# A 120 kg astronaut is at rest in space and wants to return to his space station by throwing a 2.0 kg hammer at 30 km/h directly away from?

A 120 kg astronaut is at rest in space and wants to return to his space station by throwing a 2.0 kg hammer at 30 km/h directly away from the station. What would be his returning speed to the station?

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- 冷眼旁觀Lv 72 months ago
30 km/h = (30 km/h) × (1000 m/km) × (1 h / 3600 s) = 8.33 m/s

The hammer: m₁ = 2.0 kg, u₁ = 0 m/s, v₁ = 8.33 m/s

The astronaut: m₂ = 120 kg, u₂ = 0 m/s, v₂ = ? m/s

Conservation of momentum:

m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂

2.0×0 + 120×0 = 2.0×8.33 + 120v₂

Astronaut's returning speed, v₂ = -2.0×8.33/120 = -0.139 m/s

(The negative sign shows that the direction is opposite that of the hammer.)

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