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Exercises
Exercise 0A: Models and Idealization
Equipment:
coffee filters
ramps (one per group)
balls of various sizes
sticky tape
vacuum pump and “guinea and feather” apparatus (one)
The motion of falling objects has been recognized since ancient times as an important piece of phys-
ics, but the motion is inconveniently fast, so in our everyday experience it can be hard to tell exactly
what objects are doing when they fall. In this exercise you will use several techniques to get around
this problem and study the motion. Your goal is to construct a scientific model of falling. A model
means an explanation that makes testable predictions. Often models contain simplifications or ideali-
zations that make them easier to work with, even though they are not strictly realistic.
1. One method of making falling easier to observe is to use objects like feathers that we know from
everyday experience will not fall as fast. You will use coffee filters, in stacks of various sizes, to test the
following two hypotheses and see which one is true, or whether neither is true:
Hypothesis 1A: When an object is dropped, it rapidly speeds up to a certain natural falling
speed, and then continues to fall at that speed. The falling speed is proportional to the object’s
weight. (A proportionality is not just a statement that if one thing gets bigger, the other does too.
It says that if one becomes three times bigger, the other also gets three times bigger, etc.)
Hypothesis 1B: Different objects fall the same way, regardless of weight.
Test these hypotheses and discuss your results with your instructor.
2. A second way to slow down the action is to let a ball roll down a ramp. The steeper the ramp, the
closer to free fall. Based on your experience in part 1, write a hypothesis about what will happen when
you race a heavier ball against a lighter ball down the same ramp, starting them both from rest.
Hypothesis:______________________________________________________________
Show your hypothesis to your instructor, and then test it.
You have probably found that falling was more complicated than you thought! Is there more than one
factor that affects the motion of a falling object. Can you imagine certain idealized situations that are
simpler.Try to agree verbally with your group on an informal model of falling that can make predictions
about the experiments described in parts 3 and 4.
3. You have three balls: a standard “comparison ball” of medium weight, a light ball, and a heavy ball.
Suppose you stand on a chair and (a) drop the light ball side by side with the comparison ball, then (b)
drop the heavy ball side by side with the comparison ball, then (c) join the light and heavy balls
together with sticky tape and drop them side by side with the comparison ball.
Use your model to make a prediction:____________________________________________________
Test your prediction.
4. Your instructor will pump nearly all the air out of a chamber containing a feather and a heavier
object, then let them fall side by side in the chamber.
Use your model to make a prediction:____________________________________________________
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