A2L Item 055
- Description: Identify the forces on a ball thrown vertically at a point not its maximum.
- Goal: Classify forces and hone the concept of contact force.
- Source: UMPERG
- Keywords: Forces, Mechanics
The question for students:
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 force.
- None of the above
- Cannot be determined
Commentary for teachers:
Answer
(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.
Background
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?
Suggestions
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 them).
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 “action-at-a-distance” forces.