Introduction
The law of conservation of energy tells us that energy can neither be created nor destroyed. Instead, it changes from one form of energy to another. Potential energy is energy that is stored in an object. Potential energy can transfer into other forms of energy, like kinetic energy. Kinetic energy is energy in an object because of its motion.
For example, a rubber band that is stretched has elastic potential energy, because when released, the rubber band will spring back toward its resting state, transferring the potential energy to kinetic energy in the process.
NGSS Alignment: MS-PS2-3
The disciplinary core idea behind this standard is PS3.B: Conservation of Energy and Energy Transfer. It specifically looks at when the motion of energy of an object changes, there is some other change in energy at the same time. In the investigation, students will also examine Crosscutting Concepts in Energy and Matter, specifically that energy has many different forms. By building rubber band carts students can relate the elastic potential energy stored in the rubber bands and the kinetic energy of the carts as determined by the velocity of the cart measured with a PocketLab Voyager or PocketLab One. They will see evidence of energy transfer from elastic potential energy to kinetic energy when the cart is released. The speed of the cart will continue to increase as the rubber band unwinds. The rubber bands keep transferring energy to the movement of the cart causing the speed to continue to increase. Once the rubber band is done unwinding, the cart will slow down due to the kinetic energy of the cart transferring to dissipated/thermal energy from friction.
MS-PS2-2: Construct, use, and present arguments to support the claim that when the kinetic energy of an object changes, energy is transferred to or from the object.
The standard is broken down into the three NGSS pillars below:
Science and Engineering Practices - Engaging in Argument from Evidence
Disciplinary Core Ideas - PS2.B Conservation of Energy and Energy Transfer
Crosscutting Concepts - Energy and Matter
Objective
In this exploration students will:
- Gather evidence and data to support the Law of Conservation of Energy
- Collect data to calculate the amount of energy in a system at different moments
- Determine how the total energy in a system is broken up into different types of energy at different moments and how that is related to the law of conservation of energy.
- Determine how the rubber band cart affects elastic potential energy and kinetic energy as the rubber band winds and unwinds.
Materials
1. Sensor: PocketLab Voyager or PocketLab One
2. Cart: Rubber band powered cart
3. Scale
4. Measuring tape or meter stick
Formulas and Key Words
Define the following:
| Elastic Potential Energy (EPE) Kinetic Energy (KE) Velocity (v) Law of Conservation of Energy Total Energy (TE) Thermal Energy (Therm E) Friction |
Formulas for lab: KE = ½ mv^2 |
Hypothesis
Write a prediction to answer the following question: How will the cart’s EPE, KE, TE, and Velocity be affected as you wind the rubber band? How does it change as you redesign the cart? Explain your hypothesis with either background knowledge about energy, forces, and motion and/or information gathered in the introduction.
Procedure
To measure position and velocity, use either the VelocityLab App (PocketLab One or PocketLab Voyager) or the infrared rangefinder sensor (PocketLab Voyager only).
VelocityLab App (PocketLab Voyager or PocketLab One)
Follow the steps below:
- Go to the VelocityLab Web App (in a Chrome browser) using the following address: thepocketlab.com/app and on the first menu click on the "Connect to VelocityLab" button. On iOS, use the VelocityLab app available in the App Store.
- Turn on the PocketLab Voyager by clicking the button on the top.
- For instruction on how to use the PocketLab Web App go here
- Follow the prompts on the VelocityLab setup wizard.
- Click on the "Change Graph" icon. Click to view "Position" and "Velocity".
Infrared Rangefinder (PocketLab Voyager)
Follow the steps below:
- Go to the PocketLab Web App (in a Chrome browser) using the following address: thepocketlab.com/app or open up the PocketLab mobile app.
- For instruction on how to use the PocketLab Web App go here.
- Click on the "Change Graph" icon. Click "Rangefinder" and "Rangefinder Velocity".
- When setting up your cart and ramp, you'll need to make sure the rangefinder has a "wall" (a cardboard box will do) at the bottom of the ramp to give the rangefinder a clear signal.
Part A: Exploring Elastic Potential Energy
• Design a rubber band race cart using the Teacher Geek instructions. Attach the PocketLab to the wheel of the cart and connect it to the VelocityLab app.
• Select only the velocity graph. Zero the position, and begin recording data. Wind the cart and let it go.
• Observe the cart’s velocity as it travels.
• How far did the cart travel?
• What was the cart’s top speed?
• How long did it take for the cart to get to its top speed?
• At what distance did the cart reach its top speed?
• Use the formula for Kinetic Energy to fill in Table 1. You’ll need to find the cart’s mass to do this.
| Location of Cart | Start of run | 1/4 into run | 1/2 into run | 3/4 into run | End of run |
| Distance | |||||
| Velocity | |||||
| Kinetic Energy |
Mass of cart:
• Where did the kinetic energy come from, and where did it go? Explain your answer.
Part B: Engineering Challenge
Design three different rubber band carts that each have their own speciality. More details on these designs can be found in the Teacher Geek Rubber Band Racer manual. After designing each cart, test it to determine its velocity and kinetic energy at different points during a run. Fill in the data tables provided. Think critically about how the data you are collecting is changing based on the design and therefore elastic potential energy stored in the cart and then answer the questions at the end.
Cart 1: Extreme Distance
The goal of the Extreme Distance cart is to travel as far as possible.Top speed doesn’t matter in this design. Read about using rubber bands in a series or in parallel in the Teacher Geek Rubber Band Racer manual for more help.
Draw a diagram of your design, test it out, and fill in Table 2 below.
| Location of Cart | Start of run | 1/4 into run | 1/2 into run | 3/4 into run | End of run |
| Distance | |||||
| Velocity | |||||
| Kinetic Energy |
Mass of cart:
• What do you notice that is different between the data collected with the Extreme Distance cart compared to
the original cart you tested?
• How does the data you collected (distance, velocity, kinetic energy) relate to the design of this cart and the
way the elastic potential energy was stored in the rubber bands?
Cart 2: Dragster
The goal of the Dragster is to get the top speed of the cart as fast as possible. Distance doesn’t matter in this design. Read about using rubber bands in a series or in parallel in the Teacher Geek Rubber Band Racer manual for more help. Draw a diagram of your design, test it out, and fll in Table 3 below .
| Location of Cart | Start of run | 1/4 into run | 1/2 into run | 3/4 into run | End of run |
| Distance | |||||
| Velocity | |||||
| Kinetic Energy |
Mass of cart:
• What do you notice that is different between the data collected with the Dragster cart compared to the
Extreme Distance and the original cart you tested?
• How does the data you collected (distance, velocity, kinetic energy) relate to the design of this cart and the
way the elastic potential energy was stored in the rubber bands?
Cart 3: Precision Stop/Shuffleboard
The goal of the Precision Stop/Shuffleboard cart is to travel as close to a specific distance as possible. To find your groups distance goal, average the total distance all three of your previous carts traveled. Design your cart to travel that exact distance. Draw a diagram of your design, test it out, and fill in Table 4 below.
| Location of Cart | Start of run | 1/4 into run | 1/2 into run | 3/4 into run | End of run |
| Distance | |||||
| Velocity | |||||
| Kinetic Energy |
Mass of cart:
• What do you notice that is different between the data collected with the Precision Stop/Shuffleboard cart
compared to the Extreme Distance cart, the Dragster cart, and the original cart you tested?
• How does the data you collected (distance, velocity, kinetic energy) relate to the design of this cart and the
way the elastic potential energy was stored in the rubber bands?
Conclusion
• For the rubber band carts to move they need kinetic energy. Where did the kinetic energy come from in each
cart? Why did the carts all eventually stop? Explain in terms of kinetic energy, conservation of energy, and
energy transfer.
• How did the design of your carts change as the objective for each cart changed? How did those changes in
design affect the ways in which the potential energy in the carts was stored? Explain.
• When the kinetic energy of any object changes explain where that energy is coming from, where it is going,
and how that relates to the law of conservation of energy.
Download PDF for complete lab activity
