What Does Gravity Have to Do With Weight and Mass?

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Objective

SWBAT recognize the relationship between weight, mass, and gravity.

Big Idea

Students will use each planet's gravity to calculate their weight and how far they can jump each planet.

Lesson Overview

5e Lesson Plan Model

Many of my science lessons are based upon and taught using the 5E lesson plan model: Engage, Explore, Explain, Elaborate, and Evaluate. This lesson plan model allows me to incorporate a variety of learning opportunities and strategies for students.  With multiple learning experiences, students can gain new ideas, demonstrate thinking, draw conclusions, develop critical thinking skills, and interact with peers through discussions and hands-on activities.  With each stage in this lesson model, I select strategies that will serve students best for the concepts and content being delivered to them.  These strategies were selected for this lesson to facilitate peer discussions, participation in a group activity, reflective learning practices, and accountability for learning.

Unit Focus  

The Forces and Motions unit focuses on gravity exerted by Earth on objects, while at rest or during motion. With this in mind, students will investigate types of forces and the effects it has on moving objects. They learn how forces can stop an object from moving, increase or decrease the speed of an object moving, change its direction, and put a resting object into motion. Through models, investigations, research, and the engineering and design process, students learn that gravity is a constant force that impacts an object’s motion. The unit wraps up with students using the engineering and design process to create a zip line to illustrate the effects of gravitational force.

Lesson Synopsis

This lesson, What Does Gravity Have to Do With It, begins with a teacher demonstration. I take two balls, similar in size, but different in weight and ask students to predict which one will drop first. After a few demonstrations, I have them try it themselves with a variety of items. We define the terms gravity and force as class. Next, I use an orbital model to simulate how gravity keeps the planets in orbit. The lesson move on to distinguishing between mass and weight as gravity acts upon one but not the other. Based on this understanding, they calculate their weight on other planets as gravity is not the same as Earth's. The lesson wraps up with students constructing an explanation about the gravitational force on an object.

Next Generation Science Standards  

This lesson will address and support future lessons on the following NGSS Standard(s):

5-PS2-1.  Support an argument that the gravitational force exerted by Earth on objects is directed down.

Scientific & Engineering Practices

Students are engaged in the following scientific and engineering practices:

5.) Using Mathematics and Computational Thinking- Students calculate their weight on other planets to by using an equation. They use their data to describe how gravity affects weight.

Crosscutting Concepts

The What Does Gravity Have to Do With Weight and Mass? lesson will correlate to other interdisciplinary areas.  These Crosscutting Concepts include:

2.) Cause and Effect- Students make observations of various objects falling to the ground to provide evidence of the effects of gravity from the Earth.

Disciplinary Core Ideas

Disciplinary Core Ideas within this lesson include:

PS2.A - Forces and Motion

PS2.B-  Types of Interactions

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Classroom Management Methods

Importance of Modeling to Develop Student

Responsibility, Accountability, and Independence 

Depending upon the time of year, this lesson is taught, teachers should consider modeling how groups should work together; establish group norms for activities, class discussions, and partner talks.  In addition, it is important to model think aloud strategies.  This sets up students to be more expressive and develop thinking skills during an activity.  The first half of the year, I model what group work and/or talks “look like and sound like.”  I intervene the moment students are off task with reminders and redirection.  By the second and last half of the year, I am able to ask students, “Who can give of three reminders for group activities to be successful?” Who can tell us two reminders for partner talks?”  Students take responsibility for becoming successful learners.  Again before teaching this lesson, consider the time of year, it may be necessary to do a lot of front loading to get students to eventually become more independent and transition through the lessons in a timely manner.

  • EXPLORE TEAMS (Pre-Set)

For time management purposes, I use “lab rats ” where each student has a number on the back of his or her chair, 1,2,3,4 (students sit in groups of 4)and displayed on the board.  For each activity I use lab rats, I switch up the roles randomly so students are experiencing different task responsibilities which include:  Director, Materials Manager, Reporter, and Technician.  It makes for smooth transitions and efficiency for set up, work, and clean-up.  

Engage

15 minutes

Sparking Motivation and Curiosity

(Prior to starting class, I place whiteboards in the center of each table as students will use them during this portion of the lesson.)

I begin by holding up a two balls (roughly the same size, but different weight) and pose the question: If I drop these, which do you think will hit the ground first? I have them write their prediction on their whiteboard, then hold it up so I can get a quick take on their thinking. Before dropping these items, I call on a student to explain why they made that prediction. I stand on a small step ladder and drop the items. I listen for students reactions.  I tell them I am going to repeat the test two more times to make sure they have an accurate observation. (This is also good practice and habit for students to develop and apply during investigations)

I have a few students share their observation, but I don't confirm if they're true or not as they will be trying this out themselves with their groups.

I tell them they are going to work with their groups to test a variety of objects listed listed on their data table. (I review this with them, noting they need to make a prediction prior to performing the activity and then write their observation after the activity is completed. During their activity, I circulate the room and check in with groups.

falling objects 

 

After about five minutes, I have groups share their predictions and observations of their falling objects. I am looking for students recognize that objects of different weights hit the ground at the same time.

I ask students to share what they think caused the objects to fall to the ground. I lead them to understanding that Earth's gravity caused the objects to fall to the ground. I engage them in a discussion using these questions:

  • Do you think gravity ever changes? Why or Why not? (Gravity does not change on Earth, it remains constant)
  • Why do we need to know about gravity? (Without it we would not stay on Earth; it affects motion.)

 

I explain that we are going to move on to defining gravity.

Explore

15 minutes

Defining Gravity

After our discussion, I write the question on the board and ask the class: "What is gravity?"  I have them do a turn and talk with their groups, so they all have a chance to share their thinking. Then I use my pull sticks to randomly call a few students to share their thinking.

Once we share and discuss, we create a definition for the word gravity. As I write it on the board, I have students jot it in their interactive notebook. I also define the word force as it is related to gravity.

Gravity: the force of attraction that exists between objects. 

Force: a push or pull that acts on an object

To engage them in thinking about gravity's effects, I ask them:

Have you ever wondered why the moon stay's in place as it revolves around us? And, Why do the planets all keep their place in the solar system? I tell them this is due to the gravitational pull between the sun and planets. This means that every object in the universe exerts a gravitational force on every other object. We feel the gravitational pull of the Earth because it is so strong. To further explain this, I show this video that summarizes how gravity affects planets and people.

 


Seeing the Effects of Gravity

Next, I share that I am going to demonstrate how gravity is the force which planets use to orbit the sun. I hold up an orbital model, which is made of a piece of string (about 18-24 inches long) and 2 rubber washers. through setting it up so it looks similar to this:

drawing of string threaded through a straw with washers tied to each end of the string

I start with a demonstration to show how gravity affects planets as they revolve around the sun. I hold up the orbital model and point out that the straw represents the Sun and the washers represent a planet. While holding the model vertically, I extend my arm out completely away from my body. Next, I hold the washer closest to me; I rotate the orbital model in a circular motion.While this is happening the second begins to revolve in a circular motion. Finally, I use my other hand to slowly pull on the washer closest to me, causing the other washer to revolve faster and shorten its orbital area.

As I am doing this demonstration, I ask the students questions during it to make sure they are attentive and understanding the demonstration.

  • Describe what the washer (planet) is doing while I spin it?
  • Describe what happened to the path of the planet while I pulled on one of the washer and continued rotating the handle?
  • What did you notice about the speed of the washer (planet)?
  • What do you think the string could represent in our model? 

 

I connect this model to how the gravity of the Sun pulls the planets in as they revolve around it. 

I hand out an orbital models to each group and have them try it out. I review how to do safely and they begin. They write down two observations from this activity and the demonstration in their interactive notebook and explain how it relates to planets remaining in orbit in space. 

Explain

15 minutes

How Does Gravity Affect Weight and Mass of an Object?

With a general idea about gravity, I explain to students that gravity relates to weight and mass. I display these two words in a t-chart on the board and instruct students to do the same in their interactive notebook.  I hand out a quick read that explains the difference between weight and mass. We review this together and note the difference between each one in the t-chart.

I pose the questions: On which planet in our solar system do you think an object or person would be the heaviest? On which would they be the lightest? I tell them they are going to try and figure this out by calculating their weight on another planet using multiplication. I write an equation on the board:

mass (on Earth) x gravity = weight

I use this recording sheet, that I found online and have students begin their calculations. (I let students who struggle with math use a calculator so they do not fall behind.) After their data tables are complete we analyze their calculations

I restate my questions:  On which planet in our solar system would you be the heaviest? On which would you be the lightest? and add on "Why are you heaviest on that planet and lightest on that planet?" instruct them to record their responses in their interactive notebooks. We talk about their responses and I guide them into understanding that the more mass a planet has, the more gravity it has. This means that planets with a larger mass than Earth have more gravity and vice versa, planets with a smaller mass than Earth have less gravity than Earth.

 

Elaborate/Evaluate

20 minutes

Applying Our Understanding

Once we determine our weight on other planets, I explain to students that they are going to find out how gravity affects how we move.  I begin by asking students to stand and jump. Then ask: "what did you notice about your jump?" I want them to realize how high they could go and how quickly they returned to the ground.

Now I hand out a data table and I have them add it into their interactive notebook. Then I review their task:

"You are going to determine how far you can jump on Earth and then use your data to determine how far you could jump on another planet or the moon." I show them how to mark a starting point for jumping and how to measure based on their initial landing. I review how to use a tape measure to get an accurate measurement.  I explain they are completing five jumps and will take the average of the jumps to determine their jump length on Earth. Once they do that, they will calculate their jump length on other planets and the moon using the following equation:

Earth jump length average divided by the planet or moon's gravity.  They work with their elbow partner at their group and begin the task.

Analyzing Our Data

Similar to analyzing our weight on a planet's, I ask: "Which planet can you jump the furthest?... The shortest?" Why do you think that is?  I have construct an explanation in their interactive notebook. I tell them their explanation should include details about gravitation force on an object.