7. A 2.5 kg ball moving at 4.2 m/s collides with a 2.5 kg stationary ball. If the first ball is at rest after the collision, what is the velocity of the second ball? a 2.1 m/s​

The answer and explanation to your question in a file, link below

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Explanation:

When the swing moves from the lowest point up to either peak, the main force acting is momentum; and when the swing falls from either peak to its lowest point, the main force acting is gravity. If you do not pump your legs fast enough, air resistance along with gravity will keep you down.