Eli stands on a skateboard at rest and throws a 0.5 kg rock at a velocity of 10.0m/s. Eli moves back at 0.05m/s. What is the combined mass of Eli and the skateboard

m = mass of the rock = 0.5 kg

v = velocity of the rock = 10 m/s

V = velocity of eli in opposite direction of the rock = - 0.05 m/s

M = mass of eli and skateboard together = ?

V' = velocity of combination of rock, skateboard and eli together before rock is thrown = 0 m/s

using conservation of momentum

total momentum after throwing rock = total momentum before throwing rock

mv + MV = (M + m) V'

(0.5) (10) + M ( - 0.05) = (0.5 + M) (0)

M = 100 kg