Assume that a 15-kg ball moving at 8 m/s strikes a wall perpendicularly and rebounds elastically at the same speed. What is the amount of impulse given to the wall? Show your work.

I'm inexperienced... walk me through this?

Question

Assume that a 15-kg ball moving at 8 m/s strikes a wall perpendicularly and rebounds elastically at the same speed. What is the amount of impulse given to the wall?  Show your work.

I'm inexperienced... walk me through this?

akashdeepdeb · Answer

Do you know that Force is Change in momentum / time ?
Because F = ma = m(v-u)/t = (mv - mu)/t = change in momentum/time!! :D
Thus Impulse = F*t = change in momentum * time/time = change in momentum!! :D
So Impulse is change in momentum!

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m1u1 + m2u2 = m1v1 + m2v2 [Law of conservation of momentum]
Now the wall is still so u2,v2 = 0
or 
m1u1 = m1v1
Now we know that as it is elastic!
So even kinetic energy is conserved!!
1/2m1u1^2 = 1/2m1v1^2 [u2,v2 = 0]
u1 = v1 [Just direction should be opposite]
m1u1 = 15 * 8 = 120kg.m/s
m1v1 = -15 * 8 = -120kg.m/s
Now we have to calculate impulse = Change in momentum!
= m1v1 - m1u1 = -120 - 120 = -240kg.m/s!! :D
Get it? :)

akashdeepdeb · Answer

btw in the drawing it is law of conservation of momentum!
And law of conservation of kinetic energy as the collision is elastic!! :D

anonymous · Answer

O_O Lol, thank you, much appreciated.

akashdeepdeb · Answer

:)