- #1

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**The first Question**

A 2.5 kg ball and a 5.0 kg ball have an elastic collision. The 2.5 kg ball was at rest and the other ball had a speed of 3.5 m/s. What is the kinetic energy of the 2.5 kg ball after the collision?

## Homework Equations

J=p (Impulse = momentum)

(F)t= m2v2f+m1v1i

vf(m1 + m2) = m1v1 + m2v2

1/2mv^2 + m1v1= KE ?

## The Attempt at a Solution

Well I had attempted this question twice with no avail. Mainly because I keep getting two unknowns and then i recycle the question too many times.

5 x 3.5 = 17.5

so 2.5(v) = 17.5

v = 7 m/s

kinetic energy which is 1/2 m v^2

.5 x 2.5 x 7^2

= 61.25 J

- This was wrong as it didn't match any of the multiple choice answers provided

**The Secound Question**

A 5.0 kg boy runs at a speed of 10.0m/s and jumps onto a cart. The cart is initially at rest. If the speed, with the boy on the cart, is 2.50 m/s, what is the mass of the cart?

## Homework Equations

Same as the first question

## The Attempt at a Solution

mv + m2v2 = m'v'

(5)(10) + m2(0) = (M2 + 5) (2.5)

50 = 2.5M + 12.5

M = 15 kg

-once again this is not one of the choices for the multiple choice...

Can someone please help me here? I do not know what I am doing incorrectly.

Thanks