HKDSE Physics key terms

A vector quantity giving the straight-line distance and direction from an object's starting position to its current position. It differs from distance travelled, which is a scalar and depends on the actual path taken.

The rate of change of displacement with respect to time, including direction. Average velocity is total displacement divided by total time; instantaneous velocity is the value at one moment, found from the slope of a displacement-time graph.

The rate of change of velocity with respect to time. Because velocity is a vector, an object accelerates when its speed changes, its direction changes, or both change; the SI unit is metres per second squared.

Motion under gravity alone, with air resistance neglected. Near the Earth's surface the acceleration is about 9.81 m/s² downwards, so all objects fall with the same acceleration regardless of mass.

Two-dimensional motion of an object launched into the air and then subject only to gravity. The horizontal velocity stays constant while the vertical motion accelerates at g, so the two components are analysed separately.

An object remains at rest or moves with constant velocity unless acted on by a net external force. It defines inertia: the tendency of a body to resist changes in its state of motion.

The property of a body that resists any change in its velocity. It depends only on the body's mass, not on its speed or position, and is the reason a passenger lurches forward when a bus brakes.

The net force on a body equals the rate of change of its momentum; for constant mass this is F = ma. The acceleration is in the same direction as the resultant force.

For every force that one body exerts on another, the second body exerts an equal and opposite force on the first. The pair acts on two different bodies, so the forces never cancel each other.

The single force equivalent to all forces acting on a body added as vectors. Also called the resultant force; it determines the body's acceleration through Newton's second law.

The contact force a surface exerts perpendicular to itself on an object resting on it. It adjusts to other forces, and becomes zero when the object loses contact with the surface.

A contact force opposing relative motion or the tendency of relative motion between two surfaces. Static friction adjusts up to a maximum value; kinetic friction acts when sliding occurs.

Splitting a single force into two perpendicular components, usually horizontal and vertical. It lets problems be solved by treating each direction independently, especially on inclined planes.

The turning effect of a force about a pivot, equal to force times perpendicular distance from the pivot to the line of action. The SI unit is the newton metre.

For a body in rotational equilibrium, the sum of clockwise moments about any point equals the sum of anticlockwise moments about that same point. It is used to find unknown forces or distances.

The point through which the whole weight of a body may be taken to act, whatever its orientation. For a uniform body it coincides with the geometric centre.

The ability of a body to return to equilibrium after a small displacement. It increases when the centre of gravity is low and the base area is wide, giving a larger limiting angle of tilt.

Energy transferred by a force acting through a displacement, calculated as force times displacement in the direction of the force. The SI unit is the joule.

The energy a body possesses because of its motion, equal to ½mv². Doubling the speed quadruples the kinetic energy, which is why braking distance rises steeply with speed.

Energy cannot be created or destroyed, only transferred between forms or stores. In a closed system the total energy stays constant, though energy may be dissipated as heat by friction.

The rate at which energy is transferred or work is done, equal to work done divided by time. The SI unit is the watt, where one watt equals one joule per second.

The ratio of useful energy output to total energy input, usually expressed as a percentage. The remainder is lost, typically as heat or sound, so efficiency is always below 100%.

The product of a body's mass and velocity, a vector quantity measured in kg m/s. It is changed by a net force acting over time, which is the idea behind impulse.

In a system with no net external force, the total momentum before an interaction equals the total momentum after. It applies to collisions and explosions in any direction.

A collision in which both momentum and kinetic energy are conserved. Billiard-ball collisions approximate this; in inelastic collisions kinetic energy is converted into other forms.

Motion along a circular path at constant speed. The velocity direction changes continuously, so a centripetal acceleration points towards the centre and a centripetal force is required.

The net force directed towards the centre of a circular path that keeps an object moving in that circle. It is provided by tension, gravity, friction or a normal force depending on the situation.

The rate of change of angular displacement, equal to the angle swept per unit time. It is measured in radians per second and relates to linear speed by v = rω.

The gravitational force per unit mass at a point, measured in newtons per kilogram. Near the Earth's surface it is about 9.81 N/kg, numerically equal to the acceleration of free fall.

The gravitational force exerted on a body by a planet, equal to mass times gravitational field strength. Unlike mass, weight varies with location and is measured in newtons.

A measure of the quantity of matter in a body and of its inertia, measured in kilograms. It is a scalar that stays the same wherever the body is in the universe.