Energy, Work and Power for Singapore N(A)-Level Science (Physics) 5105/5106: energy stores and transfers and the conservation of energy, kinetic and gravitational potential energy, and work done and power with efficiency

What this module covers

Energy, Work and Power explains how energy moves around and how we measure the doing of work in N(A)-Level Science (Physics) 5105/5106. It names the main energy stores, follows energy as it transfers between them under the rule that the total never changes, and gives the formulas for the two energy stores you calculate most: kinetic and gravitational potential energy. It finishes with work and power, the measures of how much and how fast energy is transferred.

These ideas link directly to forces (a force doing work) and underpin electricity later, where electrical energy is transferred. Each dot point below has full worked answers and practice questions.

Energy stores and transfers

Energy stores and transfers names the main energy stores, such as kinetic, gravitational potential, chemical, elastic and thermal, and describes how energy moves between them. The central rule is the principle of conservation of energy: energy cannot be created or destroyed, only transferred. In real devices some energy is always wasted, usually as heat, which is why efficiency is less than 100 percent.

Kinetic and potential energy

Kinetic and potential energy gives the two formulas you calculate most:

When a falling object loses height it loses gravitational potential energy and gains the same amount of kinetic energy (ignoring air resistance), which is conservation of energy in action.

Work and power

Work and power defines work done as the energy transferred when a force moves an object:

measured in joules. Power is the rate of doing work:

measured in watts ($1\ \text{W} = 1\ \text{J s}^{-1}$). Two devices can transfer the same energy, but the one that does it in less time is more powerful.

How this module is examined

• Square before multiplying. In $E_k = \tfrac{1}{2} m v^2$, square the speed first.
• Use conservation. Equate energy lost from one store to energy gained by another when air resistance is ignored.
• Separate work from power. Work is total energy transferred (joules); power is work per second (watts).

Check your knowledge

Recall and calculation questions across the module. Attempt them, then check the worked solutions.

1. State the principle of conservation of energy. (2 marks)
2. A $2.0\ \text{kg}$ object moves at $3.0\ \text{m s}^{-1}$. Calculate its kinetic energy. (2 marks)
3. A box is lifted $4.0\ \text{m}$ and gains $80\ \text{J}$ of gravitational potential energy. Taking $g = 10\ \text{N kg}^{-1}$, find its mass. (2 marks)
4. A force of $50\ \text{N}$ moves an object $6.0\ \text{m}$. Calculate the work done. (2 marks)
5. A motor does $1200\ \text{J}$ of work in $40\ \text{s}$. Calculate its power. (2 marks)