# Conservation of momentum

1. A steel ball of mass 0.400 kg is fastened to a cord 60.0 cm long and fixed at the far end, and is released when the cord is horizontal. At the bottom of its path, the ball strikes a 2.00 kg steel block initially at rest on a frictionless surface. The collision is elastic.

Assuming the positive direction is to the right, find, just after the collision,

Velocity of the ball

Velocity of the block

I don’t know how to start this one, never the less do it.

2. A body of mass 2.4 kg makes an elastic collision with another body at rest and continues to move in the original direction but with one-fourth of its original speed. The struck block has a mass of 1.44kg.

What is the speed of the two-body center of mass if the initial speed of the 2.4 kg body was 4.5 m/s?

I plugged in the numbers into the equation (m1v1+m2v2)/(m1+m2) and got 3.234375 which I know is wrong. Don’t even know how got v2.

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