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A father (mass 50 kg) and his son (mass 20 kg) stand facing each other on a frictionless floor. The father pushes the son, giving him a velocity of 0.70 m/s relative to the father. Find the speed of the father relative to the ground.

1. 0.20 m s⁻¹
2. 0.14 m s⁻¹
3. 0.47 m s⁻¹
4. 0.28 m s⁻¹

Correct answer: 0.20 m s⁻¹

Solution

Momentum conservation with the relative-velocity constraint gives the father a ground speed of 0.20 m/s.

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