Gravity (Part 1)

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Objective

SWBAT compare and contrast dropping and tossing objects and identify forces involved in both.

Big Idea

Use this hands on activity to build a conceptual model of gravity.

Getting Started

We begin a study of gravity that will continue over the next few lessons. In this lesson, you investigate how gravity works by:

1.Observing the speed of falling objects, both experimentally and with video;

2.Analyzing a ball tossed up in the air and falling back down, both experimentally and with video;

3.Carrying out a Thought Experiment about gravity from different perspectives.        

Goals for this lesson

  • Understand that gravity is a mutual force that objects exert on one another
  • Understand the effect of gravity on objects that are dropped or tossed
  • Become more fluent with velocity over time graphs

Materials

  • One or more balls
  • Other non-breakable objects to drop (e.g., stones, pencils, bean bags)
  • Transparencies and pens

Engage

15 minutes

Quick Write

 As the students enter the room, have the following prompt on the board. 

 Open your journals and write a 3 min response to the following question:

 “If you drop, at the same time, a very large boulder and very small stone from the same height will they hit the ground at the same time?

After students have a few minutes to write, ask them to turn to their neighbors and share their responses. The expectation is that they have a friendly discourse and try to reach consensus. It is okay to disagree.

Give your class 5 to 8 minutes to discuss then call them back together.

Call on a representative from each table group to share their ideas. Your job is to listen and not correct any misconceptions. Pay attention to student ideas and be sure to reiterate what you hear to clarify their ideas.

Explore Part 1

25 minutes

You've probably read that Galileo demonstrated that all objects dropped from the same height will hit the ground at the same time, but how would you convince yourself of this claim? And if it is true, why? How does gravity work? Student will perform their own Galilean experiment and accumulate some evidence about this question.

In the first activity, students begin by predicting what would happen if they dropped two non-breakable objects of varying sizes and weights from the same height, at the same time, and record their thoughts about the following questions:     

  • Do they both begin to fall at the same velocity?
  • Do they continue at the same velocity or do they change velocity as they fall?
  • Do they hit the ground going at the same velocity?

They should then draw a force diagram of each of the situations and identify where F = ma is in each.

Once students make their predictions, they investigate their thinking by dropping, at the same time, several pairs of non-breakable objects of varying size and weight from the same height and observe if they all seem to fall at the same rate or are there any exceptions? If so, can they explain the reason for the exceptions?

For each experiment, students make a video of their drops to review later. I have my students film themselves using a smartphone (usually these can be found amongst the students). These have better cameras than the school is able to provide. If your students do not have access to smartphones or other devices that have video capacity, I have provided some sample videos that you can use with your students. 

In the second investigation, students begin by predicting what will happen if they drop, at the same time, two identical balls from different heights? They record their thoughts about the following questions:

  • Do both balls begin to fall at the same velocity?
  • Do they continue at the same velocity or do they change velocity as they fall?
  • Do they hit the ground going at the same velocity?

They should then draw a force diagram for each of the situations and identify where F = ma is in each.

Once they have made these predictions, they investigate their ideas by dropping, at the same time, a pair of non-breakable objects of the same size and weight from different heights and observe if they all seem to fall at the same rate or if there are there any exceptions. Again, they make a video of these experiments to analyze the data. 


This lessons takes advantage of Science Practices 4 & 5, Analyzing and Interpreting Data and Mathematical Thinking. I describe both in the video below. 


Explore Part 2

15 minutes

Next, students test their hypotheses by looking a the videos of the objects dropped from the same height and the those dropped from different heights. They use the video evidence to gather more evidence by making strobe pictures of each video and comparing them looking for what the data indicates about how an object drops?

In their journals they return to the list of questions and evaluate their ideas. Using strobe pictures, they draw velocity-over-time graphs for each video.

Instead of using the convention of considering left to right as the positive direction and right to left as the negative direction, this time they will consider DOWN as the negative direction and UP as the positive direction. Their graphs of the ball drops will have only negative motion, as the ball is always falling down.

Explore Part 3

15 minutes

In this part of the investigation, student investigate what happens when they throw a ball straight up in the air and catch it when it comes back down.

First, they predict the behavior of the ball as it is tossed straight up in the air and comes back down by sketching a prediction strobe picture of its motion and answering the following questions:

  • What makes the ball go up?
  • What makes it change direction?
  • Why doesn't it keep going up?
  • What makes it fall?
  • When does gravity start pulling on it?
  • Does it go more slowly, at the same speed, or more quickly as it goes up?  How do you know?
  • Is the trip up the same as the trip down?
  • What would a force diagram for this situation look like? (They will have to have two "snapshots" of the ball toss to represent the forces on the ball, one that describes the forces as the ball leaves your hand and one that describes the forces on it when it's in the air.)
  • Where is F=ma in this situation?

Once students are finished predicting, they investigate by tossing the ball in the air and catching it several time then compare with their predictions. They collect video of the tossing as well. 

Then they predict what they think will happen if they toss the ball up again, but with more force than before. How is this toss similar to the first toss? How is it different?

In their journals they draw a velocity-over-time graph for this toss, then answer these questions:

  • How does this graph compare with the velocity over time graph of the previous toss? What does this mean?
  • Describe the similarities and differences between drops and tosses and post the description.
  • What are the forces on the ball in each situation?
  • Where in each of these is the ball accelerating? How do you know?
  • What is your evidence that the ball is actually accelerating? 

Using the videos, students analyze the motions and create strobe pictures. How do they compare to their predictions?


Explain

15 minutes

Gravity is not just a force that the Earth exerts on everything, pulling it toward the center of the Earth. Gravity on Earth is just a specific case of a more general phenomenon, the gravitational attraction that exists between any two masses.

The reading, Gravity as a Mutually Attracting Force, by Andee Rubin, provides more details about how gravity works from a physicist's perspective. Have your students read and then discuss the reading with their table groups. 

What questions do they have? How did the reading help deepen their understanding on gravity?

Elaborate

15 minutes

To get your students thinking deeper, have them conduct a Thought Experiment. 

As they consider this situation, ask them to draw on the evidence their group gathered so far.

The Elevator: Suppose, first, that you are near a window in a tall building. You throw a ball straight up outside the window— but stay safely inside—and catch it on the way down. What do you see?


Now imagine that your study group partner is in an elevator that is going up next to the building at a constant speed. It passes the window from which you are throwing the ball exactly when you throw it and at the same speed as the ball when it leaves your hand. It continues at that constant speed, while the ball travels up and down in its usual fashion.

What will your partner see?

They should work with at least one study group partner to come to a common understanding that they'll present to the class. If the partners disagree, it's okay to present two or more points of view. They should include the evidence and reasoning that informed their conclusion(s).