How definitions are marked
- Most definitions are worth 2 marks, and the marks sit on keywords. Leaving out one keyword loses a mark; leaving out the key idea loses both.
- Learn them word for word, but understand them — many questions ask you to apply a law immediately after stating it.
- Laws that involve proportion need both parts (e.g. “directly proportional to … and inversely proportional to …”) and conditions such as “at constant temperature” or “in an isolated system”.
Common mark losses: “speed” instead of “velocity”; “force” instead of “net force”; forgetting “in the direction of the net force”; leaving out “isolated system” or “at constant temperature”.
Forces and motion
| Term | Definition |
|---|
| Newton’s first law | An object continues in a state of rest or uniform (constant) velocity unless it is acted upon by a net (resultant) force. |
| Inertia | The property of an object that causes it to resist a change in its state of rest or uniform motion. |
| Newton’s second law | When a net force acts on an object, the object accelerates in the direction of the net force; the acceleration is directly proportional to the net force and inversely proportional to the mass. |
| Newton’s third law | When object A exerts a force on object B, object B simultaneously exerts an oppositely directed force of equal magnitude on object A. |
| Normal force | The perpendicular force exerted by a surface on an object in contact with it. |
| Frictional force | The force that opposes the motion of an object and acts parallel to the surface. |
| Free fall | Motion during which the only force acting on an object is the gravitational force. |
| Projectile | An object upon which the only force acting is the force of gravity. |
See also the momentum definitions (momentum, impulse, conservation of linear momentum, isolated system, elastic collision) in the lesson on momentum and Newton’s second law, and Newton’s law of universal gravitation in the gravitation lesson.
Work, energy and power
| Term | Definition |
|---|
| Work | The product of the force and the displacement in the direction of the force: W = FΔx cos θ. |
| Work–energy theorem | The work done by a net force on an object is equal to the change in the kinetic energy of the object. |
| Conservative force | A force for which the work done in moving an object between two points is independent of the path taken (e.g. gravitational force). |
| Non-conservative force | A force for which the work done depends on the path taken (e.g. friction, air resistance, applied force). |
| Principle of conservation of mechanical energy | In an isolated system, the total mechanical energy remains constant. |
| Power | The rate at which work is done or energy is expended. |
Doppler effect, electricity and the photoelectric effect
| Term | Definition |
|---|
| Doppler effect | The change in frequency (or pitch) of the sound detected by a listener because the sound source and the listener have different velocities relative to the medium of sound propagation. |
| Emf | The maximum energy provided by a battery per coulomb (unit charge) passing through it. |
| Internal resistance | The resistance within a battery that causes energy to be converted to heat inside it. |
| Ohm’s law | The potential difference across a conductor is directly proportional to the current in the conductor at constant temperature. |
| Faraday’s law of electromagnetic induction | The emf induced is directly proportional to the rate of change of magnetic flux (flux linkage). |
| Photoelectric effect | The process whereby electrons are ejected from a metal surface when light of suitable frequency is incident on it. |
| Work function | The minimum energy that an electron in the metal needs to be emitted from the metal surface. |
| Threshold frequency | The minimum frequency of light needed to emit electrons from a certain metal surface. |