# Forces

A force is a push or a pull which alters the state of motion of a body and is measured in Newtons (N). Here we cover balanced and unbalanced forces, friction, air resistance, impulse, force time graphs and free body diagrams.

A force is a push or a pull which alters the state of motion of a body and is measured in Newtons (N). As explained in Newton's first law of motion a force is required to make a stationary body move, change speed, direction or stop.

Impulse is simply a measure of the force applied for a specific time. Impulse = force x time and has units Ns (Newton seconds). Impulse is important in sport because many techniques, particularly throwing activities require the performer to apply as large a force possible for as long as possible. The area of a force time graph gives the impulse.

Friction forces act against the movement of one surface over another such as tennis shoes on a grass court. Friction is the force which prevents the player slipping and sliding. When air passes over a surface, particularly at speed than a frictional force called air resistance occurs.

Free body diagrams are used to show which forces are acting on a body at a particular instant in time. Arrows indicate the position, direction and size of the force acting. The most likely forces acting on a sports performer are friction, air resistance, weight and reaction forces.