Question

An 80-kg skater is coasting at a velocity of 6 m/s. She sees a small child in her way and picks him up as she skates by. Her velocity decreases to 1.12 m/s. What is the mass of the child she picked up?

Answer 1

Answer:

348.57 kg

Explanation:

From the law of conservation of momentum,

Total momentum before the small child was picked = Total momentum after the small child was picked

mu + m'u' = V(m+m')................... Equation 1

Where m = mass of the skater, u = initial velocity of the skater, m' = mass of the small child, u' = initial velocity of the small child, V = common velocity after the child was picked.

Note: Assuming the small child was stationary before he was picked, the u' = 0 m/s.

making m' the subject of the equation

m' = (mu-mV)/V........................... Equation 2

Given: m = 80 kg, u = 6 m/s, V = 1.12 m/s.

Substitute into equation 2

m' = [(80×6)-(80×1.12)]/1.12

m' = (480-89.6)/1.12

m' = 390.4/1.12

m' = 348.57 kg.

Thus the mass of the small child = 348.57 kg