We often take the force of gravity for granted, even though Earth's gravity is what keeps each of us from floating off into space! In this lesson, students begin to more fully understand and appreciate the force of gravity. They predict what will happen when a whole apple and half an apple are dropped at the same time from the same height then test their predictions. Next, they observe cannonballs of different masses being dropped out of a tower, and leaking cups being dropped into a bucket. These activities demonstrate that all objects fall at the same rate, regardless of their mass - a concept known as the law of falling bodies. Students then watch a video segment showing a NASA astronaut dropping a feather and a hammer on the Moon. They repeat the activity in the classroom then consider why these objects fall at the same rate on the Moon but not on Earth. Finally, they use what they have just learned to predict what will happen when two balls of the same mass but different volumes - and then two balls of different masses but the same volume -- are dropped at the same time from the same height.
Before you begin, you may want to review the following terms. Mass is the amount of matter in a given volume of something. Volume is the amount of space that an object or substance takes up. For example, the mass of a bag of fluffy marshmallows is the same before -- and after -- a stampede of elephants changes its volume by squishing it to the size of a bar of soap. An object's mass is what determines how much force is needed to move, speed up, or slow down the object. The greater the object's mass, the more force it takes to change its motion.
Gravity is the force that exists between any two objects that have mass. Weight is a measure of the force of gravity pulling on an object. Some people think that the mass of an object and its weight are one and the same, probably because we weigh things to determine their mass. But weight and mass are not the same. How much something weighs depends on how strongly gravity is pulling on it. So something will weigh less where the gravitational force on it is weaker (as on the Moon or in space, for example), even though its mass has not changed.
- Describe gravity as a force that exists between any two objects that have mass"
- Demonstrate that all objects, regardless of their mass, fall to the ground at the same rate"
Grade Level: 3-5
- Three 1-hour blocks
- Galileo on the Moon Video
- Galileo: His Experiments Flash Interactive
- What Is "Weightlessness"? QuickTime Video
Use these resources to create a simple assessment or video-based assignment with the Lesson Builder tool on PBS LearningMedia.
- chart paper
- two balls of same mass, different volumes
- two balls of same volume, different masses
- foam cups (optional)
- various liquids, including water (optional)
- bucket (optional)
Before the Lesson
- Cut one apple in half.
1. Find out students' ideas about gravity. Ask:
- What is gravity?
- Where is gravity?
- What does gravity do?
Give students time to explain their ideas. Record their thoughts on the board or on a piece of chart paper, so that you can return to them later.
2. Hold up a hammer and a feather and ask students to predict what would happen if you dropped them simultaneously from the same height: Would they hit the ground at the same time or at different times? Do not drop the objects at this point. Show students the Galileo on the Moon video. After screening it, ask:
- Did you expect the hammer and the feather to land on the surface of the Moon at the same time?
- Why do you think this happened?
3. Try investigating some of these questions about gravity. Ask students to predict what would happen if you dropped a whole apple and half an apple at the same time from the same height: Would they hit the ground at the same time, or would one hit before the other? Why? Have the students record their predictions and explain their thinking. Ask students to share some of their predictions. Then drop the apples. Allow time to discuss the results and for the students to try to explain the factors that produced them. Use this activity as an opportunity to discuss gravity as a force that pulls objects toward Earth.
Note to the Teacher:
It is very hard to drop objects at exactly the same time so that they hit the floor simultaneously. You may want to practice ahead of time to prevent false results that will only add to students' misconceptions. Also, even if the apples hit the floor at the same time, students may not believe it! Suggest that they listen for the number of sounds they hear - one or two - when the apples hit. Students might need to repeat this investigation many times since it most likely contradicts their preconceptions.
4. Go to theGalileo: His Experiments interactive activity (Falling Objects experiment). Ask students to predict which cannonball will hit the ground first and give reasons for their prediction. Select their choice to see if their prediction was supported or not supported. Hopefully, at this point, students are willing to accept or at least consider the idea that all objects fall at the same rate, regardless of their mass. Galileo conducted several experiments and concluded that the effect of gravity on earthly objects is the same, regardless of the mass of those objects. He argued that in the absence of other forces such as air resistance, all falling objects accelerate toward Earth at the same rate.
5. Show the Galileo on the Moon video again. Remind students of the predictions they made in step 2 (would the hammer and the feather hit Earth at the same time). Try it. Then ask:
- Why did the hammer and the feather fall at the same rate on the Moon but not on Earth?
Introduce the idea of air resistance, a force (friction) that opposes any object moving through air. Ask:
- What role did air resistance play in the rate at which the objects fell?
6. Show the video What Is "Weightlessness"?. This demonstration can be interpreted as the water floating inside the cup, but from Galileo's experiments, we know that the water and cup are falling at the same rate even though their masses are different. Review what happened in the segment, and ask:
- Were you surprised that the water stopped pouring out of the holes in the cup once the cup started to fall?
- Can you think of an explanation for this based on your understanding of the way falling objects are affected by gravity?
- Why do you think the term weightlessness is used in the title of the video? (optional)
Have students try the falling cup activity from step 6 in your classroom. Experiment with a variety of liquids. Ask students to first predict the results. Do they think they will get the same result no matter which liquid is used, or a different result? Ask them to explain their reasoning; see how well they apply what they have learned from previous investigations to these new situations.
Check for Understanding
- What will happen when two balls of the same mass but different volumes are dropped at the same time from the top of a tall ladder? Which will hit the ground first? Why?
- What will happen when two balls of different masses but the same volume are dropped from that same ladder? Which will hit the ground first? Why?
If you have time, test their predictions by dropping the balls. Ask students to record their predictions first, share some of their ideas with each other then discuss the results.
Conclude the lesson by returning to the students' ideas that were recorded at the beginning of the lesson. How would they answer those questions now? How do their new answers compare with their old ones?