Goal: Classify forces and hone the concept of contact force.
A person throws a ball straight up in the air. The ball rises to a
maximum height and falls back down so that the person catches it. What
forces are being exerted on the ball when it is half way to the maximum
height? Ignore air resistance.
- The gravitational force only.
- The force of the person’s hand only.
- Both the gravitational force and the force of the person’s hand.
- The gravitational force, the force of the person’s hand, and a third
- None of the above
- Cannot be determined
(1); nothing is in contact with the ball (we ignore forces due to the
air), and so the earth’s gravitational force is the only
“action-at-a-distance” force present.
It is common for students to think that motion requires a force; in some
cases this misconception is more specific, namely, that motion requires
a force in the direction of motion. For this assessment item, the
misconception manifests itself in the belief that there is a “force of
the hand” that propels the ball up during flight.
Questions to Reveal Student Reasoning
Ask students to state what forces are acting on the ball and what object
exerts each force.
How do you know when a force is being exerted by one object on another?
Do the sizes of the forces change? Do the directions of the forces
change? Describe how and why.
Do you have any control over the force of the hand on the ball while the
ball is in the air? Can you make it larger or smaller or change its
direction once you release the ball?
Have students brainstorm situations in which two objects interact
without touching each other. Use as the criteria for an interaction
that an object move or change shape. Have students divide their
situations into two groups: those for which the objects interact
directly, and those for which the objects interact through some other
object (e.g., two blocks “interact” through a spring placed between
Eventually raise the point that in physics the term force refers to
direct interactions only and that most objects interact only when placed
in contact. Demonstrate electric and magnetic forces as examples of