P1'' (momentum before is equal to momentum after) isn't applied in cases where u throw a ball at wall?

In this case, even is the velocity keeps the same, it will be in another direction, what makes it -v...

2. Dec 18, 2011

### JHamm

The wall now has momentum $2P_1$ and the ball has momentum $-P_1$. You always need to consider the entire system.

3. Dec 19, 2011

### SandeshPhy

The wall has gained momentum(but notice it has no velocity as mass of wall is infinite)

the conservation of momentum applies to systems where there is no external force applied.

if you only consider the ball as system,the wall applies a force on it during collision.
But with ball and wall system the force between ball and wall and vice versa simply become internal forces

4. Dec 19, 2011

### c77793

I just dont get how the wall gain momentum once it stays steady...

5. Dec 19, 2011

### SandeshPhy

The mass of wall is infinite

So momentum can be finite with even zero velocity

as mathematically, infinite multiplied by zero can be something(finite value)

6. Dec 20, 2011

### JHamm

In reality the wall doesn't stay steady.

7. Dec 23, 2011

### caleb5040

The wall does gain momentum. But really you need to consider the wall and the earth together because the wall is attached to the earth. This single component of the system has an enormous mass, so its change in velocity is imperceptible.

You can imagine, however, that if a wall is not attached to the earth, and you throw the ball sufficiently fast enough, the wall will totter and fall over. In this case the earth is not part of the system component.

I think the key realization is that a solid wall is attached to the earth and these two comprise a single component of the system.