Key concepts
Physics
Free fall
Forces
Gravity
Mass
Inertia

Introduction
Have you ever wondered how fast a heavy object falls compared with a lighter one? Imagine if you dropped both of them at the same time. Which would hit the ground first? Would it be the heavier one because it weighs more? Or would they hit the ground at the same time? In the late 1500s in Italy the famous scientist Galileo was asking some of these same questions. And he did some experiments to answer them. In this activity you'll do some of your own tests to determine whether heavier objects fall faster than lighter ones.

Background
In fourth-century B.C. Greece the philosopher Aristotle theorized that the speed at which an object falls is probably relative to its mass. In other words, if two objects are the same size but one is heavier, the heavier one has greater density than the lighter object. Therefore, when both objects are dropped from the same height and at the same time, the heavier object should hit the ground before the lighter one. Is this true?

Some 1,800 years later, in late 16th-century Italy, the young scientist and mathematician Galileo Galilei questioned Aristotle's theories of falling objects. He even performed several experiments to test Aristotle's theories. As legend has it, in 1589 Galileo stood on a balcony near the top of the Tower of Pisa and dropped two balls that were the same size but had different densities. Although there is debate about whether this actually happened, the story emphasizes the importance of using experimentation to test scientific theories, even ones that had been accepted for nearly 2,000 years.

Materials
• Two balls of the same size, but different mass. For example, you could use a metal and a rubber ball or a wooden and a plastic ball, as long as the two balls are about the same size. If two spherical balls like this are unavailable, you could try something like an apple and a similar-size round rock.
• A ladder or step stool
• A video camera and a helper (optional)

Preparation
• You will be dropping the two balls from the same height, at the same time. Set up the ladder or step stool where you will do your test. If you are using a heavy ball, be sure to find a testing area where the ball will not hurt the floor or ground when it lands.
• If you are using a video camera to record the experiment, set up the camera now and have your helper get ready to record.
• Be careful when using the step stool or ladder.

Procedure
• Carefully climb the ladder or step stool with the two balls.
• Drop both balls at the same time, from the same height. If you are using a video camera, be sure to have your helper record the balls falling and hitting the ground.
Did one ball hit the ground before the other or did both balls hit the ground at the same time?
• Repeat the experiment at least two more times. Are your results consistent? Did one ball consistently hit the ground before the other or did both balls always hit the ground at the same time?
• If you videotaped your experiments, you can watch the recordings to verify your results.
Extra: Try this experiment again but this time use balls that have the same mass but are different sizes. Does one ball hit the ground before the other or do they hit it at the same time?
Extra: Try testing two objects that have the same mass, but are different shapes. For example, you could try a large feather and a very small ball. Does one object hit the ground before the other or do they hit it at the same time?
Extra: You could try this experiment again but record it using a camera that lets you play back the recording in slow motion. If you watch the balls falling in slow motion, what do you notice about how they are falling over time? Are both objects always falling at the same speed or is one falling faster than the other at certain points in time?

Observations and results
Did both balls hit the ground at the same time?

You should have found that both balls hit the ground at roughly the same time. According to legend, this is what Galileo showed in 1589 from his Tower of Pisa experiment but, again, it's debated whether this actually happened. If you neglect air resistance, objects falling near Earth’s surface fall with the same approximate acceleration 9.8 meters per second squared (9.8 m/s2, or g) due to Earth's gravity. So the acceleration is the same for the objects, and consequently their velocity is also increasing at a constant rate. Because the downward force on an object is equal to its mass multiplied by g, heavier objects have a greater downward force. Heavier objects, however, also have more inertia, which means they resist moving more than lighter objects do, and so heaver objects need more force to get them going at the same rate.

More to explore
Elephant and Feather—Free Fall , from The Physics Classroom
Engines of Our Ingenuity: No. 166: Galileo's Experiment , from John H. H. Lienhard, University of Houston
Video: Fall of 2 Balls of Different Weights , from Matthias Liepe, Cornell University
What Goes Up, Must Come Down: Conduct Galileo's Famous Falling Objects Experiment , from Science Buddies

This activity brought to you in partnership with Science Buddies