能量守恆與效率：练习题

Which of the following is a non-renewable energy source available for use on Earth?

An electric motor has a power rating of 2.0 kW. It is used to lift a 150 kg crate through a vertical height of 10 m in 12 seconds. Calculate the efficiency of the motor system.
(Assume \(g = 10\) N/kg)

An electric motor is used to lift a 50 kg crate through a vertical height of 4.0 m in 10 seconds. The motor is supplied with 2500 J of electrical energy. Calculate the efficiency of the motor system.
(Use \(g = 10\text{ N/kg}\))

A car of mass 1200 kg is travelling at 20 m/s. The driver applies the brakes and the car comes to a stop. Calculate the total energy dissipated as heat in the brakes and to the surroundings.

In a closed system, a 2.0 kg block sliding at 4.0 m/s collides with a spring and compresses it. At the moment the block is momentarily at rest, all its kinetic energy has been transferred to elastic potential energy in the spring. Calculate the maximum elastic potential energy stored in the spring.

An object with a mass of \(5\text{ kg}\) is raised to a height of \(4\text{ m}\). Calculate the change in gravitational potential energy. (Use \(g = 10\text{ N/kg}\))

A car of mass \(1200\text{ kg}\) is travelling at \(10\text{ m/s}\). Calculate the kinetic energy of the car and explain what happens to this energy when the car brakes to a stop.