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Mechanical Energy (Gravitational Potential Energy/Kinetic... 1) The position at which potential energy is defined to be zero is a 2) A forklift raises a box 1.2 m and does 7.0 kJ of work on it. What is the mass of the box? kg 3) Tony has a mass of 45 kg and a speed of 10.0 m/s. a. Find Tony's kinetic energy. b. Tony's speed decreases to 5.0 m/s. Now what is his kinetic energy? c. Find the ratio of the kinetic energies in parts a and b. Explain how this ratio relates to the change in speed. 4) Shawn and his bike have a combined mass of 45.0 kg. Shawn rides his bike 1.80 km in 10.0 min at a constant velocity. What is the system's kinetic energy