A2L Item 020
- Description: Applying Newton's Third Law to bowling.
- Goal: Recognizing how the concept of force relates to interactions.
- Source: UMPERG
- Keywords: Forces, Mechanics, Newton's Third Law
The question for students:
A bowling ball rolls down an alley and hits a bowling pin. Which statement below is true about the forces exerted during the impact?
- The bowling pin exerts a larger force on the ball than the ball does on the pin.
- The bowling ball exerts a larger force on the pin than the pin does on the ball.
- The force that they exert on each other is the same size.
- One of the two forces is larger, but which is larger can’t be determined unless more information is provided.
- None of the above.
- Cannot be determined
Commentary for teachers:
Answer
(3); The forces are equal (independent of the masses and motions of the interacting objects), as required by Newton’s Third Law .
Background
In situations where a heavier, moving object collides with a lighter, stationary object, students have a very strong intuition that the heavier, moving object exerts a larger force on the lighter, stationary object. This intuition is based on experiences like the following: when a bowling ball hits a pin, the ball continues to move forward and the pin goes flying off the lane. Students interpret the large change in the pin’s motion as evidence that the ball (which is heavier than the pin) exerts a larger force on the pin than vice versa. Often, when a car and a truck collide, the car suffers much more damage than the truck, and so students interpret this as evidence that the truck exerts a larger force on the car. For background reading on helping students overcome this persistent misconception see Thornton and Sokoloff: Sokoloff, D.R. & Thornton, R.K. (1997), Using interactive lecture demonstrations to create an active learning environment, The Physics Teacher, 27, No. 6, 340; and Thornton, R.K. and Sokoloff, D.R. (1998), Assessing student learning of Newton’s Laws: The force and motion conceptual evaluation and the evaluation of active learning laboratory and lecture curricula, American Journal of Physics, 64, 338-352 (1998).
Questions to Reveal Student Reasoning
Which object, the bowling ball or the bowling pin, has the larger acceleration? How do you know?
Which object experiences the larger net force? How do you know?
Would your answer to the original question change if a moving pin hit a stationary bowling ball?
Suggestions
If you have MBL equipment and force probes, collide a moving cart with a stationary cart of the same mass. Ask students to compare the forces exerted on the two carts. Ask students to compare the velocities and accelerations of the two carts. Repeat using different initial conditions.
Draw a picture of a large moving cart colliding with a small stationary cart. Draw a spring between the carts. Ask students how they would determine the force on each cart given the spring constant and spring compression.
Take a bathroom scale, place it between two students (a large strong student and a slight student) and have them push as hard as they can from either end without making the scale accelerate–observe the scale reading. Repeat with the scale reversed. Ask if there is much difference in the scale reading depending on which way the front of the scale is facing. What does this imply about the forces exerted by the strong and the slight student on each other?