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Cynthia D'Souza De La Salle Institute
3455 S Wabash Ave
Chicago IL 60616
Mass remains constant but weight is the force with which the earth pulls the
body to itself. Weight is felt by the support given to us by the floor.
Weightlessness is a result of free fall. Orbital motion is a form of free fall.
analytical balance(1), pliers, string; teddy bear and ball; styrofoam cup,
water. For each pair of students: 1 styrofoam cup, rubber bands, 2 weights,
such as washers and 1 paper clip.
The concepts of mass and weight are first elicited from students and an
acceptable definition written on the board. An analytical balance is displayed
and with a nutcracker in the left pan. Discussion is generated as to how mass
can be measured on it and what would happen to the balance if the nutcracker
were to "lose" weight. Now a thread is used to connect one leg of the
nutcracker to the top of the balance and the pans are still balanced. "If the
thread were burnt now," says the teacher, "would the pans still balance? Why or
why not?" The experiment is promptly performed and results discussed. The
teacher holds a large spring balance and has students read the weight of an
object. Now he/she jumps to the floor and students discuss why the scale
reading decreased to nearly zero.
Students are given their equipment and escorted to the stairwell so that one
partner lets drop the cup with the weights suspended outside it by the rubber
bands. The other partner at the bottom notes the position of the weights and
when the "jump" occurred. Now the teacher shows a styrofoam cup with 2 holes
bored in the sides near the bottom. The cup is filled with water. With the holes
plugged by the fingers, the cup is dropped. The class observes that as long as
the cup falls, no water spills out.
Now the teacher holds teddy and the ball on the same level and lets them fall
simultaneously. Students observe that they fall together and hit the ground at
the same time.
Back in their seats, the teacher asks the following:
1) What happened to the falling weights? Why did they jump into the cups?
By comparison with the falling spring scale what must the weights have been
2) Why did the water not spill out of the falling cup? Would the fact of its
feeling "weightless" explain why it exerted no pressure on the holes?
When did the water spill out?
3) In what way is this experience similar to that of a passenger in an elevator
whose cable has been cut?
Is gravity not acting anymore on the person? How can we tell? Then gravity
is not the reason for weightlessness, but the falling away at the same rate
of the elevator.
4) How do we normally experience weight?
5) When teddy and ball fall, what fact gives teddy the illusion that he is
floating? What gives orbiting astronauts this sensation?
During free fall; the pliers, the spring scale, the weights and the water were
experiencing weightlessness though gravity's force was still acting on them all.
If their motion were suddenly stopped, their full weight would seem to return
because their supports were pressing upwards into them.
In the freely- falling elevator, a scale placed under the person's feet falls
away as fast as the person himself, the scale would read zero and the person
Persons in orbital motion are in free-fall. That's why they experience