4. A 30kg red ball is rolling to the right with a velocity of 4m/s and a green ball is rolling to the right with a velocity of 5m/s left. After the collision the red ball has a velocity of 2m/s left and the green ball is rolling to the left with a velocity of 4.5m/s left. What is the mass of the green ball?

The question is confusing for the initial direction of the green ball. We assume to the left.

The mass of the green ball is 360 Kg

Explanation:

Law Of Conservation Of Linear Momentum

The total momentum of a system of bodies is conserved unless an external force is applied to it. The formula for the momentum of a body with mass m and velocity v is

P=mv.

If we have a system of bodies, then the total momentum is the sum of the individual momentums

If some collision occurs, the velocities change to v' and the final momentum is:

In a system of two masses, the law of conservation of linear momentum

is written as:

Let's call m1=30 Kg the mass of the red ball moving to the right at v1=4 m/s, m2 (unknown) the mass of the green ball moving at v2=-5 m/s to the left (assumed).

After the collision, the red ball rolls to the left at v1'=-2 m/s and the green ball is at v2'=-4.5 m/s. Thus:

Operating:

The mass of the green ball is 360 Kg