Solve for the amount of force on an 8kg ball

[gigglin_horse]

Homework Statement

"solve for the amount of force on an 8kg ball rolling at 2m/s when it bumps into a pillow and stops in 0.5 seconds."
...and...
"how much force does the pillow exert on the ball?"

Homework Equations

net momentum before = net momentum after?
Ft=(change in) mv?

The Attempt at a Solution

8kg x 2m/s = 16N of force...or 16 kg.m/s

but what's the stopping got to do with it?
isnt both halves of this question the same?
Im so confused :S

Please help

 

[LowlyPion]

F = ma = m*Δv/Δt

 

[gigglin_horse]

...That doesn't help me...

Sorry, I'm new to physics and don't understand a lot of it

 

[LowlyPion]

Δ stands for change in something.

Δp is the change in momentum. You can also write that as m*Δv.

So initial momentum is 8 kg* 2 m/s.
Final p is 0. Δp = 16 then right?

The change in time Δt is just the .5 sec.

So F = Δp/Δt = 16 / .5 = ... ?

 

[gigglin_horse]

Ahhhhhh, I see.
32.
But what force does the pillow exert back?
Isn't it equal and opposite? But wouldn't that cancel?

Would it be 8N, but with no momentum?

 

[LowlyPion]

32 N is the force on the ball to stop it.

By the statement of the problem it was from the pillow wasn't it?

 

[gigglin_horse]

Yeah, the pillow was the reason for the ball stopping...