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Formative assessment questions for teaching physics

A2L Item 048

  • Description: Identify the sources of force on a fly ball.
  • Goal: Hone the concept of force, classify forces as contact and action-at-a-distance.
  • Source: UMPERG
  • Keywords: Forces, Interactions, Mechanics

The question for students:

A baseball is struck by a bat. While the ball is in the air, what objects exert forces on the ball?

  1. Earth
  2. Bat
  3. Air
  4. Bat, Air
  5. Earth, Bat
  6. Earth, Air
  7. Earth, Bat, Air
  8. There are no forces on the ball.
  9. None of the above

Commentary for teachers:

Answer

(6); the earth’s gravitational force (an “action-at-a-distance” force), and air resistance (a contact force) are the only two forces being exerted on the ball while in the air.

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 bat” that propels the ball up during flight.

Questions to Reveal Student Reasoning

Ask students to state what forces are being exerted 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.

Do you have any control over the force of the bat on the ball? Can you make it larger or smaller or change its direction once the ball is flying through the air?

Suggestions

Ask students if they have a way of exerting a force on an object without touching it. Invite them to move an object in the front of the room without leaving their seats and touching the item.

If Newton’s Second Law has been introduced, attempt to relate the forces exerted on the ball to the ball’s acceleration. See if students agree that, if air resistance can be neglected, the ball has a constant acceleration of 9.8 m/s2 toward the earth during its entire trajectory. If they agree ask what they can conclude about the net force on the ball while airborne.