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### KS4 Physics

Forces

What is a force?

A force is a push or a pull. A force cannot be seen but you can see how a force affects an object.

Contact and non-contact forces

When two objects or materials need to be touching for a force to have an effect, it is a contactforce.

Examples:

- friction

- air resistance

When two objects or materials do not need to be touching for a force to have an effect, it is a non-contactforce.

Examples:

- gravity

- electrostatic

- magnetic

Non-contact forces act over a distance.

Are these non-contact forces attractive, repulsive or both?

Gravitational attraction

Gravity is an attractive force that exists between all masses.

The larger the mass, the greater the gravitational attraction.

The greater the distance between masses, the smaller the gravitational attraction.

The Earth has a large mass and so produces a strong gravitational force.

The Moon is kept in orbit around the Earth by the pull

of the Earth’s gravity.

Gravity and weight

The pull of the Earth’s gravitational force on an object is called weight.

The Moon also has a gravitational force.

Why is the weight of an object

on the Moon less than the

weight of the same object on

the Earth?

The Moon is smaller than the

Earth and so the pull of the

Moon’s gravity is weaker than

the pull of the Earth’s gravity.

This means that the weight of

the object is less on the Moon.

Mass and weight

What is the difference between mass and weight?

1kg

mass = 1kg

weight = 10 N

Mass is the amount of matter that makes up

an object.

The mass of an object is always the same, wherever it is in the Universe.

Weight is a force due to the pull of gravity on an object.

The weight of an object will vary depending on where it is in the Universe.

The units of mass are kilograms (kg).

The units of weight are newtons (N).

Force diagrams

A force diagram uses arrows to show the forces acting on an object.

- The direction of each arrow shows you the direction of each force.
- The size of each arrow can be used to compare the sizes of the forces.

air resistance

What is the force diagram for this falling object when it first starts to fall?

weight

Forces on still objects

weight

reaction

force

What forces are acting on Mel’s computer?

The computer is pulled downwards by the force

of gravity and causes

it to have weight.

Thetable exerts an

equal and opposite

force pushing upwards

on the computer. This is called the reaction force.

These forces are balanced so the computer does not move.

What forces are acting on Mel as she works at her computer?

Forces on a swimming fish

weight

friction

thrust

upthrust

What forces are acting on this fish as it swims?

Upthrust is the upwards force on the fish caused by the water around the fish. Sometimes this is called buoyancy.

Thrust is the forwards force acting on the fish.

Forces on a moving boat

weight

air

resistance

thrust

friction

upthrust

What forces are acting this moving boat?

Changing forces on a ferry

An empty ferry arrives at port and is loaded with travellers.

It increases.

What happens to the weight of the ship?

What happens to the upthrust on the ship?

Which force is now largest – weight or upthrust?

It increases.

The increase in upthrust equals the increase in weight,

so both forces remain equal and balanced.

Forces and motion

- If the forces on an object are balanced, the object will
- continue to do what it is already doing without change.
- If the object isstationary, it will remain stationary.
- If the object ismoving, it will continue to move
at the same speed and in the same direction.

- If the forces on an object are unbalanced, two things
- about the object can change:
- Thespeed – the object may speed up or slow down.
- Thedirectionof motion.

Unbalanced forces and motion

If an object is stationary and unbalanced forces act on it, what will happen to the object?

- The object will start to move – its speed and direction have changed.

If an object is moving and unbalanced forces act on it, what can happen to the object?

- The speed of the object can change.
It might speed up or slow down.

- The direction of the object can change.

Balanced and unbalanced forces

How many pairs of forces can you spot?

Are they balanced or unbalanced?

What is friction?

If you rub your hands together they get warm. There is resistance to the rubbing motion.

What is the name of this resistive force called?

It is calledfriction.

What causes this force?

Your hands might look smooth, but on a microscopic level they have rough surfaces. So when you rub your hands together you feel the resistive force of friction.

In which direction does friction act?

Friction always acts in the opposite direction to which an object is moving.

An object will only start to move if the forces applied to it are greater than any frictional forces.

What is the direction of friction acting on each moving ball?

friction

friction

friction

friction

Effects of friction

Friction is a type of force that always acts in the opposite direction to which an object is moving and slows it down.

Whenever there is friction between two objects, heat is generated and their surfaces eventually wear away.

The effect of friction can be reduced by using a lubricant.

Oil is a common lubricant that is used in car engines and bike gears to reduce friction effects.

Glossary

- air resistance –A frictional force that acts against an object moving through air.
- contact force –A force that has an effect when two objects are touching.
- friction –A contact force between an object and another material. This force opposes the motion of the object.
- gravity – The force of attraction between any two objects that have mass.
- mass –The amount of matter that makes up an object, measured in grams (g) or kilograms (kg).
- unbalanced forces –Forces acting on an object that change its speed or direction.
- upthrust –An upwards force acting on an object in a liquid or a gas.
- weight –The force created by the gravitational attraction

on a mass, measured in newtons (N).

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