Work and Energy

The One Thing

Work is the transfer of energy when a force produces displacement in the direction of that force. Energy may change between forms, and power measures the rate of this transfer, but the total energy remains constant.

Reactions, Processes and Experiments

Key Terms

• Work: Work is done when a force acting on an object produces displacement in the direction of the force.
• Force: A push or pull that can change the motion, direction, or shape of an object.
• Displacement: The distance moved by an object in a particular direction.
• Kinetic Energy: The energy possessed by an object because of its motion.
• Potential Energy: The energy stored in an object because of its position, shape, or condition.
• Gravitational Potential Energy: Energy stored in an object because of its height above the ground.
• Power: The rate at which work is done or energy is transferred.
• Law of Conservation of Energy: Energy can neither be created nor destroyed; it can only be transformed from one form to another.
• Joule: The SI unit of work and energy. One joule is the work done when a force of one newton moves an object one metre in the direction of the force.
• Watt: The SI unit of power. One watt equals one joule of work done per second.

Easily Confused

• Work and energy: Work is the transfer of energy caused by a force producing displacement; energy is the capacity to do work.
• Kinetic energy and potential energy: Kinetic energy is due to motion, whereas potential energy is stored because of position, shape, or condition.
• Work and power: Work measures energy transferred, whereas power measures how quickly the transfer occurs.
• Mass and speed in kinetic energy: Kinetic energy increases with mass, but it increases greatly with speed because speed is squared in K.E. = 1/2 × m × v².
• Zero displacement and perpendicular force: Work is zero if displacement is zero, and it is also zero when force and displacement are perpendicular.
• Watt and kilowatt: One watt is one joule per second, whereas one kilowatt equals 1000 watts.
• Energy conservation and useful energy: Total energy remains constant, although some useful energy may change into heat, sound, or other forms.
• Power and total work: A more powerful machine does not necessarily do more total work; it completes the work in less time.

What Gets Asked

• Calculate work using W = F × s, ensuring that the force and displacement are in the same direction and that the answer is given in joules.
• Identify situations in which work done is zero: either displacement is zero or force and displacement are perpendicular.
• Calculate kinetic energy using K.E. = 1/2 × m × v²; the common mark-losing slip is failing to square the speed.
• Calculate gravitational potential energy using P.E. = m × g × h, identifying mass, acceleration due to gravity, and height correctly.
• Calculate power using P = W/t; distinguish the amount of work from the time taken to do it.
• Explain energy transformations, including free fall and electrical energy changing into light and heat energy in a lamp, while stating that total energy remains constant.
• Convert between units, including 1 kilowatt = 1000 watts and 1 kWh = 3.6 × 10^6 joules; confusing kilowatt-hours with a unit of power loses marks.