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.
How to draw a free body diagram
Lets take the example of our rugby player. He plays fly half and has just taken position of the ball from a stationary position and instead of passing it out to his team mates he has decided to go for glory himself. Lets follow these simple steps to identify the forces acting on him:
1. Draw on the weight force (acceleration due to gravity x his mass). Weight will always be acting and will always point straight down from the centre of mass, which for a rugby player is somewhere near his tummy.
2. Next the reaction forces where two bodies are in contact. In our example the player is in contact with the gorund (we are just considering the player for now, not the ball as well). As both feet appear to be in contact with the ground we draw vertical arrows for both feet.
3. Now lets think about the horizontal forces. Friction will be acting sideways on the feet to stop the player slipping over.
4. Finally air resistance can be added. However, for our example it is a still day with no wind and our player is not yet moving fast enough for wind resistance to be relevant.