1. FORCES & THEIR EFFECTS In This Topic We Will Learn About:
Pushes and pulls.
The different types of forces.
What is mass?
Friction.
Streamlining and reducing friction forces.
Gravity and weight.
Upthrust.

2. FORCES AND THEIR EFFECTS
A force is something that changes the way an object moves or changes the shape of an object.
On a very simple level, forces are pushes and pulls.
Forces can:
Speed things up …… including making things start to move.
Slow things down….. including making things stop.
Compress (squash) things.
Extend (stretch) things.
Twist things.

3. THE NAMES OF FORCES These are forces: Gravity. Static electricity.
Friction.
Magnetism.
Weight.
Upthrust.
Mass is not a force.
Mass is how much ‘stuff’
an object contains.
Mass is measured in
kilograms (kg).

4. FRICTION
Friction occurs when objects are moving.
Friction is a force that always slows things down.
We sometimes use friction to make things stop or slow down.
Give an example of using friction to slow or stop something.
Brakes
Sometimes friction is a nuisance because it slows things down when we don’t want them to.
Give an example of friction being a nuisance in this way.
Having to use more fuel to keep cars moving at a steady speed because friction with the road is trying to slow them down.

5. FRICTION AS A FORCE
The amount of friction between two objects depends on:
How heavy the objects are.
The area of contact.
The type of surface the objects have.
Complete these sentences:
The heavier the object, the……………………….. the friction is.
The bigger the surface area in contact the …………………. the friction is.
The rougher the surfaces are the…………………the friction is.

6. FRICTION AS A FORCE answers
The heavier the object, the greater the friction is.
The bigger the surface area in contact the greater the friction is.
The rougher the surfaces are the greater the friction is.
The amount of friction needed with the road matters in racing cars.
The tyres are made to have different amounts of friction on wet and dry surfaces.

7. REDUCING FRICTION Lubricants can be used to reduce friction.
Lubricants are often liquids.
This is because they work by smoothing out the bumps in rough surfaces.
Block of wood or metal.
Lubricant
Oil is used to lubricate the parts in a car engine.

8. AIR RESISTANCE Give an example of air resistance being useful.
The air also helps to slow down a moving object.
This force is called air resistance.
Air resistance is sometimes called drag.
Air resistance is affected by
Speed of motion
Area of contact
Type of surface
Shape of the object.
Give an example of air resistance being useful.
Parachutes.
Give an example of air resistance being a problem.
Racing cars.

9. Which is more streamlined-
STREAMLINING
There is also drag when an object moves through water or any other fluid (liquid).
The shape of the moving object affects how much drag there is.
Streamlined objects have very little drag when they move through water or air.
This means that they can move faster because drag is slowing them down less.
Which is more streamlined-
the fish or the child?

10. GRAVITY
Gravity is a force that attracts objects towards the centre of the Earth.
Your weight is a measure of how hard gravity is pulling you down.
Weight is a force and is measured in Newtons (N).
Weight is caused by the force of gravity acting on a mass.
A one kilogram mass weighs 10 Newtons.
Gravity can only attract objects.
The Earth’s gravity attracts everything.

11. ACCELERATION DUE TO GRAVITY
Gravity makes all objects accelerate (get faster) as they fall towards the ground.
The effect of gravity makes all objects accelerate at the same rate.
If there is no air resistance, all objects will get 10m/s faster every second that they fall.
Air resistance will stop the speed increasing as rapidly.
The greater the air resistance, the longer the object will take to hit the ground.
The effect of the air resistance depends on how streamlined the object is.
The way a parachute works can be explained by looking at gravity and air resistance together.

12. BALANCING FORCES
Gravity (his weight)
pulls him down.
Air resistance works
against gravity.
When the parachutist jumps out of the plane, gravity will accelerate him towards the Earth.
The large surface area of the parachute gives a lot of air resistance.
This air resistance works against gravity.
This will stop the parachutist accelerating so quickly.
This will also mean that he takes longer to reach the ground.
His speed when he hits the ground will be a lot less than if he had fallen without the parachute.
How much he is slowed down and how fast his final speed will be, depends on how much air resistance there is.

13. FALLING AT A STEADY SPEED
Question:
What will happen if the air resistance force equals the weight of the parachutist ?
Gravity (his weight)
pulls him down.
Air resistance works
against gravity.
The parachutist will fall at a constant (steady) speed.
This is because the two forces are balanced.
This means that they cancel each other out.
If there is no force, an object will not change the way it moves.
So the parachutist will keep falling at the same speed – he will not accelerate.

14. SPEEDING UP AND SLOWING DOWN
Balancing forces:
What will happen to each of the cars shown?
The size of each arrow represents the size of the force.

15. SPEEDING UP AND SLOWING DOWN ANSWERS
This car will slow down, because there is a bigger force in the direction opposite to the one it’s moving in.
This car will stay at the same speed, because the forces on it are balanced.
This car will speed up, because there’s a bigger force in the direction it’s moving in.

16. UPTHRUST Weight Upthrust
Upthrust is a force that liquids exert on floating objects.
Objects appear to weigh less in water than they do in air.
This is because the upthrust cancels out some of the weight of the object.
Weight
Upthrust
This is why heavy boats are able to float in water.
In air there is almost no upthrust to help cancel out the weight.
Much of the weight is
cancelled out by the
upthrust.

17. SPEED Speed = distance time
The speed of an object tells you how far it will travel in a given time.
For example, if an athlete is running at a speed of 10 metres per second (10m/s), it means that he can run 10 metres in one second.
You can calculate the speed of a moving object by using this equation:
Speed = distance
time
distance
speed
time

18. SPEED
What is the speed of a car that travels 300 metres in 20 seconds?
Speed = 300 = 15 m/s 20
Don’t forget to show your calculations and
put the correct units in your answer.
distance
speed
time

19. Check your learning How many different types of force can you name?
Are mass and weight the same thing?
What can you do to reduce friction?
Which has less air resistance; a bird or a child?
What is the name of the force that allows ships to float?
How do you find out the speed of an object?

20. Check your learning - answers
Gravity. Static electricity. Friction. Magnetism. Weight. Upthrust.
Mass and weight are different. Mass is the amount of substance that is there. Weight is the force of the mass due to gravity.
You can streamline an object to reduce friction. You could use oil or a lubricant on the surface to smooth it out.
A bird would have less air resistance as it is streamlined to move through the air.
The name of the force that allows ships to float is UPTHRUST.
To find out speed you divide the distance moved by the time taken.