Key Concepts
Free-fall, forces, gravity, mass, inertia

Introduction

Have you ever wondered how fast a heavy object falls compared to 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 C.E. in Italy, Galileo was actually asking some of these same questions and did some tests to answer them.  In this activity, you’ll do some of your own tests to determine whether heavier objects fall faster than lighter ones. 

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

In the 300s B.C.E. in Greece, the philosopher Aristotle theorized that the speed at which an object falls is relative to its mass.  In other words, if two objects are the same size, but one is heavier (it has a greater density) than the other object, then when both objects are dropped from the same height and at the same time, the heavier object should hit the ground before the lighter object.  Is this true?  

In the late 1500s C.E. in 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 into the crowd below.  While there is much debate about whether this actually happened, the story emphasizes the importance of using experimentation to test scientific theories, even ones that have been accepted for nearly two thousand years.   

Materials

  • 2 balls of the same size, but different mass.  For example, you could use a metal ball and a rubber, wooden, or 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 similarly-sized round rock.
  • A ladder or step stool
  • A video camera and a helper (optional)

Preparation

  1. 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.
  2. If you are using a video camera to record the experiment, set the camera up now and have your helper get ready to record.
  3. Be careful when using the step stool or ladder.

Procedure

  1. Carefully climb the ladder or step stool with the two balls.
  2. 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. 
  3. Did one ball hit the ground before the other, or did both balls hit the ground at the same time?  
  4. 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?
  5. If you video taped your experiments, you can watch the recordings to verify your results.
  6. Can you explain 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 see 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 hotly debated whether this actually ever happened.  If you neglect air resistance, objects falling near the surface of earth fall with the same approximate acceleration (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.  However, heavier objects 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. 

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Credits

Teisha Rowland, PhD, Science Buddies

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Key Concepts
Free-fall, forces, gravity, mass, inertia
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