Galileo Projectile Motion Experiment
Student Instructions
Galileo used an inclined plane to study the motion of
projectiles as well. He used the inclined pane to vary the speed of the object,
by starting it at different heights. He discovered not only the parabolic path
of all projectile motion, but also the mathematical law that governs the
relationship between the starting point on the ramp and the distance traveled.
Set up: The inclined
pane should be set up on a table with the end of the plane near the end of the
table so the ball can roll off. It should be set up much higher (at a greater
pitch or angle) than in the acceleration experiment. You should attach the
metal shunt to the nail at the end of the ramp and line up the end of the shunt
with the end of the table. The shunt makes sure that the ball is traveling
horizontally when it starts to drop, rather than pointed down at the angle of
the ramp. (See the Video
Instructions for how to attach the shunt.)
Preliminaries:
1. Vertical and Horizontal motions are independent.
First have a student stand at the end of the ramp holding a
ball identical to the one being rolled down. Have another student release the
ball from the top of the ramp. The first student will release that ball from
exactly the height of the table at exactly when they see the ball on the ramp
reach the end of the table. You may have to try a couple times to get the
release point right.
The two balls will fall exactly the distance, though one
will be moving horizontally as well. Listen for the sound as they hit the
floor. Do they hit together? What does this show? Why is it important?
A. Seeing the shape of parabolic motion:
1. Use some of the plastic balls provided and have the
students throw them back and forth. Note the shape of the balls path. If the
students toss it back and forth at the same angle and speed you should get a
good view of the path as the ball traces it back and forth. Try changing the angle you throw it at. Try
changing the speed of the throw. How does the path change? What do all paths
have in common? Can you figure out why they are all called parabolas?
2. Line up as many balls as you can at the top of the ramp
and have one student release them one after another as fast as they can. Have
the rest of the students kneel down and watch the shape of the path the balls
take. Try it again starting the balls further down the ramp. Does the shape of
the path change? How are these paths similar to the ones you saw throwing the
balls? How are they different?
Galileo traced out the path by letting the balls fall onto a
moveable platform and raising the platform little by little and marking where
the ball hit at higher and higher planes. Can you think of any other way he
could have measured the path given his available technology.
B. Measuring the Distances. What determines how far the ball
travels before it hits the ground? Galileo also discovered a mathematical
answer to this question.
Horizontal distance |
1 unit |
2 |
3 |
4 |
5 |
6 |
Trial 1 distance |
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Trial 2 distance |
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Vertical distance |
1 unit |
2 |
3 |
4 |
5 |
Trial 1 distance |
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Trial 2 distance |
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Compare how the distance changes with horizontal distance
with how it changes with vertical distance? Which of these is the better
measurement or experiment? How can you decide? Galileo discovered that the
distance varies with the square of the height. Did you get that result? How did Galileo know which measurement was
the important one?