JCE Physics key terms

A vector quantity describing the straight-line change in position from a starting point to an ending point, including both distance and direction. It differs from distance, which is scalar and path-dependent.

The rate at which displacement changes with time, a vector with both magnitude and direction. Average velocity is displacement divided by elapsed time; instantaneous velocity is its value at one moment.

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

Motion under gravity alone, with air resistance neglected, so every object accelerates downward at the same rate regardless of mass. Near Earth's surface this acceleration is about 9.8 metres per second squared.

The curved path of an object launched into the air and then moving only under gravity. Horizontal and vertical motions are independent: horizontal velocity stays constant while vertical velocity changes uniformly.

An object remains at rest or moves with constant velocity unless a net external force acts on it. It defines inertia and explains why seatbelts and friction matter in everyday motion.

The net force on an object equals its mass multiplied by its acceleration, written F = ma. It links force, mass and acceleration, and shows that larger mass needs more force for the same acceleration.

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

The single force obtained by vector addition of all forces acting on an object. Only the net force determines acceleration; balanced forces give zero net force and therefore no change in velocity.

A sketch showing one object isolated with arrows for every external force acting on it, such as weight, normal reaction, friction and tension. Resolving these arrows into components gives the net force.

A contact force that opposes relative sliding or the tendency to slide between surfaces. Static friction adjusts up to a maximum; kinetic friction stays roughly constant while surfaces slide.

The perpendicular contact force a surface exerts on an object resting on or pressing against it. It is not always equal to weight, for example on an incline or in a lift accelerating vertically.

The pulling force transmitted along a string, rope or cable when it is stretched. In an ideal light string it is the same throughout and always acts away from the object being pulled.

The state of a body when the net force and net moment on it are both zero, so it stays at rest or moves with constant velocity. Forces and torques balance in every direction.

The turning effect of a force about a pivot, equal to force multiplied by the perpendicular distance from the pivot to the force's line of action. It is measured in newton metres.

The single point through which the whole weight of a body may be considered to act. For a uniform object it lies at the geometric centre; for irregular shapes it can be found by balancing.

The gravitational force on an object, equal to mass multiplied by gravitational field strength and measured in newtons. Unlike mass, it changes with location, such as between Earth and the Moon.

A measure of the quantity of matter in a body and of its resistance to acceleration, measured in kilograms. It is scalar and stays the same wherever the object is in the universe.

The tendency of an object to resist any change in its state of rest or uniform motion. It depends only on mass, so a more massive object needs a larger force to change its velocity.

The product of an object's mass and velocity, a vector measured in kilogram metres per second. In a closed system with no external force, total momentum is conserved in collisions and explosions.

The product of a force and the time for which it acts, equal to the change in momentum it produces. Extending impact time with crumple zones or airbags reduces the peak force.

In a system with no net external force, the total momentum before an event equals the total momentum after it. This applies to collisions, explosions and recoil in every direction.

A collision in which both momentum and kinetic energy are conserved. Objects bounce apart without permanent deformation or heat loss, as with ideal gas molecules or steel ball bearings.

The energy transferred when a force moves an object along its line of action, equal to force times displacement in the direction of the force. It is measured in joules.

The rate of transferring energy or doing work, equal to work divided by time and measured in watts. One watt equals one joule per second, and power also equals force times velocity.

The energy an object has because of its motion, equal to half its mass times its speed squared. Doubling the speed quadruples the kinetic energy, which is why stopping distances rise sharply.

Energy stored because of an object's position or condition, such as height in a gravitational field or the compression of a spring. It converts to kinetic energy when the object is released.

Energy cannot be created or destroyed, only transferred between stores or transformed from one form to another. The total energy of a closed system therefore remains constant.

The ratio of useful energy or power output to total energy or power input, often expressed as a percentage. The remainder is usually dissipated as heat or sound and is never zero in real machines.

The gravitational force per unit mass at a point, measured in newtons per kilogram. Near Earth's surface it is about 9.8 N/kg and equals the free-fall acceleration there.

Another name for the turning effect of a force about an axis, calculated as force times perpendicular distance. A couple produces torque from two equal, opposite, parallel forces.

Mass per unit volume of a substance, measured in kilograms per cubic metre. It explains floating and sinking: an object floats if its average density is less than that of the fluid.

The force acting normally per unit area on a surface, measured in pascals. In a fluid it increases with depth and depends on density, and it is transmitted equally in all directions.

The upward buoyant force a fluid exerts on an immersed object, equal to the weight of fluid displaced. It arises because pressure is greater at the bottom of the object than at the top.

Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid and the container walls. Hydraulic brakes and jacks use this to multiply force through different piston areas.

The extension of a spring or elastic material is directly proportional to the applied force, provided the limit of proportionality is not exceeded. The constant of proportionality is the spring constant.

Energy stored in a stretched or compressed elastic object, such as a spring. For a spring obeying Hooke's Law it equals half the spring constant times the extension squared.

The capacity of a system to do work or cause change, measured in joules. It exists in stores such as kinetic, gravitational, elastic, chemical, thermal and nuclear, and is always conserved overall.