HSC Physics key terms

The straight-line distance and direction from a starting point to a finishing point. It is a vector, so reversing direction reduces its magnitude even when total distance travelled keeps increasing.

The rate of change of displacement with respect to time, including direction. Its magnitude is speed; a change in direction alone counts as an acceleration because velocity is a vector.

The rate at which velocity changes with time. It is a vector, so it occurs whenever speed changes, direction changes, or both, and is measured in metres per second squared.

A measurable quantity that requires both magnitude and direction to be fully described, such as force, velocity or displacement. Vectors are added by resolving into perpendicular components.

A measurable quantity described fully by magnitude alone, with no direction, such as mass, speed, distance, energy or time. Scalars add arithmetically; vectors cannot be added this way.

Splitting a single vector into two perpendicular components, usually horizontal and vertical, using trigonometry. It simplifies problems such as projectile motion and forces on inclines.

The two-dimensional motion of an object launched into the air and then acted on only by gravity and air resistance. Horizontal velocity is constant while vertical velocity changes at the acceleration due to gravity.

The acceleration experienced by an object in free fall near a planet's surface, directed towards the centre of the planet. Near Earth's surface its value is approximately 9.8 metres per second squared, ignoring air resistance.

An object remains at rest or moves with constant velocity unless acted on by a net external force. It defines inertia and explains why passengers lurch forward when a vehicle brakes suddenly.

The net force on an object equals the product of its mass and acceleration, so acceleration is directly proportional to net force and inversely proportional to mass. It links force, mass and acceleration in one equation.

For every action force there is an equal and opposite reaction force, acting on a different object. The pair never cancels because the two forces act on separate bodies.

The single vector sum of all forces acting on an object. It determines the object's acceleration; a net force of zero means the object is at rest or moving at constant velocity.

The tendency of an object to resist changes in its state of motion, determined by its mass. A larger mass means greater inertia and a smaller acceleration for the same applied net force.

A measure of the amount of matter in an object and of its inertia, measured in kilograms. It is constant everywhere, unlike weight, which depends on the local gravitational field.

The gravitational force exerted on an object by a planet, equal to mass multiplied by the local acceleration due to gravity. It is a force measured in newtons and changes with location.

The perpendicular contact force a surface exerts on an object resting against it. On a horizontal surface it balances weight, but on an incline it is smaller than the weight.

A contact force that opposes relative motion or the tendency towards relative motion between surfaces. Static friction prevents sliding up to a maximum value; kinetic friction acts during sliding.

A simplified diagram showing one object as a point with all external forces drawn as labelled arrows from it. It helps identify the net force before applying Newton's laws.

The energy transferred to or from an object when a force moves it through a displacement. It equals the product of displacement and the force component along that displacement, measured in joules.

The energy an object possesses because of its motion, equal to half its mass times its speed squared. Doubling speed quadruples kinetic energy, which is why braking distance rises steeply.

The energy stored in an object because of its position in a gravitational field. Near Earth's surface it equals mass times gravity times height above a chosen reference level.

The principle that energy cannot be created or destroyed, only transferred or transformed between forms. In mechanical systems the total of kinetic and potential energy stays constant when only gravity does work.

The rate at which energy is transferred or work is done, measured in watts, where one watt is one joule per second. It equals work divided by time or force times velocity.

The ratio of useful energy or power output to total energy or power input, usually expressed as a percentage. It is always less than 100 percent because some energy is dissipated, often as heat.

The product of an object's mass and velocity, a vector quantity measured in kilogram metres per second. Changes in momentum are caused by impulse, the product of net force and time.

The product of a net force and the time over which it acts, equal to the change in momentum produced. Extending collision time with crumple zones reduces the force experienced.

In a closed system with no external net force, total momentum before an interaction equals total momentum after it. It applies to collisions and explosions in both one and two dimensions.

A collision in which both momentum and total kinetic energy are conserved. Ideal gas molecules collide elastically; in practice most everyday macroscopic collisions are partly inelastic.

A collision in which momentum is conserved but total kinetic energy is not, because some energy becomes heat, sound or deformation. Objects may stick together in a perfectly inelastic collision.

Motion in a circle at constant speed. Although speed is constant, velocity changes direction continuously, so the object accelerates towards the centre.

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, not an extra force.

The time taken for one complete cycle of a repeating motion, such as one full orbit or oscillation, measured in seconds. Frequency is its reciprocal.

The number of complete cycles of a repeating motion or wave passing a point each second, measured in hertz. It equals one divided by the period.

The turning effect of a force about a pivot, equal to the force multiplied by the perpendicular distance from the pivot. It is measured in newton metres and causes angular acceleration.

The state in which the total clockwise torque about a pivot equals the total anticlockwise torque, so an object does not rotate. It is separate from translational equilibrium.

The ratio of load force to effort force in a simple machine. A value greater than one means the machine multiplies force, though energy is still conserved.