Teacher Guide

Falling Physics

Experimenting with forces and variables of falling objects

How does mass affect how fast an object falls?

This resource was originally published in PhysicsQuest 2020: Force & Motion.

This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.

View the student guide: Falling Physics

How does mass affect how fast an object falls?

• 2 equal-sized pieces of paper
• Chair or table (or both)
• A ruler or metric tape (optional)
• Beam balance (optional)
• A camera to record the experiment (optional)
• A chronometer or something to measure the time (optional)
• 2 balls similar in size but different mass 2 aluminum tart pans

Students will experiment to understand the relationship between mass, weight, velocity, acceleration and air resistance.

• Total time
45 - 60 minutes
• Education level
• Content Area
Force & Motion
• Educational topic
Gravity, force

For more information and ideas on how to implement the activity in your classroom check out the video.

If someone showed you two spheres of the same size but with different masses, say 1 g and 10 kg, and asked which would hit the ground first after being dropped from the Leaning Tower of Pisa, what would you say? If you’re like most people, you would say the 10 kg sphere would hit the ground first. Aristotle said so too, and for 1,000 years everyone believed him. But doing the experiment would show you that in fact, both spheres hit the ground at the same time.

This is exactly what Galileo did, showing the world that objects of different masses fall at the same rate. To start understanding why Galileo was right, we need to understand the difference between several physics words that are often jumbled together and confused: mass, weight, speed, velocity, acceleration, and force.

When something falls, it falls because of gravity. Because that object feels a force, it accelerates, which means its velocity increases as it falls. The strength with which the Earth pulls on something in the form of gravity causes this acceleration. However, when mass increases, the force of gravity from the Earth has a higher magnitude. When mass increases, force of gravity increases, and therefore acceleration of all objects on Earth are equal a = F/m. Weight (W=m*Fg) is the force that acts on the mass due to gravity, because it is how much stuff there is times the acceleration at which is pulled towards the Earth, or any planet or moons. Because Earth gives everything the exact same acceleration, objects with different masses will still hit the ground at the same time if they are dropped from the same height.

But what about a marble and a feather, they don’t seem to be accelerating towards the Earth with equal acceleration. That is not because of differences in the acceleration - which is constant on Earth, it is because air is pushing against the object in the opposite direction the Earth is pulling. This force is caused by air resistance.

The less massive the object is, the more the force of air resistance slows the object down as it falls. If two objects were dropped on the moon, where there is no air, they would fall at the same rate no matter how much they differ in mass. The shape of the object can impact how much it is affected by air resistance. For example, if you drop a piece of paper horizontally, it has a lot of surface that is exposed to the air resistance. But if you drop the paper vertically, on the thin side, then there is less surface exposed to the air resistance. This means that, in that position, the paper will feel less push from the air and the same pull from the Earth. Two pieces of paper with the same mass dropped from the same height but with one in the horizontal position and the other in the vertical position will not hit the floor at the same time.

Astronaut Neil Armstrong did an experiment on the moon to convince everyone that Galileo was right, that two objects of different mass and shape — in this case a feather and a hammer — in the absence of air resistance will hit the ground at the same time.

Key terms students should know by the END of the activity(s). Try not front loading, see here for reasoning.

Key terms

These are the key terms that students should know by the END of the two lessons. They do not need to be front loaded. In fact, studies show that presenting key terms to students before the lesson may not be as effective as having students observe and witness the phenomenon the key terms illustrate beforehand and learn the formalized words afterwards. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterwards in the context of what they learned.

However, if these words are helpful for students on an IEP, ELL students, or anyone else that may need more support, please use at your discretion.

• Mass: A measure of the amount of stuff (or matter) an object has. Not to be confused with weight or volume. Mass only says how much actual stuff there is, not how big an object is or how hard something is pulling on it.
• Weight: Mass (amount of stuff) times how hard the planet is pulling on it (gravity). This means that your weight on the moon will be 1/6 of that on Earth (gravity on the moon is 0.166 times of that on Earth). However, your mass will still be the same.
• Force: The push or pull an object feels because of interactions with other objects. If the interaction stops, then there is no force. It is formally defined as mass times acceleration. For example, gravity is a force that represents the pull the Earth has on all objects.
• Velocity: A measure of how fast something is going in some specific direction. Not to be confused with speed, which is only how fast something is moving. “The car was going 65 mph south on I-95” is a measure of velocity. “The rollercoaster was moving at 65 mph when Billy got sick” is a measure of speed.
• Acceleration: How fast the velocity is changing. When something accelerates it changes how fast it is going or the direction in which it is moving. For a positive change in acceleration means that the object is moving faster, a car going from 30 mph to 40 mph. A negative change means the object is moving slower, the car is going from 40mph to 30 mph. Finally, a change in the direction of the object’s velocity without changing speed, such as if a car is moving North and turns East still moving, then the car accelerated because the direction of the car’s velocity changed. Remember that velocity is a vector with direction and magnitude, therefore changes in any (or both) of those factors will produce an acceleration.
• Air resistance: The force air exerts on something moving through it. When an object with a bigger surface falls through air, it feels more air resistance. Air resistance does not depend on the mass of the object.
Experiment 1
Objective

Students will experiment to understand the rate of falling objects and variables that affect them.

It is important to understand that student goals may be different and unique from the lesson goals. We recommend leaving room for students to set their own goals for each activity.

Before the experiment
• Show students two objects with the same size and shape but with different masses.

Use the turn and talk protocol to Ask & Discuss: A famous experiment was done at the Leaning Tower of Pisa (perhaps show pictures and description for context). Which object do you think would hit the ground first after being dropped from the Leaning Tower of Pisa?

1. Pair students up
2. Give them a minute to think quietly
3. Give students 2 minutes to discuss their thinking
4. Have students record their answers or share out to the whole group
• Once students have shared their initial ideas, show a video of the debate between Aristotle and Galileo.

• Then have them complete the BEFORE activities in their student manual.

• After students share their ideas, first with a partner, then with the class, sort students into two groups: Aristotle and Galileo.

Setting up
• Place the two tart pans on the floor, bottom up, about a foot apart.

• Select two equal sized balls.

• If you have a beam balance, find the mass of the two balls and write those values on your notes.

• Make sure to drop the balls at the same time.

• How many bangs did you hear on the tart pans each time you dropped the balls?

• Check your slow motion video. Did the video confirm what you heard?

During the experiment
Collecting data
• Make experiment groups based on who students agree with (Aristotle or Galileo).

• In the student’s guide we have asked the students to design their own experiment to test if two objects of the same mass but different shape hit the ground at the same time. The idea is to encourage them to be creative, to understand how to design experiments, and to think like scientists and engineers. They are given a set of materials that they can use to do their experiments. This is to prompt them, but they should be allowed to use other materials in their design. Make sure you inform students of invisible variables they should control for like drop height, etc.

• Slow motion videos of their dropping objects work really well for this experiment.

Analyzing data
• The goal of the experiment is for students to understand that mass is not a factor that affects how objects fall, that they notice the shape matters and why it matters. Crumpling the paper or changing the direction in which the paper is dropped can support those ideas. They need to figure out which variables they should control for, for example dropping the papers at the same time or the presence of a strong air current, and consistency of the repeated experiments.

Teacher tip

Suggested STEP UP Everyday Actions to incorporate into activity:

• When pairing students, try to have male/female partners and invite female students to share their ideas first.
• As you put students into groups, consider having female or minority students take the leadership role.
• Take note of female participation. If they seem to be taking direction and following along, elevate their voice by asking them a question about their experiment.
• Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.
• As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.

Consider using the RIP protocol (Research, Instruct, Plan) for lab group visits and conferring.

Consider culturally responsive tools and strategies and/or open ended reflection questions to help push student thinking, evidence tracking, and connections to their lives.

Conclusion
• After students have had a chance to discuss key ideas from the lesson, you can now clarify and give concise definitions to the forces they experimented with.

• Watch the video, Was Aristotle Right? No, he wasn't. And yet, for 1,000 years everyone believed him. But doing the experiment would show you, besides a great view of Pisa, that in fact, both spheres hit the ground at the same time.

• This is exactly what Galileo did, showing the world that objects of different masses fall at the same rate. This is also a good example of why it is important to do experiments yourself and not to just take someone else’s word for it.

• To start understanding why Galileo was right, we need to understand the difference between several physics words that are often jumbled together and confused: mass, weight, speed, velocity, acceleration, and force.

Experiment 2
Objective

Students will experiment to understand the relationship between mass, weight, velocity, acceleration and air resistance.

Before the experiment
• Think about the experiment we did yesterday, draw, write, or explain our conclusions? Discuss and compare with your elbow partner.

• Today, you will have two objects with different mass but similar shapes. What do you think will happen?

• Use the Turn and Talk Protocol to have students discuss what their hypothesis will be, then have them write or draw it.

1. Pair students up
2. Give them a minute to think quietly
3. Give students two minutes to discuss their thinking
4. Have students record their answers or share out to the whole group
Setting up
• Take the two pieces of paper and crumple one into a ball.

• If you have a beam balance, find the mass of each piece of paper. Write those values in your notes and observations section.

• Measure the height from which you will drop the pieces of papers. Make sure it is the same height. Write the height value on your notes and observations section.

• Drop the pieces of paper at the same time.

During the experiment
Collecting data
• Make sure students are put into intentional groups. See above.

• Students will complete the experiment using the student guide where we have outlined the experiment for students and along the way, they record results and answer questions.

Analyzing data
• In the student guide, they will answer questions that help them understand what rust does to objects.

• Continue to listen in on each group’s discussion, answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later.

Conclusion
• Use the All Class Science-Talk to discuss the question: Does mass affect the rate of a falling object? Why or why not?

1. The teacher poses a question for students to answer.
2. If necessary, give students time to think or write.
3. The teacher asks students to turn their bodies toward the center of the room for a Science Talk.
4. The teacher explains:
5. “In a Science Talk, knowledge is held by your fellow scientists, and you should talk to each other. The goal of a Science Talk is to help each other understand a phenomenon. You can help your fellow scientists understand by: sharing results, using data, being as clear as possible, and listening carefully to deeply understand what your fellow scientists are saying.
6. Teacher facilitates the student discussion by using Michaels and O’Connor’s Talk Science Primer and avoiding telling answers or asking closed-ended questions.
7. Optional: At the end of the Science Talk, students can record their thinking.

PhysicsQuest Activity: Introduction

PhysicsQuest Activity: Falling Physics

• Create a video for younger students, how would you explain these concepts to them?
• Have students write their own definition for key terms using evidence from experiments to support their definitions.
• Create mind maps connecting all the key terms and connect to evidence
• Begin students on drawing free body force diagrams to account for gravity and air resistance. Discuss Newton’s Second Law and introduce equations.

Astronaut Neil Armstrong did an experiment on the moon

Astronaut Neil Armstrong did an experiment on the moon to convince everyone that Galileo was right, that two objects of different mass and shape -in this case a feather and a hammer - in the absence of air resistance will hit the ground at the same time.

Public Experiment

Videos for younger children

Was Galileo Right?

Investigate the effect of gravity on objects of various masses during free fall. Predict what the position-time and velocity-time graphs will look like. Compare graphs for light and heavy objects.

Free Fall Model

This simulation allows students to examine the motion of an object in free fall.

Free Fall Fall Air Resistance

This simulation allows students to compare the motion of free falling objects with and without the influence of air resistance.

Credits

Coordination, Research, Text, and Editorial Review Claudia Frachiolla, Jamie Liu, Leah Poffenberger, James Roche, Laurie Tangren, Rose Villatoro, David Voss

Graphic Design and Production Meghan White

Illustrations Isabel Bishop

Updated in 2023 by Sierra Crandell, M.Ed. partially funded by Eucalyptus Foundation

Extension by Jenna Tempkin with Society of Physics Students (SPS)