Question

A boy of 50kg hops with a horizontal velocity of 5m/s on stationary skateboard. The skateboard weights 4 kg . Find the velocity when the board starts rolling.

Answer 1

Boy's weight = 50 kg

Velocity = 5 m/s

Weight of skateboard = 4 kg

Initial velocity of skateboard = 0 (stationery)

Total mass after jumping on the skateboard = 50 + 4 = 54 kg

Law of conservation of momentum:

==> m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

==> (50)(5) + (4)(0) = v(m₁+ m₂)

==> 250 + 0 = v(54)

==> 250 = 54v

==> v = 250/54

==> v = 4.62 m/s

Therefore, the velocity when the board starts rolling is 4.62 m/s.

Answer 2

Answer:

The skateboard starts rolling at the speed of 4.62 m/s.

Step-by-step explanation:

Mass of the boy = 50 kg

Intial velocity of the boy = 5 m/s

Weight of the skateboard = 4 kg

Intial velocity of skateboard (the skateboard is stationery) = 0 m/s

According to Law of conservation of momentum:

$\boxed{\sf{m_a u_a +m_b u_b = m_a v_a + m_b v_b}}$

$\longrightarrow$ (50 × 5) + (4 × 0) = v(50 + 4)

$\longrightarrow$ 250 + 0 = 54v

$\longrightarrow$ 250 = 54v

$\longrightarrow$ v = 250/54

$\longrightarrow$ v = 4.62 m/s

Therefore, the skateboard starts rolling at the speed of 4.62 m/s.