Energy：练习题

An electric lamp has a power rating of \(60\text{ W}\). How much electrical energy is consumed by the lamp in \(2\text{ minutes}\)?

A ball of mass \(0.50\text{ kg}\) is released from rest at a height of \(20\text{ m}\) above the ground. Air resistance is negligible. What is the kinetic energy of the ball when it has fallen to a height of \(5.0\text{ m}\) above the ground? (Take \(g = 10\text{ m/s}^2\))

An electric motor of efficiency \(75\%\) lifts a mass of \(120\text{ kg}\) vertically through a height of \(5.0\text{ m}\) in \(8.0\text{ s}\). What is the electrical power supplied to the motor? (Take \(g = 10\text{ m/s}^2\))

A worker pushes a crate with a constant horizontal force of \( 50\text{ N} \) for a distance of \( 4\text{ m} \) across a horizontal floor. How much work is done by the worker?

A hoist lifts a load of mass \( 60\text{ kg} \) vertically through a height of \( 2.0\text{ m} \) in a time of \( 4.0\text{ s} \). What is the average power output of the hoist? (Take \( g = 10\text{ m/s}^2 \))

A ball of mass \(0.50 \text{ kg}\) rolls along a horizontal floor at a constant speed of \(4.0 \text{ m/s}\). Calculate the kinetic energy of the ball.<\/p>

A worker pushes a crate with a constant horizontal force of \(25 \text{ N}\). The crate moves a distance of \(6.0 \text{ m}\) in the direction of the force. Calculate the work done by the worker on the crate.<\/p>

State the Principle of Conservation of Energy.<\/p>

The diagram shows a roller coaster car of mass \(400\text{ kg}\) at point A, which is at a height of \(20\text{ m}\) above the ground. The car starts from rest and moves down a frictionless track to point B at ground level, before rising to point C at a height of \(12\text{ m}\). (Take \(g = 10\text{ m/s}^2\))
(a) Calculate the gravitational potential energy of the car at point A.
(b) Using the principle of conservation of energy, state the value of the kinetic energy of the car as it passes point B.
(c) Calculate the speed of the car when it is at point B.
(d) Calculate the gravitational potential energy of the car when it reaches point C.