When a skateboarder performs an ollie, there is an upward force (the long blue arrow) created by the ground when the board strikes against it, and a downward force (the black arrow) exerted by gravity.

Simply put, a force will always produce an acceleration unless the force is completely opposed by another force or forces. For example, an apple that falls from a tree branch feels only the force of gravity. Gravity is unopposed by any other force; therefore, the apple accelerates toward the ground (as Newton presumably discovered.)

A book sitting on a table also feels a downward-acting force of gravity, but does not accelerate. Why not? Because the force of gravity is opposed by an upward-acting normal force exerted by the table. A force that is completely opposed, and therefore produces no acceleration, is known as a BALANCED FORCE. A force that is not completely opposed (as with Newton's apple) is known as an UNBALANCED FORCE.

The arrow starts from the object and points in the direction of the force. The size of the arrow indicates the force's magnitude, with larger arrows implying stronger forces. Some people use longer arrows for stronger forces, and some people use fatter arrows (either the whole arrow or just the head).

When constructing an FBD, think about all the forces acting on the object. Is gravity pulling on it? Is the normal force pushing it? What about friction? Are any "applied forces" pulling or pushing the object? It takes a little practice to become proficient in drawing free-body diagrams, but once mastered, FBD's are a big help in calculating force.

Word to the WiseAn object will always accelerate if there is an unbalanced force is acting on it.

Example: Is an unbalanced force acting on this car? Assume the trail is made of oil drops leaking from the car at a constant rate of 1 drop per second.

Is there an unbalanced force acting on this car?

Physicists define stress as force per unit area. The rest of humanity defines stress as physics.

Consider a yacht being towed by four boats, each tugging with a force F. Which direction is the acceleration? If the towing boats are all moving in the same direction, the acceleration is also in that direction. Is it possible to produce the same acceleration in the yacht with only one force (equal to 4F)?

The arrows indicate the forces exerted on a plane as it tows a glider.

Intuition (or experimentation) indicates that one big towing boat tugging with a force 4 * F would produce the same acceleration. This single force represents the combined, or net, effect of all four forces acting on the yacht and is therefore called the NET FORCE. In this example, the net force is equal to the sum of the four towing forces.

The net forces acting upon a ship are the same forces that act upon any other object.

What if each of the smaller towing boats pulled in different directions — one due north, the second due east, the third due south, and the fourth due west? What would be the net force then? In that case, both the north-south pair of forces and the east-west pair would be balanced (i.e., would cancel each other), and the yacht would feel zero net force.

The net force on an object is the vector sum of all the forces acting on the object.

Vector addition will be discussed in greater detail in the section on two-dimensional motion. For now, just remember that a nonzero net force always produces an acceleration in the direction of the force. Zero net force never produces acceleration. Armed with this new understanding of forces, we're ready to embark on the journey that made Newton famous and launched humans into space. But do try another example first.