Elevators and Newton's Second Law
Lesson 6 of 10
Objective: Students will be able to identify the net force acting a person in an elevator based on his or her acceleration.
The goal of this lesson is for students to see how Newton's second law applies to the real-life situation of riding up and down in an elevator. This allows students to connect to the material and participate in higher level thought process of applying the Newton's second law model they developed from the lab (HS-PS2-1). Students engage in multiple science and engineering practices including developing and using a model for any elevator situation (SP2), using mathematics and computational thinking to find the acceleration and forces acting on a person in an elevator (SP5), and constructing explanations of what is happening to the forces and acceleration during each situation (SP6).
To begin class and the week, I have students gather in their Physics Families. This is a great way to start out the week and brings a positive culture to my classroom.
For this Physics Families activity, I have students complete a base group grid to learn more about their group members. To complete the base group grid, I have students fold a piece of paper so it has 16 boxes on it.
In the first column I ask them to write the names of all of their Physics Family members and then I have them write the three questions they need to answer in the other three columns. Before they write the questions on the paper, I ask that they tell their answer to someone else in the group who then writes in down. This ensures that everyone in the group focuses on the answer to each question.
The questions that I ask them are:
1. What do you think you will be doing in 10 years?
2. If you were able to choose one superpower, what would it be and why?
3. What is one goal you want to complete before the end of the semester?
To begin the elevator activity, I ask students to make predictions of how they feel when they are in an elevator. I am trying to help my students make connections with their prior knowledge and experiences of being on an elevator. So I have students answer the following questions individually based on their own experiences:
1. Describe the times in an elevator that you feel your "normal" weight.
2. Describe the times in an elevator that you feel heavier than your "normal" weight.
3. Describe the times in an elevator that you feel lighter than your "normal" weight.
Below is a prediction from one student:
After students have a few minutes to individually come up with their predictions. I have students share what they predict with a partner. Then I have groups share out what they think. Usually the class is split on when they think they feel heavier and when they think they feel lighter, so I have them raise their hands to see what everyone's prediction is in the class before we watch the video.
After students make their predictions, I show students a video of a student on an elevator that records their weight at different points of the elevator ride. I show them from 0:45 to 2:00 to collect the data. I make sure to tell students to watch carefully to record all of the data for each point listed on their worksheet. I show the video twice before I ask for the numbers they recorded to write on my copy of the Stages of an Elevator WS shown on the projector screen.
When they have the weight at each station, I have them go through each situation and identify if the person felt heavier or lighter based on their "normal" weight. Then I ask them to compare what they found to their what they predicted. Students give me a thumbs up if they their prediction was correct and a thumbs down if their prediction was incorrect.
Stages of an Elevator WS
After we have collected the data from the video, I walk students through two problems where we identify the directions of velocity and acceleration as well as the net force acting on the person riding the elevator and the acceleration of the elevator. The two problems are shown below.
To start each problem I have them draw the person in the elevator and identify which direction the elevator is moving because that is the direction that the velocity will be in. Then I have students calculate the weight, which will be the same for each situation because the weight does not change. Then, using the data collected from the video, I have students calculate the normal force. When they have both forces calculated I ask them to draw and quantitatively label the forces on the diagram they drew earlier. Students know how to find net force so I ask them to do that next. I remind them that they should always do the larger force minus the small force and then look at which is larger to identify the direction of the force. Since they know they direction of the net force, they can identify the direction of the acceleration since they are in the same direction. Then I ask them if we have enough information to solve for acceleration, and they find the qunatitative acceleration.
After the two problems that we do together, I give them the rest of the period to work with their partner or group at their tables to complete the remaining situations using the same process that we used on the Stages of an Elevator WS. I like to model two examples so they can figure out the process of how to solve for net force and acceleration using the data from the video and then let them try some on their own. At the end of class, I remind students that we will go over these problems as a class in a whiteboard discussion during the next lesson.