Also on this page:   Mass and weight     Friction    

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A force is any kind of PUSH or PULL.
The unit of force is the newton (N).

A force can be measured using an instrument called a force meter (sometimes called a newton meter or spring balance)

force_meterUsing a force meter to measure weight
force_meter2Using a force meter to measure friction






A force will bring about one of the following changes

  • i. Make an object move faster (accelerate)

  • ii. Make a moving object slow down

  • iii.Make a moving object change direction

  • iv. Make an object change shape. eg cause a spring to stretch

Examples of different kinds of force:

  • Magnetism

  • Gravity (which causes an object to have weight)

  • Electrostatic (static charges cause objects to attract or repel each other)

  • Friction which will try and make any moving object to slow down.


Forces usually work in pairs
When a ball is in water the water pushes up on the ball helping it float.
This upward force is called upthrust.

Upthrust pushes UP.
Gravity pulls DOWN.

If the ball is floating then these two forces are  the same strength.upthrust

Balanced and unbalanced forces

Most objects have TWO forces on them.
If these two forces are the same the object will not move
(Or will stay moving at a steady speed)
A book on the table does not fall to the ground because the table pushes up

A man falling with a parachute has two forces on him
Gravity pulls him down. Air resistance pushes up.
If the man is falling at a steady speed then these two forces must be the same.


A rocket balloon has two forces on it.
The air coming out of the back pushes the balloon forwards.
Air resistance slows the balloon down.
if the thrust pushing forwards is larger than the air resistance then the rocket speeds up.

Click on the image below to see a simulation of forces in action

Mass and weight
The mass of an object will never change. Mass is measured in kilograms

Weight depends on the pull of gravity. If the pull of gravity increases or decreases then so will the weight. The Earth’s gravity pulls with a force of 10 N on every 1 kg.

A block of butter has a mass of 2 kg. on the Earth.
What is its weight? 
The pull of the Earth’s gravity is 10N/kg
so the weight of the butter is 2 x 10 which = 20N
On the Moon (where the gravity is less than the Earth the mass will still be 2 kg but it’s weight will be LESS than before.

Lunar Lander game .
The thrust from the motor balances the pull of Gravity. Click on the Image below

Lunar Lander Game

A moving car
The car will remain stationary until a force P pushes it forwards.

If the car is moving at a steady speed then the force P will equal the force F.

The force W (the weight of the car) caused by gravity, must be balanced by the road pushing upwards supporting the car.



Friction is a force which tries to slow down a moving object.

It always pushes in the OPPOSITE direction to the direction of movement Friction can be lowered several ways (eg when sliding a wooden block across the table:

Lubricating the surface between the block and the table. Placing bearings (rollers) between the block and the table.

Changing the surface of the block (eg coating with PTFE or nylon) Making the block smoother.

Friction causes the release of heat energy when two surfaces are rubbed together
Click on the image above to see a friction simulation

Disadvantages of friction: overheating in bearings, increase fuel consumption in cars, lowers the top speed of cars, bicyclists or skiers, tries to slow down any moving object, overheating in objects moving through the air at high speed.

Advantages of friction: Allows cars, bicycles, etc to speed up, slow down or change direction. Without friction we would not be able to walk, run, stop, or change direction. Friction helps to stop things (eg furniture) from sliding around. Air resistance can be used to slow down a fast moving object (eg using a parachute).

Investigation to discover if the more expensive trainer has a better grip
Measuring the friction between a trainer and the bench.

Prediction: The more expensive brand will perform better

Method: The trainer is being pulled by the spring balance.

We can repeat the experiment using different trainers but to keep it a fair test each trainer must be loaded with weights so that they all weigh the same. The trainers should also all be the same shoe size

Independent variable (what we are testing): The make of trainer (ie the material in contact with the bench)

Dependent variable (what we are measuring): The force (in newtons) required to pull the trainer at a steady speed across the bench

Other variables (to be kept the same for each experiment): Size and mass of  trainer



Type of shoe Force required to
pull the trainer (N)
A 5 £25
B 8 £70
C 7 £120
D 4 £40


Bar Graph

Force (N)
  A B C D E

Type of shoe

The trainer that has the best grip is trainer B.
our prediction was that this would be the most expensive trainer, but we see that this is not the case so our prediction was wrong.

Of course there may be other factors as well that make the most expensive trainer better such as comfort and durability but they would need to be investigated in separate experiments.

Thinking, braking and stopping distances


When driving sometimes the driver to brake very rapidly (eg if a dog runs out in front of the car).

The distance it takes to stop depends on two factors:

  • Thinking distance – This is the distance the car travels between spotting the hazard and actually applying the brakes. It is caused by time it takes the driver to react to the situation and realise that he needs to stop.
  • Braking distance – The braking distance is the distance you travel between applying the brakes and actually coming to a stop.
  • Stopping distance = thinking distance + braking distance


Factors that might increase stopping distance:

Thinking distance can be increased by:

  • feeling tired
  • alcohol and drugs
  • driving faster
  • getting distracted


Braking distance can be increased by:

  • Driving faster
  • Roads being icy or wet
  • something wrong with the car (eg bald tyres or poor brakes)


NOTE: You do not need to know what the actual stopping distances are, just what might affect them.

These distances may double in wet conditions