Student Conception: Gravity stronger higher above earth

[Matt Anthes-Washburn]

Student Conception: Gravity stronger higher above earth
The force of gravity is stronger for objects further away from the earth. (This is combined with the idea that forces are proportional to velocity, not acceleration).

Identifiers
Students will likely accurately identify that objects reach higher speeds when they fall from greater heights.  They will not accurately note that all objects are accelerating as they fall.
Students may also indicate that the force of gravity is zero once an object gets outside of the Earth’s atmosphere, directly conflicting with their rule for gravity but accommodating their knowledge of space travel and satellites.  This may be linked to conceptions that gravity is related to the air present in the atmosphere.
(Driver, R. et al., (1994) Making Sense of Science; research into children’s ideas. Routledge, London, pp.164)

Confounding Experiences
Objects dropped from higher heights have higher speeds when they reach the ground.

Formative Assessment Question
Ask students to consider a 10 Kg mass held one meter above the ground.  When it is dropped it will strike the ground with a velocity of 5 m/s downward.  If you drop the same mass from a height of 10 m (Equivalent to the third floor roof), which do think is most likely to be the velocity when the object strikes the ground: A) 5 m/s downward, B) 14 m/s downward, C) 1 m/s downward?  (14 m/s is correct)
What is the weight of the 10 Kg mass at the height of 10 m? A) 100 N, B) 300 N, C) 33 N.

Interventions
During the activity have students analyze a ticker tape connected to a mass that is dropped from a height in the classroom.  Ask students to describe what happens to the spacing between dots along the tape.  Student’s prior efforts working with ticker tapes to measure speed will help students evaluate these results.
See Activity #1 for information about constant horizontal motion.