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:
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.
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 = 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.
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.
|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 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)
Experiment to stretch a spring
A spring is clamped near a metre rule.
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.
Springs in series
Springs in parallel