Student Conception: Gravity and constant velocity

[Matt Anthes-Washburn]

Student Conception: Gravity and constant velocity
Students may believe that the force of gravity is a constant, causing constant velocity of objects falling toward the Earth.
(Driver, R. et al., (1994) Making Sense of Science; research into children’s ideas. Routledge, London, pp.163)

Identifiers
Students may say that any object that is dropped will fall at a constant speed because gravity is constant. 

Confounding Experiences
Humans do not have a good method of visually detecting acceleration.  Falling objects fall quickly and we do not perceive their acceleration.  We know that jumping or falling from a higher height causes more speed, but we do not associate that with constant acceleration.

Formative Assessment Question
Demonstrate and draw a diagram of a ball thrown vertically into the air. Define positive velocity as moving upward.  Ask students to choose one of the following three answers to describe the motion of the ball on its way up, at the top, and on the way down: A) Zero Velocity, B) Positive Velocity, C) Negative Velocity.
Draw three Velocity vs. Time graphs and ask students to select the one that represents the motion of the ball.  Draw an upward facing “V”, a downward facing “V”, and the proper graph showing a continuously downward sloping line crossing the X axis when the ball reaches the top of the trajectory.

Interventions
Measuring the different tick intervals on the ticker tape attached to a falling object and calculating the independent speeds for each interval in the For You To Do will provide students with evidence that the vertical speed of the falling object is increasing.
Ask students to focus on writing or other details on an object they toss into the air.  Letters or numerals on the object will be easier to identify at the top of the trajectory where the object has slowed down, compared to when the object is close to the person tossing the object.