Question

A 60 kg boy on roller skates moves with a speed of 5 m/s. He runs into a 40 kg girl on skates. Assuming they cling together after the collision, what is their speed?How much kinetic energy was lost in the collision?

Answer 1

you have:

m1 = 60 kg
v1 = 5 m/s
m2 = 40 kg
v2 = 0 m/s (I presume she's not moving because there's no mention of her velocity)

so

m1v1 = (m1 + m2)V

V = m1v1 / (m1 + m2)

solve for v

once you find v, you plug it into your kinetic energy formula: K = 1/2mv^2
where you'll have Kf - Ki = your lost energy

Answer 2

60 kg * 5 m/s + 40 kg * 0 m/s = (60 kg + 40 kg) * v_final
300 kg*m/s = 100 kg * v_final
3 m/s = v_final