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# KS3 Physics - PowerPoint PPT Presentation

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

7K Forces and

their Effects

7K Forces and their Effects

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.

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.

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.

In which direction will these stationary objects move?

7K Forces and their Effects

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?

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.

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.

pedal bearing

wheel bearing

wheel bearing

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

air resistance or “drag”

One more? Probably the most important…

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

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

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

upthrust or buoyancy

air resistance

thrust

friction

weight

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

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

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.

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.

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”.

7K Forces and their Effects

Feel the force

Friction

Forces in water

Weight and mass

Summary activities

• 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).