Forces make things happen! To squeeze toothpaste from a tube you apply a force to the tube to squeeze it, changing its shape. This pushes toothpaste out of the end of the tube.
A force is a push or pull that is caused when one object interacts with another. The two objects do not necessarily need to touch for a force to take effect.
They are always measured in Newtons (N)
If two objects must touch to interact, for example you pushing a domino over to start a domino rally, the forces are called contact forces. The contact may need to be continuous (for example you pushing a block along the floor), or momentary which is known as an impulse force (for example you kicking a football).
If two objects do not need to touch to interact, for example two magnets, the forces are called non-contact forces.
|Contact Forces||Non-Contact Forces|
|Air Resistance||Electrostatic Force|
|Applied Force (eg Push)||Gravitational Force|
Forces often occur in pairs - you might be asked to analyse which forces are acting given a particular scenario. For example you might be asked to state what forces are demonstrated by a rubber duck floating.
The water is exerting an upward force on the duck (upthrust)
The weight of the duck acts downwards (weight)
These two forces are called balanced forces because they are equal and opposite and nothing is changing. By this we mean they have the same magnitude but in exactly the opposite direction, and neither object is changing position.
Remember that although when two objects interact they exert equal and opposite forces on each other, the mass of the objects will determine the effect that occurs. For example if a hammer hits a nail with 50N of force downwards, the nail also hits the hammer with 50N of force upwards. Because the mass of the hammer is so much bigger than that of the nail, the forces have the effect of moving the nail downwards. It is important that you do not confuse forces and effects!