Scientists describe force as a push or a pull. Pushes and pulls can get things to move. They can also speed up or stop a moving object and change its shape and the direction in which it is moving. Forces cannot be seen and sometimes cannot be felt. Consequently, it is not surprising that many children and adults do not think about forces. Practical experiences that help children to become familiar with the terms 'pushing', 'pulling', 'floating' and 'sinking' would be a useful starting point for work on forces. These experiences will provide the basis for the work in the middle and senior classes, when children will develop an understanding of forces and their effects. At all stages it will be important that children can relate their science investigations to everyday experiences; they will be aware that force is applied when they pull the top from a pen, open and close doors, roll a marble or push a supermarket trolley.
Safety
Care should be taken during work on forces, because of the risk of injury resulting from moving objects.
Exemplar 30 illustrates an approach to the teaching of the topic of floating and sinking in infant and junior classes. Suggestions are offered for the organisation of the lesson and for the employment of strategies for the development of scientific skills.
Exemplar 31 describes how a unit of work on pushes and pulls can be structured for infant classes.
EXEMPLAR 30 - Floating and sinking (infant and junior classes)
EXEMPLAR 31 - Approaches to learning about forces (infant and junior classes)
Friction
Push a toy car or roll a marble across a table and observe as it slows down and comes to rest. The car stops because there is a force acting on it. Friction is the force that opposes the movement of an object. There are several forces acting on the moving car:
- the forward force from the push that was given to the car. This causes the wheels of the car to turn. The wheels then push on the surface of the table
- the force of friction between the tyres and the surface
- air resistance.
Objects that move through the air experience the frictional force of the air, which acts in the opposite direction to their movement. Air resistance acts against gravity on falling objects. As the speed of the falling object increases, the air resistance increases.
Exemplar 32 illustrates some approaches to exploring friction with children in third and fourth classes. This activity is also appropriate for senior classes.
Gravity
All objects attract one another. The force of attraction which an object exerts is in proportion to its mass. The Earth has a large mass, and so the force of attraction between it and other objects is big, and this force pulls objects to the Earth. This force is called the 'weight' of an object. The weight of an object is a measure of how much the Earth pulls on it.
When you lift a bucket two forces are at work. These are the upward force or pull exerted by you and the downward pull of gravity.
Falling objects
Two objects, such as a pebble and a block, released from the same height above ground will fall at the same rate and reach the ground at the same time. If air resistance is removed all objects fall to Earth at the same rate. Children will find this concept difficult to test, as air resistance cannot be removed. They will discover that a pen and a feather will fall at different rates, as the surface area of the feather is much greater than that of the pen; the feather is therefore affected to a much greater extent by air resistance.
Rolling down a slope
Gravity makes it more difficult to force things to move uphill and easier to force them downhill. Children will find that carrying, sliding and rolling are easier downhill. A slope is a simple machine that diverts part of the force sideways. The extent to which the slope diverts gravity sideways depends on its angle, so that objects take longer to roll down a shallower slope. Children can compare sliding a brick using a rubber band along a level surface, measuring the extent to which the rubber band is stretched. Tilt the surface and force the load to slide uphill. Measurements of the extent to which the rubber band is stretched are taken. Observe and compare the measurements of the force required to slide the load downhill.
Exemplar 33, through focusing on the role of wheels and rollers in movement, provides opportunities for children to investigate slopes and the effects of movement downhill and uphill.
Levers
A lever is a simple machine that turns on a pivot. Exemplar 34 describes how children might begin to develop a simple understanding of levers in the senior classes.
Wheels, axles, belts and chains
Wheel belt systems can be made from thread spools and rubber bands. Children will find that
- an anti-clockwise movement of one wheel creates a clockwise movement of the other
- there is a correlation between the number of turns the small wheel makes and the size of the large wheel.
EXEMPLAR 32 - Friction (third and fourth classes)
EXEMPLAR 33 - A unit of work on machines -- wheels, gears, chains and belts (third to sixth classes)
EXEMPLAR 34 - Levers (fifth and sixth classes)