1.2 Forces

Moment of a force

turning effect produced when force(s) cause object to turn/ rotate about a pivot

Pivot (fulcrum)

The point about which moments act in a clockwise or anticlockwise direction

The equation linking moment of a force, force and distance

M = Fd

M

The symbol for moment of a force

F

The symbol for force

d

The symbol for distance

Unit for moment of a force

Newton-metre (Nm)

Unit for force

Newtons (N)

Unit for distance

Metres (m)

Principle of moments

the total clockwise moments about a pivot is equal to the total anticlockwise moments in equilibrium

Centre of gravity

The point at which the entire weight of an object can be considered to act

Stable

object resists tipping over as line of action falls within its base

Newton's First Law

An object remains in the same state of motion unless acted on by a resultant force

Resultant force

single force with the same effect as original forces acting together

When the resultant force is zero

object is stationary or continues to move at the same speed/direction

Newton's Second Law

acceleration is proportional to the resultant force acting on the object, and inversely proportional to the mass

The equation linking resultant force, mass and acceleration

F = ma

F

The symbol for force

m

The symbol for mass

a

The symbol for acceleration

Unit for mass

Kilograms (kg)

Unit for acceleration

Metres per second squared (m/s²)

Newton's Third Law

for every action, there is an equal and opposite reaction

Inertia

an object continues in their state of rest or in uniform motion

Inertial mass

how difficult it is to change the velocity of an object or the ratio of force ÷ acceleration

Deformation

change of shape/ size as a result of forces being applied

Conditions required to deform a stationary object

must be more than one force acting on the object

Extension

increase in length/ size as a result of an object being stretched

Compression

reduction in length/ size as a result of an object being squashed

Hooke's Law

extension is directly proportional to force applied, provided limit of proportionality isn’t exceeded

Limit of proportionality

point beyond which Hooke's law is no longer obeyed

The equation linking force, spring constant and extension/compression

F = ke

F

The symbol for force

k

The symbol for spring constant

e

The symbol for extension (or compression)

Unit for spring constant

Newtons per metre (N/m)

Unit for extension

Metres (m)

Spring constant

measure of the stiffness of a spring

Relationship between spring constant and extension

higher the spring constant, the stiffer the spring

Scalar quantity

quantity that has a magnitude (size) only

Examples of scalar quantities

Speed, temperature, mass, energy, distance, density

Vector quantity

quantity that has a magnitude and direction

Examples of vector quantities

Velocity, force, displacement, acceleration, momentum

Representing vector quantities

Vectors are represented by an arrow to indicate its size and direction

Contact forces

Forces between two objects that are physically touching each other

Examples of contact forces

friction, air resistance, tension

Non-contact forces

Forces between two objects that are not physically touching each other

Examples of non-contact forces

gravitational force, electrostatic force, magnetic force

Free body diagram

diagram showing all the forces acting on an object

Friction

contact force that always opposes the motion of an object

Fluid

A liquid or a gas

Pressure

The force per unit area

the equation linking pressure, force and area

p = f/a

p

The symbol for pressure

F

The symbol for force

A

The symbol for area

Unit for pressure

Pascals (Pa)

Unit for area

Square metres (m²)

A unit used for pressure that is equivalent to pascals (Pa)

Newtons per metre squared (N/m²)

How to increase pressure

Increase the force or decrease the area

How to decrease pressure

Decrease the force or increase the area

Mass

the amount of matter in an object

Weight

force acting on an object due to gravity

Relationship between weight and mass

directly proportional to one another

The equation linking weight, mass and gravitational field strength

W = mg

W

The symbol for weight

m

The symbol for mass

g

The symbol for gravitational field strength (gravity)

Unit for gravitational field strength

Newtons per kilograms (N/kg)

Centre of mass

point the weight of an object may be considered to act at

Newton meter

calibrated spring-balance used to measure an object's weight