Superman must stop a 115-km/h train in 160 m to keep it from hitting a stalled car on the tracks. The train's mass is 3.6 × 105 kg. Determine the force that must be exerted on the train.

  1.148×10^6 N

Explanation:

The kinetic energy of the train is ...

  KE = (1/2)mv^2 = (1/2)(3.6×10^5 kg)(115 km/h × (1000 m/km)/(3600 s/h))^2

  ≈ 1.83681×10^8 J

This is the work required to stop the train, so is ...

  F·d = KE

  F = KE/d = (1.83681×10^8 J)/(160 m) = 1.148×10^6 N

The force required to stop the train is 1.148×10^6 newtons.

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