A net force will cause a change in motion of an object. This is obvious in the case of contact forces; a push or a pull can make an object speed up, slow down or turn through an angle. Sometimes though the motion of an object can change and there is no obvious contact with anything else to cause this change. This is when the concept of a force field becomes useful. Objects that have mass are attracted to each other, e.g. the ball and the planet Earth. Objects that have charge are either attracted to or repelled by each other e.g. plastic brush and hair as you brush your hair. Magnetic objects will try and orientate themselves so they are attracted to each other e.g. fridge magnets. In each case one object can be considered to be producing a field or a volume around it extending from the surface to infinity. If another object with similar characteristics, mass, charge or magnetism, enters this field it will experience a force, the strength of which is in some way related to its distance from the surface of the object producing the field. There can be more than one field in a space at any one time e.g. fields due to mass and charge. The field can come from more than one object and it is then the sum total of all of the fields of that type at each point in space. It is only when an object is placed at any particular point in space that the net force on that object can be calculated because the force the object experiences also depends upon its own mass / charge / magnetic properties as well as the properties of the force field. This video introduces the idea of force fields, and can be used to start the discussion about them with students, but it is not enough on its own to help students fully understand the concept.