Free Body Diagrams
A free body diagram illustrates all the forces acting on a particular object and leaves us in a position to apply Newtons second law –– easily.
Often the forces will be balanced so the forces vertically and horizontal are equal, Generally this will mean that the velocity in any direction where the net force is zero will be conat. Often though, especially where the forces are resolved perpendicular to a surface, this constant velocity will be zero.
Example: When a parachutist fall from a plane, the air resistance is initially zero. As the vertical speed of the parachutist downwards starts to increase, the air resistance increases. The Force of air resistance increases, while the force of gravity is constant. Eventually the forces balance and the vertical acceleration decreases to zero:Terminal velocity is reached. At any instant before terminal velocity is reached (right below) the application ofproduces the equationwhereis the force of air resistance.
Example: For a book resting on a table, the force of gravity (W=mg) equals the reaction force.