FORCES
Forces are everywhere but they are invisible. A force is a push or a pull. Different things cause pushes and pulls. It could be the muscles in your body, an engine, gravity, magnetism or many other things.
Forces cause objects to move, to stop, to change speed, to change direction or change shape. A stationary object resists moving, so a force is needed to start it moving. The amount of force needed depends on things such as the weight of the object, the material it is made of and the surface on which it is resting. The stronger a force, the stronger its effects.
For every force there is an equal and opposite one. If one force is greater than the other, something moves. If forces balance each other, they stay still.
Friction is a force that acts between to surfaces and tends to prevent movement. The lower the force of friction between two surfaces, the easier movement becomes. The higher the force of friction, the more difficult it is to move. Friction can be useful in, for example, making wheels grip the road. It can sometimes be a nuisance, for example, when you want your sledge to go faster down a hill!
Gravity is the force that pulls objects towards the centre of the earth. The amount of force pulling objects down like this is what we call weight. This force can be measured in units called newtons. 1 newton = 100 grams. 
Magnetism is a force that only works between two magnets or a magnet and certain metals. Magnets can exert force without actually touching an object.