Work, Energy, and Power

A trolley runs from P to Q along a track. At Q its potential energy is 50 kJ less than at P.

At P, the kinetic energy of the trolley is 5 kJ. Between P and Q, the trolley does 10 kJ of work against friction.

What is the kinetic energy of the trolley at Q?

• 35 kJ

• 45 kJ

• 55 kJ

• 65 kJ

A hydroelectric power station uses the gravitational potential energy of water to generate electrical energy.

In one particular power station, the mass of water flowing per unit time is 1.5 x 10^5 kg s^–1. The water falls through a vertical height of 120 m.

The electrical power generated is 100 MW. What is the efficiency of the power station?

• 5.6 %

• 43 %

• 57 %

• 77 %

Which amount of energy is not 2400 J?

• the decrease in gravitational potential energy of a mass of 60 kg when it moves vertically downwards through 40 m near the Earth’s surface

• the energy transferred in 15 s by a machine of power 160 W

• the kinetic energy of a mass of 12 kg moving at a speed of 20 m s^–1

• the work done by a gas expanding against a constant external pressure of 120 kPa when its volume increases by 0.020 m^3

A train of mass 300 000 kg is accelerating at 0.80 m s^–2. At one instant, the speed of the train is 5.0 m s^-1 and the resistive force to its motion is 15 kN. At this instant, what is the rate of increase of kinetic energy of the train?

• 0.075 MW

• 1.2 MW

• 1.3 MW

• 3.8 MW

A ball is thrown vertically upwards. Air resistance is negligible.

Which statement is correct?

• By the principle of conservation of energy, the total energy of the ball is constant throughout its motion.

• By the principle of conservation of momentum, the momentum of the ball is constant throughout its motion.

• The kinetic energy of the ball is greatest at the greatest height attained.

• The potential energy of the ball increases at a constant rate during its ascent.