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**Velocity Calculation -- ball bouncing off a wall at an angle**

Question:

"A ball is locked at 45 degrees towards a wall at 4 m/s. It bounces away from the wall at an angle of 45 degrees at 3 m/s. Find the change in velocity of the ball at the bounce."

My attempt:

So I know that change in velocity = v-u.

Therefore I thought that the answer would be 3 - -4 (3+4) = 7 m/s at an angle of 90 degrees, however my book says the answer is 5 m/s.

I'm a little confused and would be grateful for some clarification. I'm thinking it must have something to do with it being the velocity at the bounce but I'm not sure?

Thanks in advance!

Molly