The following Activities are for the Box-on-Table applet. Make sure you know how the applet functions by consulting Help, Assumptions, and ShowMe under Applet Help on the applet's Help menu.

Activity 1. The purpose of Activity 1 is to learn the proper way of describing forces verbally and to place forces in free-body diagrams.

The term "force" in physics is related to the interaction between two objects. A phrase like "a moving truck has a lot of force" has no meaning in physics because there is no force inherent in a truck no matter how fast the truck may be moving. It does have meaning to talk about the "force exerted by the truck on a tree" if the truck collides with the tree. This phrase refers to two objects, the truck and the tree. In a collision, the truck and the tree are interacting with each other and can exert forces on each other.

The proper way to refer to a force is by saying "the force exerted by ... on ... ", where the dots stand for the two objects that are interacting. The word "force" may be modified by an adjective, e.g., "gravitational" or "normal", to further specify which force one is talking about.

Exercise 1. RESET the applet. Make sure the applet is in the Force mode.

In your Notebook, make a drawing of the box-table-earth system. Also, start making free-body diagrams for the box, the table, and the earth by drawing a dot for each of these elements of the system, labeling the dots "Box", "Table", and "Earth" as in the applet.

Both in your drawing of the box-table-earth system and in the appropriate free-body diagram, draw the arrow representing the gravitational force exerted by the earth on the box. Keep in mind that gravitational forces acting on an extended object like the box effectively act at the center of gravity of the object. Thus, the tail end of a gravitational force vector should be placed at the center of gravity of the object on which the force is acting, the box in this case.

Use the Force drop-down menus to select the gravitational force exerted by the earth on the box.

SUBMIT your force selection, and compare the applet's response with what you have drawn.

Exercise 2. Continuing from Exercise 1, one of the two force vectors included in the applet's free-body diagram for the box, the gravitational force vector, is shown selected in green while the other vector is unselected and shown in grey. Select the unselected grey vector via the Force drop-down menus, and add this other force vector to your drawing of the box-table-earth system.

SUBMIT your selection, and compare the applet's response. Does the applet select the other vector in the free-body diagram of the box? If so, you have correctly identified this force by its verbal description. If not, change your selection and SUBMIT again.

Also, does the applet draw the vector in the image of the system the way you have done in your drawing?

Exercise 3. Repeat Exercises 1 and 2 for the forces acting on the table and the earth.

Activity 2. The purpose of this Activity is to provide further practice with free-body diagrams and with the verbal description of forces. This time, the applet will randomly select a force and you are expected to identify it.

Exercise 1. RESET the applet. Select the Force Review mode.

The applet selects a force either by selecting a force vector in one of the three free-body diagrams or by selecting a verbal description of the force via the Force drop-down menus.

Respond by selecting the force by the "other" method, the one not chosen by the applet, and SUBMIT your response.

If the applet responds with "Correct" in the Answer Quality text field, click NEXT and continue with another force. Otherwise, change your response and SUBMIT again.

Exercise 2. Continue in this manner until the applet has chosen all forces indicated in the three free-body diagrams.

The applet will go through the eight forces indicated in the free-body diagrams in random order until all eight forces have been selected.