# A ball with a mass of 15 kg moving at 4 m/s hits a still ball with a mass of 24 kg. If the first ball stops moving, how fast is the second ball moving? How much kinetic energy was lost as heat in the collision?

Jan 17, 2016

Speed = $6.25 m {s}^{-} 1$
Energy lost = 45J

#### Explanation:

Momentum is conserved so:

Momentum before = momentum after
$15 \cdot 4 = 24 \cdot v$
$v = \frac{60}{24} = 2.5 m {s}^{-} 1$

Kinetic energy = $\frac{1}{2} m {v}^{2}$

KE Before = $\frac{1}{2} \cdot 15 \cdot {4}^{2} = 120 J$

KE After = $\frac{1}{2} \cdot 24 \cdot {2.5}^{2} = 12 \cdot 6.25 = 75 J$

Energy lost = 120-75=45J