You have some decisions to make. A physical situation is described and one of the six 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 information and the described situation to determine the types of forces present and their direction. Second, consider the fact that the net force must be directed inwards. If there is an outward force, then it must be weaker than the inward force. Finally, toggle through the five choices to identify the one that matches the description. If you need more specific information about this situation, read the Some Finer Details section nearer the end of this section of the page.
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 ... and directed downward.
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.
Some Finer Details
This situation describes a rider that is in a coaster chair at the top of the loop and upside down. Normally chair surfaces are below us; we push down on the chair and the chair pushes up on us. But in this situation the rider is upside down, the chair is above the rider. The rider pushes up on the chair and the chair ...
Hopefully you get it from the above narrative. Now be careful about gravity; it does what it always does.