The Basic Idea

A free-body diagram displays the type, the relative magnitude, and the direction of all forces that act upon an object in any given situation. An arrow is drawn for each force in the direction of the force. The arrow is labeled with a force label that indicates the type of force. And finally, the length of the arrow is proportional to the strength of the force.

There are two versions of this question. Both versions involve a bundle of hay attached to a rope and pulled at a constant speed across dirt. One of the versions is shown below.

Version 1:
A bundle of hay, attached to a truck by a rope, is moved across the dirt to the right at a constant speed.

You have some decisions to make. A physical situation is described and one of the five free-body diagrams describes all the forces acting upon the object. You need to decide what forces are present, which direction they act, and whether the opposing forces are of equal or unequal strength. The listing below describes the most common force types. First, use the informatoin and the described situation to determine the types of forces present and their direction. Then toggle through the five choices to identify the one that matches the description.

Force Types

Normal Force: A normal force exists whenever two surfaces are pressed against one another.  Sit on a chair, stand on the floor, lean against the wall ... in all cases your body and another surface are pressing against each other. This pressing together results in a force on your body (and on the object). This is called a normal force.

Gravity Force: Gravity falls into the category of a non-contact force. It is the result of the interaction between two masses. All objects with mass attract. And when the Earth or some other very massive object is present, the gravity force is significant in size. For an object on Earth, you can always count on this force being present.

Applied Force:  This force is the result of person pushing or pulling on an object. A careful reading of the description of the situation will often describe a person "applying a force." This force could be considered a sub-type of the normal force. For our purposes, we will think of the applied force resulting when another person is doing the pushing or pulling on the object.

Friction Force: Friction force results when two touching surfaces are sliding across each other (sliding friction) or attempting to slide across each other (static friction). Read the problem carefully to determine if the object's surface is in contact with another surface. Then determine if the object is moving across the surface; if it is, then there is a sliding friction force. Or determine if the object is stationary on an inclined surface; if it is, then there is a static friction force holding it in place.

Tension Force: Tension is the force that results when a rope, string, cable, or wire is attached to an object and exerting a push or pull on the object. Read the description of the situation carefully and look for words that describe a rope, wire, string, or cable pulling on the object.

Air Resistance Force: Air resistance results when there is relative motion between the object and the surrounding air. This force is caused by the collisions between air particles and the moving object. Whenever an object is moving (and there is air present around it), there will be an air resistance force. But sometimes iit is not included in a diagram because it is so small. It is small for slow-moving and/or streamlined objects.