Coulomb's Law and Newton's Law of Gravitation

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Students will be able to understand relationships between force & distance and force & mass and solve problems using Newton's Law of Gravitation.

Big Idea

Students see the connection between Coulomb's Law and Newton's Law of Gravitation equations.

Physics Families

10 minutes

The goal of this lesson is to introduce Newton's Law of Gravitation and make a connection to Coulomb's Law (HS-PS2-4). Students use computational thinking (SP5) to solve problems and to explain their solutions with others (SP6). To start out class, I have students get into their Physics Families for the last time of the semester. Once they are in their families I have one students from each group to get a post-it note for each student. Then I ask students to draw a line down the middle of their post it. They reflect on something that they did that was fun this past semester and draw a picture to represent that and to draw a picture of something they are looking forward to in the next semester or the summer. Then they show their pictures to their group members and they try to guess what they drew. Afterwards, students post it on the board. I do this so students see something good in the past and good in the future.


Coulomb's Law Mega-Whiteboard

10 minutes

After Physics families, students return to their seats and take out the Coulomb's law problems they completed in the previous class. I randomly select 8 students to show their work for the problems up on the board. After students write the problems on the board, as shown below, I talk through the problems and ask students if they have any questions about the problems. As we go through each problem, I ask students questions like "If the distance was doubled, how would that change your answer?" or "If one charge or both charges doubled, how would that change your answer?" to help them think about the relationships between the variables. Students do well on these problems in class and most students get most to all problems correct. 

Newton's Law of Gravitation Guided Notes

25 minutes

After we go over the Coulomb's Law problems, I ask students to take out their Guided Notes and turn to the last page on Newton's Law of Gravitation. The video below explains how I go through the notes to help students learn about a new equation.

As stated in the video, I start with relating Newton's Law of Gravitation to Coulomb's Law. After that I provide to pictorial examples that students need to compare and figure out which one they think will have more force; the Earth and moon closer together or the Earth and moon further apart. They turn to a partner and discuss and then I have them raise their hand for the first situation or the second situation. We discuss that the relationship between force and distance are inversely proportional. Then, I do the same thing with a picture of the Earth and moon and the Earth and Sun at the same distance apart. They turn to their partner and discuss and then I have them raise their hand for the first situation or the second situation. We discuss that the relationship between force and mass are directly proportional. 

After discussing the relationships, I ask students to help me determine the equation for Newton's Law of Gravitation based on the relationships. Once we determine the equations, we go through the first problem together. I emphasize that parentheses are crucial when entering values into a calculator. Then I ask students to work on two problems on their own. After about 5 minutes, I show them the answers for the two problems and I ask them to give me a thumbs up if they got both right, thumbs to the side if they got 1 right and thumbs down if they got both wrong. I see that almost all students have their thumbs up or to the side, so we can move onto the group practice problems.

Group Problem Solving Using Newton's Law of Gravitation

25 minutes

After the example problems in the guided notes, I have students complete 4 additional practice problems to make sure that the students can use Newton's Law of Gravitation on their own. The problems that I have them complete require them to find all of the different variables (mass, force and distance) and are found on the physics classroom website. The Newton's Law of Gravitation Practice Problems are shown below:

1. Tyrone and Mia have masses of 84 kg and 59 kg respectively. They sit 1 meter apart in the front center of Mrs. Meyer's physics class. Determine the magnitude of this force of gravitational attraction.

2. The gravitational force between two objects that are 2.1 x 10-1 meters apart is 3.2 x 10-6 N. If the mass of one object is 5.5 x 101 kg, what is the mass of the other object?

3. If two objects, each with a mass of 500 kg, produce a gravitational force between them of 3.7 x 10-6 N, what is the distance between them?

4. Determine the force of gravitational attraction between the Earth and the moon. Their masses are 5.98 x 1024 kg and 7.26 x 1022 kg, respectively. The average distance separating the Earth and the moon is 3.84 x 108 meters. Determine the force of gravitational attraction between the Earth and the moon. 

To complete these problems, I have each student do all four problems on his or her own sheet of paper. Each member of a 4 person group is in charge of showing their work and answer for their assigned problem on the group answer sheet for one problem. Before they can write their answer down on the group page, they must check with the other group members to make sure that everyone agrees with their answer. When students complete all four problems I check their work. As shown below, I put a check if the answer is correct and an x if it is wrong. If students get the problem wrong, I don't tell them what they did wrong but they have to discuss with their group and figure out what they did wrong until they get the problem right. This is a great end to the lesson because it helps students to use Newton's Law of Gravitation that they learned about earlier in the lesson. It also helps students to work together in their group and come to a consensus.