The best way to use textbooks: to build ramps

When I was applying for my most recent teaching position part of the interview included a tour of the school with the students where the students asked me their questions and reported back to administration as to whether or not I met with their approval. I remember vividly that one of the students asked me “What do you think about textbooks?” and my answer was “They are very useful for supporting ramps for doing experiments.” Apparently the students liked that answer (and hopefully other things too) that I was hired.

My perspective today remains similar, so let’s see how those textbooks can be put to good use for working toward mastering the Next Generation Science Standard

MS PS3-1 Construct and interpret graphical displays of data to describe the relationships of kinetic energy to the mass of an object and to the speed of an object.

This particular Next Generation Science Standard doesn’t have an exact correlate within the Texas standards but, since I teach 8th grade, I use it as a review of the 6th grade objective

TEKS 6.8A compare and contrast potential and kinetic energy.

Potential and kinetic energy are another of those topics that we over simplify and, in the teacher’s “expert” mind seems easy to grasp. Potential energy = stored energy, able to perform work; kinetic energy = energy of motion. For students, though, not so easy.

The most basic form of potential energy that is obvious to students and instructionally useful is gravitational potential energy – the energy that comes from an object’s position. An object high up has the “potential” to fall down as gravity pulls the object toward a larger mass. In high school physics, students will learn to calculate gravitational potential as

PE = mgh

potential energy = mass (kg) x gravitational acceleration (9.8 m/s2) * height (m)

In theory, gravitational potential energy should convert to kinetic energy, calculated as

KE = 1/2 mv^2

kinetic energy = (1/2) * mass (kg) x (velocity squared) (m/s)

In middle school, we simplify the idea and leave out the math to talk about how the higher an object starts, the faster it goes at the end, so that is what we will do here in this activity.

We start the lesson by reviewing what students already know about potential and kinetic energy using a card sort activity derived from BrainPop (thanks Tim and Moby!).

The card sort usually reveals many student misconceptions, especially that promoted by some teachers that potential energy is energy of objects at rest. NO! Objects can be moving and still have potential energy. The sort also includes other types of potential energy besides just gravitational potential energy such as chemical energy. Thermal energy is a type of kinetic energy that is it’s own cup of tea (pun intended) so we leave it out here.

Once we have established some basics about potential energy (stored) and kinetic energy (in motion) it’s time to start the lab.

Materials needed:

• Marble (see end note about using different marbles)
• Ruler with center groove
• Styrofoam cup with 1” square cut out
• Books
• Measurement template
• Calculator (optional)
• Instructions

You’ll notice on the materials list there is a “measurement template.” This is something I created using a legal size piece of paper with markings where the book and ramp and cup should be placed AND a line with (fictitious) speed markings (see below)

In the experiment, the students roll a marble down the ramp and vary the height of the ramp. The marble will go faster the higher it starts (more kinetic energy coming from more potential energy) but, in a low resources classroom, we don’t have a way to measure the speed of the ball. Fortunately, the faster the ball is going (higher ramp) the farther the ball will push the cup when the ball hits the cup at the end of the ramp. We can use the distance that the cup moves as a substitute for the speed. So I just make up various speeds for the students to use and mark those theoretical positions on the measurement template. I feel like as long as they get the “big idea” relationship, the fact that the speed isn’t exactly perfect will be okay.

Provide students with instructions and have them perform the lab to gather data. You’ll notice that my instructions include specific lab roles:

• Materials manager – pick up and return materials, “launch” marble
• Setup supervisor  – manage lab setup, ensure correct placement of materials
• Instructor – read directions, read measurements to group
• Data Director – record data and calculate averages

I find it helpful to have students assigned to specific jobs to help everyone take ownership and participate. Data is recorded by each participant in the data table template provided on the notes page and, after the lab, students answer the conclusion questions included on that same page.

You have to do a bit more work to fully meet the Next Generation Science Standard expectations since those relate kinetic energy to mass and speed. I usually do this lesson over two days with slightly different labs each day. I also many times include the PhET simulation “Energy Skate Park” to further clarify the relationship between potential and kinetic energy.

Also, students should create a graph of the data. Graphing skills are another challenge at the middle school level and, depending on where we are in the year, I provide different levels of support. Most of the time we just graph the data as a class. Feel free to create your own version of a graph for this lesson and share it here.

NOTE….when I do this activity I give different marbles to each group. Not obviously different, just subtly different in MASS (i.e. some glass marbles, some heavy metal, some wooden, some plastic). Each group then gets different data for the “final speed.” The marbles with more mass had a higher final speed, so students also discover the relationship between mass and acceleration (at least a little bit) which can be referenced later in future lessons about Newton’s Laws.