In these diagrams we will call the force due to gravity the gravitational force. We know that the acceleration due to gravity (if on Earth) is approximately g = 9.8m/s2 . The force of gravity, by Newton's Second Law is the product of the object’s mass and the acceleration due to gravity (F=ma). 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. 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. You may see this force called the “push or pull” on the body. 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 image, the wind pushing on the ship. 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. Which are the forces we can show in a free-body diagram? *
