Topic A - Space, Time and Motion

Scalar

A quantity with size (magnitude) but no direction.

Vector

A quantity with a size (magnitude) and a direction.

Position

The placement of an object is given in relation to a reference point.

Displacement

A change in position. It is a vector quantity.

Distance

The length of the path followed. It is a scalar quantity.

Speed

The rate of change of distance measured in m/s. Speed is a scalar quantity.

Velocity

The rate of change of displacement measured in m/s. Velocity is a vector quantity.

Acceleration

The rate of change of velocity. A vector quantity.

Instantaneous Velocity

Velocity of an object at a specific point in time. It is the gradient of the tangent line in a displacement-time graph.

Free-fall

The motion of a body that experiences only the force of gravity.

Trajectory

The path followed by a projectile that is launched at an angle. When there is no air resistance, the path is parabolic.

Terminal velocity

The top velocity reached by any object experiencing air resistance or a similar resistance force.

Force

An interaction between bodies that can change the motion of a body, or change the shape or size of a body.

Resultant Force

The vector sum of all the forces acting on an object.

Newton's third law

Forces never occur on their own - there is always a pair of forces in nature.

Field

A region of space where a particle experiences a force. Examples include an electric field, magnetic field and a gravitational field.

Gravitational force

All bodies with mass exert a gravitational force on other bodies and is also known as weight.

Electric charge

The property of matter that is affected by electric fields. Measured in coulombs, symbol C.

Electric force

An interaction between bodies that can change the motion of a body, or change the shape or size of a body.

Magnetic force

A non-contact force caused by the interaction between a magnet and a magnetic material, or between two magnets.

Field forces

Non-contact forces between bodies.

Normal force

The contact force that acts on a body that is perpendicular to the surface in contact with the body. Its magnitude is equal to the weight exerted by the body on the surface.

Frictional force

A force between two surfaces when in contact. It slows down motion of an object and results in heating.

Static friction

The frictional force that acts between two bodies at rest.

Dynamic friction

The frictional force that acts between two bodies moving relative to each other.

Tension force

The force experienced by a rope (or wire, etc.) attached to a fixed point when the rope is pulled from the other end, or the force experienced by a rope when it is pulled from both ends.

Elastic restoring force

The force that counteracts the force extending or compressing a spring and restores the spring to its natural length.

Hooke's law

The displacement of a spring is directly proportional to the force exerted on the spring.

Viscous drag force

The resistive force opposing the motion of a body inside a fluid.

Buoyancy

The force exerted by a fluid on an object partly or wholly immersed in the fluid, that counteracts the weight of the body.

Newton's first law

A body that experiences a zero resultant force will remain at rest or continue to move at a constant velocity.

Newton's second law

The acceleration of a body depends on the mass of the body and the resultant force acting on the body.

Linear momentum

The product of the mass and velocity of a body.

Impulse

The change in momentum of a body, usually caused by a collision or impact.

Elastic collision

A collision where the momentum and the kinetic energy of the system are conserved.

Inelastic collision

A collision where the momentum of the system is conserved, but the kinetic energy of the system is not conserved.

Explosion

When two (or more) bodies are initially at rest and exert forces on each other, moving apart rapidly.

Period

Time taken to complete one revolution.

Frequency

Number of complete revolutions/periods done in one second.

Angular velocity

Rate of change of angle of an object moving in circular motion.

Centripetal acceleration

The acceleration experienced by a body as the body moves in circular motion. The direction of centripetal acceleration is towards the centre of the circle.

Centripetal force

The force exerted along the radial axis on any object that moves on a circular path.

Kinetic energy

The energy stored in a moving mass.

Gravitational potential energy

Energy stored in a mass due to its position within a gravitational field.

Elastic potential energy

Energy stored in an object that has been deformed, such as a stretched spring.

Thermal energy

Energy associated with molecular motion.

Chemical potential

Energy associated with molecular arrangement.

Nuclear energy

Energy stored within the atomic nucleus.

Electric potential energy

Energy associated with a charge due to its position in an electric field.

Work

The amount of energy transferred when an external force moves an object over a displacement.

Conservation of energy

Sum of the useful and wasted energy outputs of an isolated system is always equal to the total energy input. Energy can only be transferred, it cannot be created or destroyed.

Sankey diagram

A scale diagram used to represent the total input, the useful output and the wasted output energies within a system.

Mechanical energy

The sum of the kinetic energy, gravitational potential energy and elastic potential energy of an object.

Power

The rate of work done (or the rate at which energy is transferred).

Efficiency

The ratio of useful energy output to total energy input (or the ratio of useful power output to total power input).

Fuel

A substance that can be made to react with other substances in order to transfer chemical energy into thermal energy or mechanical energy that can be used to do work.

Energy density

The ratio of energy stored within a fuel to the volume of a fuel. It is measured in joules per cubic metre (J m−3).

Moment of inertia

A measure of the ability of a body to resist changes in its rotational motion.

Extended body

A large or irregular object in which the centre of mass is one of an infinite number of points inside the object. An object that cannot be treated as a point mass.

Simple pendulum

A mass suspended from a fixed point pivot so that it can swing freely.

Angular displacement

The angle, measured in radians, that an object rotates around its axis.

Angular acceleration

The rate of change of angular velocity with time. It is measured in radians per second squared (rad s-2).

Uniformly angularly accelerated motion

The motion of a body in a circle with constant angular acceleration.

Line of action

A line with the same direction as the force vector that passes through the point at which the force is applied.

Pivot

The point about which an object rotates (or could rotate). The axis of rotation passes through this point.

Turning effect

The effect a force has on rotating an object around a pivot.

Torque

A measure of the turning effect of a force. It is given by the product of the force's magnitude and the perpendicular distance between the line of action of the force and the pivot. Torque is measured in newton metres (N m).

Couple

A pair of parallel forces of equal magnitude and opposite directions with different lines of action.

Translational motion

A motion during which there is no change in the orientation of the moving object relative to a fixed point.

Translational equilibrium

A body's state when the resultant force on it is zero.

Rotational equilibrium

A body's state when the resultant torque on it is zero.

Angular momentum

The product of a body's moment of inertia and angular velocity.

Angular Impulse

Equivalent to a body's change in angular momentum usually upon collision or impact.

Inertial reference frames

A coordinate system, which is not accelerating, used to describe the motion of a body in space-time.

Galilean transformation

A method of describing the position or velocity of a moving body, relative to a stationary body.

2 postulates of special relativity

The laws of physics are the same in all inertial reference frames. The speed of light is the same for all observers, regardless of their relative motion.

Time dilation

Where the duration of an event, as measured by an observer moving relative to the event, is found to be less than the proper time.

Proper time interval

The duration of an event, as measured by an observer who is stationary relative to the event being timed meaning that observer is in the moving reference frame.

Proper length

The length of a stationary object measured by an observer who is stationary relative to the object.

Length contraction

Where the length of a moving object, as measured by a stationary observer, is found to be less than the proper length.

Muons

Subatomic particles that are created when high energy cosmic rays from space interact with the upper atmosphere of the earth.

Muon decay

Muons are unstable particles that have a very short mean lifetime of about 2.2 μs before they decay into other particles.

Distance travelled by muons in earth's reference frame

It is the distance between the upper atmosphere and the surface of the Earth which is known as the proper length since there is no relative motion between the scientists and the atmosphere.

Distance travelled by muons in their own reference frame

The distance between the upper atmosphere and the surface of the Earth undergoes length contraction.

Lorentz transformation

A method of finding the position and time of an event in the reference frame of a moving observer, based on the position and time of the event in the reference frame of a stationary observer.

Invariant quantity

Quantity which is constant across all reference frames.

Space-time interval

The interval between two events happening at different points in space and/or time. The space-time interval is an invariant quantity.

World line

The trajectory of an object through space and time, with each point representing a specific event (position and time).

Simultaneity

When two events happen at the same time in the reference frame of an observer.

Concept of Simultaneity

Events at different points in space-time that are simultaneous for one observer will not necessarily be simultaneous for an observer in a different reference frame.

Hooke’s law

The extension of the material is directly proportional to the applied force (load) up to the limit of proportionality