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
If you have never been on such a ride, then it might be more diffitult to answer this question. One of the biggest clues in the description is that Phil is lifted off his seat. The seat below him cannot push upward on him because his is not touching it.
The second clue in the description is that Phil is held in his car by a lap bar. Phil is at the top of a hill. And before reaching that point on a hill, Phill had been moving upward. An upward moving object would continue moving upward (the law of inertia). So without any downward force on Phil, he could simply be launched out of the coaster car while it makes the turn around the hill. But the lap bar pushes down on him so that there is sufficient centripetal (inward) force to make the turn. So study the list of force types and pick the force that the lap bar exerts on Phil; and get its direction correct.