Energy: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 6 · Read time: ~9 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Work done
Work is done whenever a force causes a displacement in the direction of that force. Formula: W = F × d, where d is the distance moved in the direction of the force, not just any distance travelled.
Unit: joule (J), equivalent to N m.
No work is done if there is no displacement (pushing against a wall that doesn't move), or if the force acts perpendicular to the direction of motion (carrying a bag while walking horizontally, gravity does no work on you).
Trap: A very common MCQ trap: a person holding a heavy bag and standing still does zero work on the bag, no matter how tired their arm gets, because there's no displacement. Work is about force and displacement, not effort.
Kinetic and potential energy
Kinetic energy (KE) is the energy a body has because of its motion: KE = ½mv². Doubling the speed quadruples the kinetic energy, since v is squared, a very common calculation trap.
Gravitational potential energy (GPE) is the energy a body has because of its position above a reference level: GPE = mgh. Only the vertical height change matters, the path taken to get there (a ramp vs a ladder) makes no difference to the GPE gained.
Principle of conservation of energy: energy cannot be created or destroyed, only converted from one form to another or transferred from one body to another. The total energy in a closed system stays constant.
Trick: For 'falling object' or 'swinging pendulum' questions, the fastest route is often GPE lost = KE gained (ignoring air resistance), rather than working through suvat equations. Check whether the question allows you to ignore air resistance before using this shortcut.
Power and efficiency
Power is the rate of doing work, or the rate of transferring energy: P = W/t = E/t. Unit: watt (W), equal to one joule per second.
Efficiency measures how much of the energy put into a system is usefully converted, rather than wasted (usually as heat due to friction): Efficiency = (useful energy output ÷ total energy input) × 100%.
No real machine is ever 100% efficient, some energy is always lost to friction, air resistance, or heat, these losses don't disappear, they're still accounted for by the conservation of energy, just not in a useful form.
Trap: Students sometimes think a more powerful machine always does more total work. Power tells you how *fast* work is done, not how much total work is eventually done, a low-power motor can still do the same total work as a high-power one, just taking longer.
Worked Example
A toy car of mass 0.50 kg is released from rest at the top of a smooth ramp 1.2 m high, and reaches the bottom of the ramp with a speed of 4.0 m/s. Take g = 10 m/s².
(a) Calculate the gravitational potential energy lost by the car as it descends the ramp. [2]
GPE lost = mgh
= 0.50 × 10 × 1.2
= 6.0 J
(b) Calculate the kinetic energy gained by the car at the bottom of the ramp. [2]
KE gained = ½mv²
= ½ × 0.50 × 4.0²
= 4.0 J
(c) Account for the difference between the GPE lost and the KE gained, even though the ramp is described as smooth. [2]
The GPE lost (6.0 J) is greater than the KE gained (4.0 J), a difference of 2.0 J.
Even on a 'smooth' ramp, some energy is lost to air resistance and to sound as the car moves, converting that 2.0 J into other, non-useful forms of energy.
Energy is still conserved overall, the 'missing' energy hasn't vanished, it has just been transferred to forms other than the car's kinetic energy.
Why this question is a good test of the topic: it's tempting to assume GPE lost = KE gained exactly whenever a ramp is called 'smooth', since that usually means frictionless. This question tests whether a student can still account for the small, real-world energy loss rather than just asserting the two values must be equal.
Frequently Asked Questions
What is the formula for work done in O Level Physics?
W = F × d, where d is the distance moved in the direction of the force, not the total distance travelled.
What happens to kinetic energy if speed doubles?
Kinetic energy quadruples, since KE = ½mv² and speed is squared, not simply doubled.
Does the path taken affect gravitational potential energy?
No. GPE = mgh depends only on the vertical height change, not the path taken to get there.
Can a machine be 100% efficient?
No real machine is ever 100% efficient, some energy is always lost, usually as heat due to friction or air resistance.
What's the difference between power and energy?
Energy (or work done) is the total amount transferred. Power is the rate at which that energy is transferred, P = E/t.
Still mixing up work, energy and power?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.