A child runs at 3.0 m/s and jumps onto a sled, initially at rest. If the child's mass is 36 kg, and if the child and sled slide off together at 2.0 m/s after the collision, the sled's mass is
A) 12 kg.
B) 18 kg.
C) 36 kg.
D) 48 kg.

Question

Quizplus · Accepted Answer

We can use the conservation of momentum equation, which states that the momentum before the collision equals the momentum after the collision:

(mass of child)(velocity of child before) + (mass of sled)(velocity of sled before) = (mass of child + mass of sled)(velocity after)

Plugging in the values we know, we get:

(36 kg)(3.0 m/s) + (mass of sled)(0 m/s) = (36 kg + mass of sled)(2.0 m/s)

Simplifying and solving for the mass of the sled, we get:

108 kg m/s = (36 kg + mass of sled)(2.0 m/s)
54 kg = 36 kg + mass of sled
mass of sled = 18 kg

Therefore, the sled's mass is 18 kg, which is choice B.

A Child Runs at 3