Goal: Recognizing how the concept of force relates to interactions.
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
(3); The forces are equal (independent of the masses and motions of the
interacting objects), as required by Newton’s Third Law .
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?
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
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
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?