Free-Body Diagrams Worksheet
No doubt you are aware of free body diagrams (otherwise known as FBD's). These are
simplified representations of an object (the body) in a problem, and includes force
vectors acting on the object. This body is free because the diagram will show it without
its surroundings; i.e. the body is 'free' of its environment. This eliminates unnecessary
information which might be given in a problem.
Gravity
The first force we will investigate is that due to gravity, and we'll call it the gravitational
force. We know that the acceleration due to gravity (if on Earth) is approximately g = 9.8
m/s . The force, by Newton's Second Law is
F = m g
Normal
The normal force one which prevents objects from 'falling' into whatever it is they are
sitting upon. It is always perpendicular to the surface with which an object is in contact.
For example, if there is a crate on the floor, then we say that the crate experiences a
normal force by the floor; and because of this force, the crate does not fall into the floor.
The normal force on the crate points upward, perpendicular to the floor.
Friction
Related to the normal force is the frictional force. The two are related because they are
both due to the surface in contact with the body. Whereas the normal force was
perpendicular to the surface, the frictional force is parallel. Furthermore, friction opposes
motion, and so its vector always points away from the direction of movement.
Push and Pull
Another force which may act on an object could be any physical push or pull. This could
be caused by a person pushing a crate on the floor, a child pulling on a wagon, or in the
case of our example, the wind pushing on the ship.
Tension
Tension in an object results if pulling force act on its ends, such as in a rope used to pull a
boulder. If no forces are acting on the rope, say, except at its ends, and the rope itself is in
equilibrium, then the tension is the same throughout the rope.
