Student Conception: Constant force, constant motion

[Matt Anthes-Washburn]

Student Conception: Constant force, constant motion
Constant forces produce constant motion, as soon as a force is removed from a projectile it is slowing down or stopping.

Identifiers
Students may argue that a ball thrown through the air does not have a uniform horizontal velocity.  They will describe projectiles as speeding up or slowing down as they travel. They may explain that air resistance is slowing down a football for example because they observe it falling to the ground as it gets further away.

Confounding Experiences
While everyone has some experience with projectiles, incorrect conclusions are drawn from the available evidence.  Students will improperly associate the more curved portion of a trajectory as the projectile approaches the ground as evidence that objects do not fall until they have little or no horizontal velocity.  This is reinforced by the baseball pitching example, where the fast moving ball is not perceived to fall relative to the Earth.

Formative Assessment Question
What happens to a ball on a flat, frictionless surface when you stop pushing it? A) It stops moving, B) It speeds up, C) It keeps moving at about the same speed.

Interventions
Refer to the results for Activity #4 with the falling coins.  Comparing the coins, a baseball, and football should provide enough contradiction to cause students to seek a resolution.  The results from Activity 5 will also help students to see that the speed of the falling object is increasing for each measured interval.
It will be difficult to get students to attribute the correct amount of negative acceleration to air resistance.  Students will think that some objects only fall after they have slowed down significantly (the football example).  Students need to compare trajectories of objects with different amounts of air resistance to see that trajectories become more parabolic as the force of air friction is reduced.
In PTG #5, students equate Carl Lewis’s running speed with his ability to jump far in the long jump. In #9b, students are asked the horizontal velocity of a projectile in flight.