 Science revision for 11 - 14 year olds |
PHYSICS |
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| Forces |
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Mass and weight Friction Levers Springs
A force is any kind of PUSH or PULL.
The unit of force is the
newton (N).
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.
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 = 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. |
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.
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A moving car 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. |
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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 fulcrum Notice how the direction of the force can be represented by an arrow. |
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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)
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
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).
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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.
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Experiment to stretch a spring
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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.
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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.
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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
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| 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.
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