1.2 Forces 🏗️

What is a force?

a push or a pull, either contact or non-contact

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

Moment = force * 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

Stablility

An object resists tipping over if line of action falls within its base and it has a low centre of gravity

What will happen to the bus

Position 1

• Does not topple over

• Centre of gravity is inside the wheel base, producing a clockwise moment to pull the bus back onto its base

Position 2

• Topples over

• Centre of gravity is outside the wheel base, producing an anticlockwise moment to pull the bus off its base and on to its side

Newton's First Law

in the absence of unbalanced forces an object will continue to move in a straight line at constant speed

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

Resultant force = mass * 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

Inertial mass

how difficult it is to change the velocity of an object, a heavier object is harder

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

Acceleration of free fall (gravity)

when ignoring air resistance, object falling freely from rest will accelerate at rate of 10 m/s² regardless of mass

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

force = spring constant * extension

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

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

Pressure

The force per unit area

the equation linking pressure, force and area

pressure = force ÷ 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

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

Weight = mass * gravity

Unit for gravitational field strength

Newtons per kilograms (N/kg)

Newton meter

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