KS3 Physics

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20051 of 24

KS3 Physics

7K Forces and their Effects



Contents

Feel the force

Friction

Forces in water

Weight and mass

Summary activities


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

What is a force?


What type of force?


10 N 10 N

If you link two newtonmeters and pull equally hard from both ends, the forces recorded on both will be the same.

balanced forces

When forces are balanced

The forces acting on the central hooks cancel each other out – they are equal in magnitude and opposite in direction.

Because the forces are balanced the hooks do not move.


11 N 10 N

What happens if the pull on one end is greater than the pull on the other end?

unbalanced forces

movement

When forces are unbalanced

The forces acting on the hooks are no longer balanced. Both hooks will start to move to the left – their speed will change. This is called acceleration.

Unbalanced forces lead to a change in speed or direction.


In which direction will these stationary objects move?

Balanced or unbalanced forces?



Contents

Feel the force

Friction

Forces in water

Weight and mass

Summary activities


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?

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.

What is friction?

It is called friction.


Friction always tries to slow moving objects down. We say it opposes motion.

There is a maximum value for the frictional force which depends on:

More about friction

Friction is created whenever two touching objects or surfaces move past each other.

Friction also occurs when things move through air. This is called air resistance or drag.

the force pushing the two surfaces together;

the state of the surfaces in contact.


One more? Probably the most important…

Label all sources of friction that can act on this bike.

tyre and road

brake pad and rim

wheel bearingwheel

bearing

pedal bearing

links in chain

air resistance or “drag”

What are the sources of friction?


Effects of frictional forces



Contents

Feel the force

Friction

Forces in water

Weight and mass

Summary activities


upthrust

frictionthrust

weight

What is thrust? The force that pushes an object forward.

What is upthrust? The force that pushes an object upwards in a liquid or a gas, also called buoyancy.

Forces in water


upthrust or buoyancy

thrust

air resistance

friction

weight

Label the forces acting on the ferry as it sails to France.

Ferry forces


What happens to the weight of the ship?

What happens to the upthrust on the ship?

Which is the largest force - weight or upthrust?

increases

both equal

increases

The ferry arrives at Calais and is loaded with travellers.

Changing forces



Contents

Feel the force

Friction

Forces in water

Weight and mass

Summary activities


Mass and weight are not the same!

What are mass and weight?

Mass is the amount of matter in an object and is measured in kilograms. Mass is not a force.

Weight is a force and is caused by the pull of gravity acting on a mass . Like other forces, weight is measured in newtons and has both magnitude and direction.

Mass will have the same value anywhere in the Universe, including space.

Weight has different values depending on where you are in the Universe.


Weight and mass on the Moon

The force of gravity is less on the Moon than it is on Earth. This is because the Moon has a smaller mass than the Earth.

However, the astronaut still has the same body and the same mass: he just weighs less because gravity is weaker on the Moon.

Any object will weigh less on the Moon than it does on Earth.

An astronaut could jump 20 feet into the air on the Moon because gravity is less.


A scientist should never say:

“She weighs 50 kilograms,”

but should always say:

Talking about weight and mass

“She has a mass of 50 kilograms”.

“Her weight is about 500 newtons”.

Alternatively, the scientist could say:

“The gravitational force acting on her mass is about 500 newtons”.

This is the same as saying:



Contents

Feel the force

Friction

Forces in water

Weight and mass

Summary activities


Glossaryair resistance – A frictional force that acts against an

object moving through air.force – A push or a pull, measured in newtons (N).friction – A force that occurs between two surfaces

rubbing against each other.gravity – The force of attraction between two objects.mass – The amount of matter that an object is made of,

measured in kilograms (kg).thrust – A force that pushes an object forwards.upthrust – A force that pushes an object upwards.weight – The force caused by gravity acting on an object,

measured in newtons (N).


Anagrams


Multiple-choice quiz

KS3 Physics

Documents

Transcript of KS3 Physics