Answer to Question #162401 in Physics for Andu

Question

2. A boy with a mass of 60kg runs at a speed of 3m / s behind a cart weighing 160kg, which moves in the same direction at a speed of 1m / s.
When they reach the cart, it is thrown into it. Find the size and direction of the cart as the boy jumps on it. Friction is not taken into account.

Answer 1

From the law of conservation of momentum, we get:

$$m_1v_1+m_2v_2=(m_1+m_2)v,$$ $$v=\dfrac{m_1v_1+m_2v_2}{m_1+m_2},$$ $$v=\dfrac{60\ kg\cdot3\ \dfrac{m}{s}+160\ kg\cdot1\ \dfrac{m}{s}}{60\ kg+160\ kg}=1.54\ \dfrac{m}{s}.$$

The sign plus means that the cart and boy moves in the same direction (to the right).