- When several forces act on a body, the resultant (overall) force on the body can be found by adding together forces which act in the same direction and subtracting forces which act in opposite directions:
Diagram showing the resultant forces on three different objects
- When the forces acting on a body are balanced (i.e. there is no resultant force), the body will either remain at rest or continue to move in a straight line at a constant speed
When the forces acting on a body are balanced the body will remain at rest or continue to travel at a constant speed in a straight line
- Friction is a force that opposes the motion of an object caused by the contact (rubbing) of two surfaces. It always acts in the opposite direction to the direction in which the object is moving
Friction opposes the motion of an object
- Air resistance (sometimes called drag) is a form of friction caused by a body moving through the air
- Friction (including air resistance) results in energy loss due to the transfer of energy from kinetic to internal (heat)
The resultant force is sometimes also known as the net force or the unbalanced force.
Avoid referring to air resistance as wind resistance or air pressure – these are incorrect terms and will lose you marks if you use them when you actually mean air resistance.