**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?

I did

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

I got 61.25 J, however the answers have to be..

a) 1.7 J b) 8.1 J c) 3.4 J d) 14 J e) 27 J

What is the answer? and how do you come to it?

**2nd 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?

mv + m2v2 = m'v'

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

50 = 2.5M + 12.5

M = 15 kg

is what I did, but the answers have to be either....

a) 150J b) 210J c) 175J d) 260 J e) 300J

what is the answer and what am I doing wrong? can someone help??

thanks