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What are forces?


Push and pull


All forces involve interactions between objects. There are several different types.

Types of forces

What forces are acting during this rocket launch?

All types of forces can occur whether objects are still or moving.

Gravity and magnetism are forces that can act over distances.

Friction and upthrust are forces that involve direct contact between objects.


Forces affecting objects


Forces changing an object’s shape


There are two wind machines applying forces to the ice skater. The forces acting on the skater are equal in magnitude and opposite in direction.

Balanced forces

The forces are balanced, so they cancel each other out. The skater does not move.


Unbalanced forces

Unbalanced forces lead to a change in speed or direction.

What if only one wind machine is blowing on the skater?

The forces acting on her are no longer balanced so she will start to move to the left. Her speed will change – this is called acceleration.


Measuring forces


Force pairs


500 N 500 N

Imagine a car travelling at a constant speed of 50 km/h.

Balanced and unbalanced forces

The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed.


A crosswind acting on the car produces a sideways force.

The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. The car will veer sideways.

Balanced and unbalanced forces

If the car turns right so that the wind is now behind the car, what will happen to the speed?

100 Ncross wind

500 N 500 N


The resultant force is calculated by working out the difference between opposing forces in each direction.What is the resultant force on this truck?

The sum effect of more than one force is called the resultant force.

A resultant force of 100 N is accelerating the truck.

Resultant force

500 N400 N


Balanced and unbalanced forces – summary

and the object is stationary, it will remain stationary

and the object is moving, it will continue to move at the same speed and in a straight line.

If the forces are unbalanced, two things can happen:

If the forces on an object are balanced:

In other words, the object will continue to do what it is already doing without any change.

The speed can change. This is called acceleration.

The direction of motion can change.


20 N

1. What is the resultant force on the satellite?

Resultant force = 20 N – 10 N= 10 N down

The satellite will accelerate downwards.

Resultant forces – question 1

5 N 5 N


5 N

2. What is the resultant force on the bird?

5 N 5 N

The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction.

Resultant force = 5 N – 5 N = 0 N


The vertical forces are not balanced, the bird will accelerate in a downwards direction.

Resultant force = 5 N – 0 N = 5 N down


The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction.

The horizontal forces are not balanced, so the yacht will accelerate to the right.

10 N

13 N

3. What is the resultant force on the yacht?

10 N

10 N

Resultant force = (20 N +10 N) – 13 N = 17 N right


20 N


When the netball is thrown, why does it fall back down?

What is gravity?

The Earth and the ball are pulling each other together.

There is a gravitational force pulling it towards the Earth.

Gravity is a force that occurs between all objects.

Gravity always acts to pull objects towards each other.

However, the ball moves much more than the Earth because it has a much smaller mass.


Sir Isaac Newton is a very famous physicist who lived in England 1643–1727.

Sir Isaac Newton

Newton wrote down his ideas in the Philosophiae Naturalis Principia Mathematica; a very important book about forces and gravity.

Some accounts suggest that one of Newton’s greatest discoveries occurred when an apple fell on his head and it made him think about the reason it fell…


Gravity and Newton

He wrote in the Philosophiae Naturalis, “It is an attractive force that makes apples fall from trees and the planets orbit the Sun.”

Other scientists had already noted the effects of gravity but Newton was the first to calculate the force of gravity on objects.

Newton realized that the motion of falling objects and objects orbiting in space must be caused by the same force – Gravity!


Gravity is an attractive force that acts between all objects that have mass. The size of the force depends on the masses of the objects.

When you jump, the gravitational force of the Earth pulls you down. Your gravitational force also pulls the Earth towards you, but you don’t notice it because the Earth is too heavy to be visibly affected by your gravity.

What is gravity?

All objects produce a gravitational force. This is very large for huge masses such as planets.

Earth’s gravity

Satellite’s gravity


The force of gravity between two objects depends on their masses and the distance between them.

Gravity and distance

Why do spacecraft lose their large fuel tanks and booster rockets once they have left the Earth’s surface?

As a spacecraft gets further away from Earth, the force of gravity gets smaller.

Spacecraft produce a very large force, called thrust, to overcome the force of gravity.


Gravity during a rocket launch


Mass is the amount of matter in an object and is measured in kilograms.

What are mass and weight?

Mass is not a force.

An object, such as this satellite, has the same mass at any point in the Universe.

Weight is a force and is caused by the pull of gravity acting on a mass.

Weight is measured in newtons and has both magnitude and direction. An object’s weight changes depending on where it is in the Universe.


Investigating mass and weight


The force of gravity on the Moon is only one-sixth of that on Earth because the Moon has a much smaller mass.

Mass and weight on the Moon

However, the astronaut still has the same mass – they just weigh less because gravity is weaker.

Any object on the Moon weighs one-sixth of the amount it would weigh on Earth.

Astronauts can jump up 20 feet on the Moon due to there being such a low gravitational force.


Mass and weight on different planets


Weight and mass activity


Missing words about gravity

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