Mechanical Energy and Mass

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Objective

SWBAT identify the relationship between mechanical energy and mass by completing a hands on lab activity.

Big Idea

As mass increases, so does mechanical energy. Students will find this out through inquiry and the use of cars and ramps.

Engage

5 minutes

This lesson builds upon a previous understanding of mechanical energy as the energy of an object's motion.  Students will determine the distinction between kinetic and potential and how they relate to an object's mass. The standard MS-PS3-1 is addressed during the EVALUATE section of the lesson when students are asked to reflect on their data and determine the relationship between mass and mechanical energy.

For the first 5 minutes of class, students will answer the following question in their student notes sheet:

Which will move faster down a ramp, a car with a mass of 50g or a car with a mass of 100g? Explain your answer.

As the students are thinking and writing, I am moving around with 2 different cars, one that I claim has a mass of 50g and one with a claimed mass of 100g.  This gives the kids a visual for that they are analyzing.  I also have a sample ramp on the table they can see should they not know what a ramp looks like.

After 3-4 minutes, I ask one or two students to share their answers with the class. Here are some sample student responses from students. 

Explore

5 minutes

For this section, I have the students observe an online simulation of a ramp and various objects with different weights/masses. This website (PhET) is amazing and offers various other simulations, as well. 

This demonstration explains how to use the simulation and exactly what to do in order to show the students mass and mechanical energy levels. 

  

 I ask questions which we answer as a class:

  • Which has more mass, the ball or the skater? How do you know that?
  • What do you notice about the levels of potential energy?
  • What will happen when we let each of them move down the ramp? Which will go faster? Which will go slower?

Explain

15 minutes

Now, the students work independently to answer the following questions based on the simulation.

Based on what you’ve seen so far, which car will move faster down the ramp, the one weighing 50g or the one weighing 100g? Why?  Use must use the evidence from the skateboard simulation.

  1. Which has more potential energy? The ball or the person? Why?

  1. Which has more kinetic energy? The ball or the person? Why?

  1. When does the person have the most potential energy?

  1. When does the person have the most kinetic energy?

If the students are struggling, I pull up the demo and move through the questions one-by-one, showing them on the demo and student responses recorded (rather than shared) in their notes. After they have answered, the students will share their responses with one another at their tables. 

Elaborate

30 minutes

Now comes the fun part! Students collect data about mass and mechanical energy using the following materials: 1 ramp, 1 plastic cart, 3 weights, and a timer.

I use materials from the 8th grade Foss kits for force and motion. I imagine you can use any ramp (ruler and book set up) and any car as long as the car can move freely down the ramp and you can attach weights to the car.  

Students follow the procedure and complete the data charts in the notes sheet independently.  Sometimes I stop the class and make sure everyone is on the same page, but the procedure is simple enough that the students generally are able to follow it.  Before the students start on their own, I do show them a quick demo of putting the car on the ramp at the top release point.  Points to emphasize: You must release it from the same spot every time.  You do not give it a push. (This could be a good place to ask why. Reminding students of the importance of accuracy and consistency is not a bad idea in 6th grade.)

I tape the ramps to the table and measure out 30 cm from the bottom of the ramp and put a piece of tape at that point, marking the end of the track.  This is where the students will stop the timer. 

Evaluate

10 minutes

To close, students write a conclusion using the following prompt:

Explain the relationship between mass and mechanical energy.

When mass is increased (bigger), mechanical energy is…

My evidence for this is…

When mass is decreased (smaller), mechanical energy is…

My evidence for this is...

I ask two or three kids to share their responses and have the other kids comment, agree or disagree.