Goal: Distinguish between mass, gravitational force and weight.
An astronaut floats inside an orbiting spacestation. Which of the
following are true?
- No forces act on the astronaut.
- The astronaut has no mass.
- The astronaut has no
- A only
- B only
- C only
- A and B
- A and C
- B and C
- all are true
- none are true
The only possible answers are #3 and #8. The issue turns on the
definition of weight. At the surface of the earth weight and the
gravitational force are often considered equivalent. Further, since the
gravitational force depends upon the mass, mass and weight are
proportional and mass units are sometimes used as a measure of weight.
In orbit bodies still experience a gravitational force but are said to
have no weight. Is it any wonder that students are confused? Invoking
scale readings as weight is not a solution either as one’s weight would
change in an elevator. The best solution to this is to sensitize
students to these issues and charge them with the responsibilty of
determining how to interpret these quantities in context.