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KS3 Physics. 7K Forces and their Effects. Contents. 7K Forces and their Effects. Feel the force. Friction. Forces in water. Weight and mass. Summary activities. What is a force? .

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ks3 physics

KS3 Physics

7K Forces and

their Effects

contents
Contents

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

what is a force
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.

when forces are balanced
When forces are balanced

10 N

10 N

balanced forces

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

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

Because theforces arebalancedthe hooksdo not move.

when forces are unbalanced
When forces are unbalanced

10 N

11 N

unbalanced forces

movement

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

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 calledacceleration.

Unbalanced forces lead to a change in speed or direction.

balanced or unbalanced forces
Balanced or unbalanced forces?

In which direction will these stationary objects move?

contents8
Contents

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

what is friction
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 called friction.

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.

more about friction
More about friction

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

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

Friction also occurs when things move through air. This is calledair resistanceordrag.

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

  • the force pushing the two surfaces together;
  • the state of the surfaces in contact.
what are the sources of friction
What are the sources of friction?

brake pad and rim

pedal bearing

wheel bearing

wheel bearing

links in chain

tyre and road

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

air resistance or “drag”

One more? Probably the most important…

contents13
Contents

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

forces in water
Forces in water

What isthrust?

The force that pushes an object forward.

What isupthrust?

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

upthrust

friction

thrust

weight

ferry forces
Ferry forces

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

upthrust or buoyancy

air resistance

thrust

friction

weight

changing forces
Changing forces

The ferry arrives at Calais and is loaded with travellers.

increases

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

contents17
Contents

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

what are mass and weight
What are mass and weight?

Massis theamount of matterin an object and is measured inkilograms. Mass is not a force.

Weightis aforceand is caused by the pull of gravity acting on a mass. Like other forces, weight is measured innewtonsand has both magnitude and direction.

Mass and weight are not the same!

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
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.

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.

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

talking about weight and mass
Talking about weight and mass

A scientist should never say:

“She weighs 50 kilograms,”

but should always say:

“She has a mass of 50 kilograms”.

Alternatively, the scientist could say:

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

This is the same as saying:

“Her weight is about 500 newtons”.

contents21
Contents

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

glossary
Glossary
  • air 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 forceof 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 objectupwards.
  • weight –The force caused by gravity acting on an object,measured in newtons (N).
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