Using just two different sized and weighted balls, you can reenact Galileo's famous experiment demonstrating "universal attraction" on all objects. This is an ideal and fun science project/experiment for a teacher working with grades 3 through 5.
What is Gravity?
Gravity, or gravitational attraction, is the natural force that governs all falling (or accelerating) objects on Earth. It is the same force that controls the Moon's orbit around the Earth. Gravity ensures that all falling object fall at the same rate (that is, the same speed per unit of time).
Using two different sized and/or weighted balls, you can duplicate Galileo's famous experiment and demonstrate that the force of gravity acts the same on all objects.
Galileo's Experiment with Gravity - Part 1
Gather a collection of different sized and weighted balls (for example, baseball, basketball, ping pong ball, golf ball, volley ball, rubber ball, even a bowling ball). Have your students handle the different balls so they can get a sense of their different weights.
Depending upon the age of the students, and their science knowledge level, you can do this while discussing the relationship between an object's weight and its mass.
Have one student select a heavy ball, and another student select a light (low weight) one.
Mention the famous astronomer and scientist Galileo and how he was curious about objects of different weights (or mass). Galileo wanted to know if the force that made objects fall to earth (like acorns falling from a tree, or an arrow shot into the air) acted the same upon all objects, or, if an object's heavier weight made gravity's pull "stronger." (Note: The term "gravity" was not in use in Galileo's day; that would come with Isaac Newton and his falling apple observation.)
To find out, Galileo conducted a famous experiment at the tower of Pisa using a heavy iron cannon ball and small stone (mention that you don't own a cannon ball, which would damage the floor, and so are using another ball instead).
Ask your students if they would like to reenact Galileo's famous experiment at the tower of Pisa--with the balls that they selected.
Galileo's Experiment with Gravity - Part 2
To finish the experiment, you will need a small step ladder. Tell the students that the ladder represents the tower of Pisa that Galileo climbed to the top of to test his theory of "universal attraction" (for safety, it's probably best if you or an assistant does this part--and remember to wear flat-heeled shoes).
Take the two selected balls (and this demonstration works best if they are very different in size, like a basketball and a ping pong ball), and carefully climb up a few steps of the ladder--just high enough that you can hold the balls a good seven or eight feet off the ground, saying (something like), "I am Galileo climbing to the top of the tower of Pisa to conduct an important scientific experiment. I am going to drop these two differently weighted balls to see which has the strongest force of attraction."
At this point, holding a ball in each hand, at the exact same height, arms extended, ask your students to guess which of the two will hit the ground first.
Wait a few moment as they shout out their guesses (and they will have many guesses--including, maybe, the right one). After a few moments of guessing, you announce: "It's time for the experiment to begin. On the count of three, I will release the two balls at the SAME TIME...one...two...three!"
Make sure to release the balls at the same time. They will hit the floor at exactly the same time (this can be repeated with different balls). Allow the balls to bounce for a bit, and for the realization of the truth to sink in.
Then, ask the students what just happened (or ask: Was this just an accident?). You might even try the experiment a second time with two different balls. Let them come up with the answer on their own (or maybe have them write down what happened and then try to explain it). They should figure out quickly that no matter how much heavier one ball is compared to another, the force of gravity is "universally" the same for all falling objects, and so, the two balls hit the ground at the same time, every time.
One important note: The use of balls here is not accidental; sphere-shaped objects have low and equal "drag" (air resistance) and so will always fall at the same rate. This is especially important if conducting this experiment outdoors where there is wind and variable air pressure. An observant student might discover that certain objects--like a cotton ball or a feather--do not hit the ground at the same time as a heavier (more compact) object, and question this experiment. This is a good, instructive opportunity--and possibly the basis for a second experiment--to explain how a parachute works (for example), or, how gliding flight is possible.
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