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)

Using a force meter to measure weight

Using 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.
Most objects have TWO forces on them.
If these two forces are the same the boject 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 spoeed 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 iir resistance then the rocket speeds up.

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.

eg 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 = 20NOn 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.

Forces will usually work in pairs.
eg. A weight hanging from a spring: gravity is acting on the weight pulling it DOWNWARDS. The spring is acting on the weight pulling it UPWARDS. These two forces will be equal to each other and cancel each other out.

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  
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.

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).

Measuring the friction between a trainer and the floor.
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 wiegh the same.

 

LEVERS and PULLEYS can be used to change the magnitude (strength) and direction of a force.

A crow bar is an example of a lever.The effort is less than the load because the load is NEAR to the fulcrum and the effort is a long way from the fulcrumNotice how the direction of the force can be represented by an arrow.

Other examples for the application of levers are: pliers, scissors, wheel barrow. These are all examples of simple MACHINES.

The Lever law
(The law of moments) (Level 2 only)

Imagine a ruler pivoted at the centre
If the ruler is balanced the lever law states that:

The force on the left x its distance from the pivot = the force on the right x its distance from the pivot
ie 2 x 6 = 3 x 4

The unit of a moment is a newton metre (or newton centimetre)

Springs

If some weights are hung from a spring the spring will stretch. The amount the spring stretches is called the extension. There will be two forces on the spring: R = the ceiling pulling on the spring. This direction of this force is up. W= the weights pulling on the spring. This force is caused by gravity. The direction of this force is down.

Experiment to stretch a spring

A spring is clamped near a metre rule. The position of the unstretched spring is noted. 1N weights are added to the spring, one at a time, and the total extension for the new load is recorded. A graph is drawn plotting extension against load.

It is noticed that for small loads the extension of the spring is proportional to the load. During this time the spring is obeying Hooke’s Law, and the line on the graph is straight. Beyond a certain load the spring acquires a permanent stretch. This load is called the elastic limit of the spring. At this point the line on the graph starts to curve as the extension gets longer.

Springs in series

Two springs joined end to end (in series) will have twice as much extension as a single spring. Eg if one spring stretches 3cm with a load of 1N then two springs in series will stretch 6 cm. This is because each of the springs stretch 3cm making 6cm all together. If each spring were 10cm long to start (with no load) the total length would be 10cm + 10cm + 6cm = 26cm

Springs in parallel

Two springs joined side by side (in parallel) will have half the extension of a single spring.eg if one spring stretches 3cm with a load of 1N then two springs in parallel will stretch 1.5cm. This is because the load is shared out between the two springs, so each spring is only receiving 0.5N. Half the load means half the stretch.