[Phys-L] Aristotelian thinking among modern students
From: marx at PHY.ILSTU.EDU (David T. Marx)
Date: Thu Oct 20 12:30:04 2005
I am presently teaching a general ed course for mostly non-technical undergraduates (mostly
freshmen). Two-thirds of the students had high school physics. One half had calculus in high
school. The course is taught as primarily conceptual with examples and practice problems that use
algebra and trig. I gave a written exam last week covering Newton's laws, gravity, projectile
motion, work and energy. I give written tests rather than multiple choice tests so that I can see
student's thought processes at work. Sometimes, as below, I give them choices and then ask for an
explanation of their answer.
The first question on the exam was the following:
A rock is thrown straight up from the Earth's surface. Which one of the following statements
concerning the net force acting on the rock at the top of its path is true?
(a) The net force is instantaneously equal to zero newtons.
(b) The net force is greater than the weight of the rock.
(c) The net force is less than the weight of the rock, but greater than zero newtons.
(d) The direction of the net force changes from up to down.
(e) The net force is equal to the weight of the rock.
The majority of students answered the question incorrectly. Here is a sampling of some of the
incorrect explanations:
Student #1: choice a
"The force pushing the rock up equals the force of gravity pulling it down so at the top its
acceleration is zero and the net force is zero."
Student #2: choice b
"The net force is greater than the weight of the rock because in order for the force to push the
rock up into the air, the net force has to be greater than the net weight."
Student #3: choice a
"At the top of the path at that instant both the velocity and acceleration of the rock is 0. It is
not until the rock starts its fall towards the ground that the net force downward is greater than
the weight of the rock."
Student #4: choice e
"There is always an equal an opposite force acting on an object. Therefore the weight of the rock
which is pulling it down is equal to the force upward."
I can understand students' misunderstanding with regard to velocity being zero and acceleration not
being zero, but I am amazed that students would think that some force other than gravity was acting
on the object to make it move upward, even after it left the hand that threw it. It very much
reminded me of Aristotelian thinking. Any comments?