A free body diagram models the forces acting on an object. The object or 'body' is usually shown as a box or a dot. The forces are shown as thin arrows pointing away from the centre of the box or dot.
Free body diagrams do not need to be drawn to scale but it can sometimes be useful if they are. It is important to label each arrow to show the magnitude of the force it represents. The type of force involved may also be shown.
Drawing of situation
Free body diagram