Leaving Certificate Physics - Mechanics Definitions & Laws

Displacement

Distance in a given direction

Velocity

The rate of change of displacement with respect to time

Acceleration

The rate of change of velocity with respect to time

Vector Quantity

A quantity which has both magnitude and direction

Scalar Quantity

A quantity which has magnitude only

Newton's First Law

An object will remain at rest or travelling with constant velocity unless acted on by a resultant force

Newton's Second Law

The rate of change of an object's momentum is directly proportional to the resultant force and takes place in the same direction as that force

Newton's Third Law

For every action there is an equal and opposite reaction

Force

Anything that causes the velocity of an object to change

Momentum

mass x velocity

Friction

A force that opposes motion between two surfaces that are in contact

Principle of Conservation of Momentum

Total momentum of a system remains the same before and after a collision, providing no external forces are acting on the system

Centripetal Force

The force towards the centre of a circle needed to keep the object moving in a circle

Angular Velocity

The rate of change of angle with respect to time

Newton's Law of Universal Gravitation

The force felt between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

Density

Mass per unit volume

Pressure

Force per unit area.

Boyle's Law

At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.

Archimede's Principle

When an object is fully or partially submerged in a fluid, it experiences an upthrust equal in magnitude to the weight of the fluid displaced

Law of Flotation

The weight of a floating body is equal to the weight of the fluid it displaces

Moment of a Force

Force x perpendicular distance from the axis

Hooke's Law

When an object is bent, stretched or compressed by a displacement s, the restoring force F is directly proportional to the displacement, provided the elastic limit is not exceeded

Simple Harmonic Motion

1. Acceleration directly proportional to displacement from the mean position

2. Acceleration always directed towards the mean position

Work

Work is done when a force moves an object through a displacement in the direction of the force

Energy

The ability to do work

Kinetic Energy

The energy an object has due to its motion

Potential Energy

The energy an object has due to its position in a force field

Principle of Conservation of Energy

Energy cannot be created or destroyed but can only be converted from one form to another

Power

The rate at which work is done / The rate at which energy is converted from one form to another

Newton (unit)

1 N of force gives an acceleration of 1 m/s^2 to a mass of 1 kg

Joule (unit)

1 J of work is done when a force of 1 N displaces an object by 1 m

Conditions for Equilibrium

1. The vector sum of the forces in any direction is zero

2. The sum of moments about any point is zero

Speed

The rate of change of distance with respect to time

Weight

The force of the Earth's gravity acting on an object

Centripetal Acceleration

The acceleration an object moving in a circle has toward the centre of the circle

Period of an Orbit

The time taken for a satellite to go once around the central body