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You can solve a page of Newton's-law numericals and still believe, deep down, that a moving object needs a force to keep it moving. That is the uncomfortable finding behind the Force Concept Inventory (FCI), a short multiple-choice test that physicist David Hestenes and two high-school physics teachers published in 1992.
What is the Force Concept Inventory?
The FCI was published in *The Physics Teacher* by David Hestenes, Malcolm Wells and Gregg Swackhamer (1992). It grew out of earlier work by Ibrahim Halloun and Hestenes (1985), who found that students' common-sense beliefs about motion had a large effect on their performance in physics, and that conventional instruction changed those beliefs only a little. A companion paper, "Common sense concepts about motion", built a catalogue of beliefs that conflict with Newton's theory.
PhysPort, a physics-education resource site, describes the FCI as a multiple-choice test of the most basic Newtonian concepts, written in everyday language, with common-sense wrong answers as the options. It takes about 30 minutes and is usually given before and after a course.
In the 1992 paper, the authors make a point worth remembering: "The Inventory, therefore, is not a test of intelligence; it is a probe of belief systems." They also note that some of these beliefs were held by great thinkers of the past, including Galileo, so they are reasonable guesses from everyday experience. They just happen to be wrong.
Which misconceptions about force are most common?
The 1992 paper lists 28 misconceptions in six groups. The main ones:
Impetus. Objects carry a store of "go power" that keeps them moving and slowly runs out. This contradicts Newton's first law: nothing is needed to keep velocity constant.
Active force. Motion implies a force in the direction of motion, and only "active" agents, usually living things, exert forces. This mixes up velocity and acceleration: force is proportional to acceleration, not velocity.
Dominance in action and reaction. In a collision, the bigger, heavier or more active object pushes harder. Newton's third law says the two forces are equal and opposite. The authors call it one of the last misconceptions to be overcome.
Obstacles do not push. A table or wall "just gets in the way" instead of exerting a normal force.
Gravity. Heavier objects fall faster, or gravity grows stronger as an object falls.
How widespread were these beliefs?
In the 1992 data, regular high-school physics classes in Arizona (612 students, 15 teachers) averaged 27% on the test before instruction and 48% after (Hestenes et al., Table III). University classes in the same table scored between 63% and 77% after instruction. The paper's summary of the research is blunt: conventional physics instruction produces little change in these beliefs.
The FCI went on to become a standard measuring tool. PhysPort notes it has been used in more than 50 published studies covering over 35,000 students, including Richard Hake's 1998 survey of more than 6,000.
Quick self-check: six questions
These are our own questions in the FCI's spirit, not FCI items. Answer each in one line, then check. Ignore air resistance.
A ball is thrown straight up. At the highest point, what forces act on it?
A heavy truck hits a small car head-on. Which exerts the larger force on the other?
A book rests on a table. Does the table exert a force on the book?
A puck slides across frictionless ice at constant speed. Is any horizontal force needed to keep it going?
A puck tied to a string slides in a circle on frictionless ice, and the string snaps. What path does the puck take?
Two balls of the same size, one twice as heavy, are dropped together from a balcony. Which lands first?
Answers. (1) Only gravity, even though the velocity is momentarily zero; "an upward force that has run out" is impetus thinking. (2) Equal forces; the car accelerates more because its mass is smaller. (3) Yes, an upward normal force equal to the book's weight. (4) No; constant velocity needs zero net force. (5) A straight line along the tangent at the moment the string snaps, not a curve. (6) They land at the same time.
If you got one wrong, write down the belief behind your answer using the names above. Then explain in two sentences why Newton's laws disagree. Our guide to common mistakes in JEE mechanics problems has more practice, and our profile of Carl Wieman and active learning in physics looks at how teaching can change.
Seeing forces act often shifts a belief faster than reading about them. The Fermi Simulation Lab has interactive simulations for ideas that are easier to see than to read, and the Fermi Concept Library gives every physics concept, Newton's laws included, its own lesson.
