Free 125 Physics Projects for the Evil Genius by Jerry Silver

toward the bottom at a faster rate than it would fall on its own. Also, some electrostatic drag might develop between the feather and the plastic, which slows the descent of the feather in a vacuum.)

    Figure 18-3
With air removed from the tube, both objects fall at the same speed
.
Why It Works
     
    There is no doubt that the gravity of the Earth exerts a greater force on a more massive object. However, the more massive object requires precisely that same amount of larger force to cause it to accelerate. The upshot is that all objects on the surface of the Earth accelerate at the same constant rate.
Other Things to Try
     
    This experiment has a number of variations, including:
Compare the descent of a crumpled sheet of paper with an unfolded sheet of paper (both of the same mass).
Compare the descent of a single pencil with several pencils bundled together.
Tie a weight (such as a large stainless steel nut) to a string at the following intervals: 125 cm, 80 cm, 45 cm, 20 cm, 5 cm. Hold the string vertically. When dropped, each weight hits the floor in the same time interval. This is because the distance each weight falls is proportional to the
square
of the time that it is falling. These intervals are built into the spacing of the weights, so they should hit at the same time interval.
Which falls faster (in air): a book or a dollar bill? Certainly, if they’re dropped side by side, the book will fall fastest. However, if the dollar bill is placed on top of the book or below the book, the effect of air resistance will be eliminated and they will fall together.
The Point
     
    Gravitational acceleration (in a vacuum) is a constant. Specifically, it does not depend on the mass of the falling object.

Project 19
How fast do things fall ?
     
The Idea
     
    Objects exposed to the force of gravity accelerate at the same rate. We proved that in the previous experiment. Here, we measure the rate of gravitational acceleration for all objects on the earth.
    You measure acceleration two different ways in this experiment. In the first method, you use a stopwatch. We call this a
ballpark experiment
, which means we expect it to give a rough approximation rather than a very accurate result.
    The second method involves the use of a motion sensor, which offers a greater degree of precision.
What You Need
     
Stopwatch method
     
various objects: baseballs, golf balls, bowling balls, your physics textbook
stopwatch
tape measure
Motion sensor method
     
motion sensor with DataStudio software
ring stand or other support to orient the motion sensor vertically, looking downward
basketball, softball
Method
     
Stopwatch method
     
     
Use the tape measure to identify the distance the object will be dropped.
One person drops the object and the other person times the trip down.
Start the timer just as the object is released and stop it at the precise time it hits the ground. Try to avoid anticipating the release that will give too large a time measurement and an understated value for gravitational acceleration.
Calculate the gravitational acceleration using the equation g = 2d/t 2 , where
d
is the distance in meters and
t
is the time in seconds. Gravitational acceleration is measured in m/s 2 , which is read as meters per second squared or meters per second per second.
Motion sensor method
     
1. Set up a motion detector mounted on a table with an unobstructed view of the floor, as shown in Figure 19-1 .
2. Set up the motion detector to read distance versus time and velocity versus time. This can be accomplished by selecting the “velocity” file that comes with the DataStudio software package.

    Figure 19-1
Motion sensor aligned to measure vertical motion. Courtesy PASCO
.
3. This measurement works best by increasing the frequency of the motion sensor measurement by increasing the sampling from 10 per second to 50 per second.
4. Align the motion sensor in the vertical direction.
5. Hold the ball just under the motion sensor, as shown in Figure