Allison (of mass 20 kg) is coasting in her wagon (of mass 20 kg) at a constant velocity of 7 m/s. She passes her mother, who drops a bag of toys (of mass 2 kg) into the wagon. What will happen to the speed of the wagon?

6.67 m/s

Explanation:

mass of Alison, m = 20 kg

mass of wagon, m' = 20 kg

velocity, u = 7 m/s

Let the speed of wagon is v.

mass of toys, m'' = 2 kg

Use conservation of momentum

(m + m') x u = (m + m' + m'') x v

(20 + 20) x 7 = (20 + 20 + 2) x v

280 = 42 x v

v = 6.67 m/s

Thus, the speed of wagon is 6.67 m/s.