# Forces

## The basics

• Forces can either be 'push' or 'pull', and they work in pairs (have a look at the diagram below).
• Forces are also either balanced or unbalanced, and they are measured using Newtons (N)
• The magnitude of a force is shown on a diagram by the size of the arrow

## What are the pairs of forces on this diagram? • REACTION - WEIGHT are a pair
• DRAG - THRUST are a pair

Can you identify which forces are push, and which forces are pull?

The forces on this race car are balanced, this means one of two things....

1. The car is not moving, OR...
2. If its already moving then it will stay at a constant speed.
Check out this picture below.... ## How do we make an object move?

• Make the forces unbalanced! When forces become unbalanced and object will move, this movement includes acceleration (speeding up), decceleration (slowing down), changing direction.
• When this guy jumps out of the plane the forces on him become unbalanced, and he changes direction and speed.

## How do the forces change as you jump out of a plane?!

As you leave the plane the forces acting upon you become unbalanced, but how?

GRAVITY

Gravity is the force that pulls you down towards the Earth, but what is gravity?

• A force that attracts objects with mass towards each other.
• Any object with mass (KG) creates gravity
• The more mass something has, the stronger the gravitational pull it has.

The skydiver has mass (KG) and so he has his own gravitational field, but because the Earth has a much bigger mass it pulls him downwards (really fast!).

## How much would your WEIGHT be on other planets? Use this calculator to find out....

So we know that MASS (how much matter/stuff your made of) doesn't change on other planets, but WEIGHT does. This is because WEIGHT depends on GRAVITY, and gravity changes on different planets because each planet has a different MASS (remember how we said that gravitational strength depends on how much MASS there is)

## Density ## Why are there layers in this picture?

Why are there distinct layers in the picture? DENSITY! Each substance has a different density.

Density is how much MASS there is in a specific AREA. For example, the density of water is 1.0g/cm2.

• g=MASS
• cm2=AREA

Honey is the substance with the highest density, and lamp oil has the lowest density.

## You can calculate DENSITY using this handy formula triangle ## Practise Questions

A block of wood has a density of 0.6g/cm3, and a volume of 1.2 cm3, what is the mass?

An 800g boulder has a density of 8g/cm3, what is the volume?

## Pressure • F = FORCE
• p=PRESSURE
• A=AREA

## Example Questions

James weighs 700N and wearing shoes with an area of 0.2m2, what pressure does he exert?

A birthday present has an area of 2m2 (2 metres squared), and exerts a pressure of 20N/m2 (Newton metres squared), how much does it weigh?

## Moments

A moment is a turning force that acts around a pivot (think of how a see-saw moves).

See if you can identify where the pivot is in these diagrams below....