## Rubber Band CarsBrady Connolly and Mike Deminico

### What are Rubber Band Cars?

The materials that we used to make rubber band cars were cardboard, rubber bands, CD's, duct tape and wooden skewers. For our base we cut out a small square in the cardboard and used the cardboard to hold the skewer and the skewer to hold CD tires. In order to make the wheels spin we had to feed the skewer through the corigation of the cardboard which acted as our axis. Then we used the rubber band to spin around the axis causing increased potential energy, but when we released the band the potential energy turned into kinetic energy pushing the car forward.

### Potential and Kinetic Energy

When the elastic band on the cardboard car is stretched and wound up, there is elastic potential energy added to the band, and when it is released the potential energy turns into kinetic energy. Potential energy is always associated with a force and in this case the force is provided by the elastic band. Kinetic energy is the energy of motion, thus meaning that when the elastic is released, the car begins to move, and the potential energy turns into kinetic energy.

As illustrated in the photo above, the blue boxed-in area represents the potential energy on the car. When the rubber band is wrapped around the skewer it creates potential energy. When you let the rubber band go the potential energy turns into kinetic energy pushing the car forward as the arrows show above.

### Top Four Successful Modifications

#### 1. Adding tubes of tape in the CDs' holes

This modification kept the wheels stable allowing the car to build up more speed becuase it wasn't wobbling around. The car went 8.1 feet with this modification.

#### 2. Adding rubber bands to the wheels

The rubber bands allow the wheels to have more tension than if it was just the plastic CD material. The added traction lets the car get more grip on the road and go further. The car went 6 feet with this modification.

#### 3. Moving the rubber band further back on the cardboard

This modification drastically increased the amount of elastic potential energy that could be built up by winding the rubber band around the skewer. This was due to the fact that more tension could be exerted on the rubber band becuase of the larger distance between the rubber band and the skewer. The car traveled 16 feet with this modification.

#### 4. Adding the stick onto the back and placing the rubber band onto it

Placing the stick on the back of the car lifted the cardboard off the ground, and caused less friction to be present. Also, moving the rubber band onto the stick allowed for even more elastic potential energy to be built up because of the larger amount of tension on the band. This modification allowed for our car to travel 27 feet.

### Velocity and ACceleration

#### Video Physics App

By using the video physics app we were able to not only record our car, but also caculate the velocity and the acceleration. After we took the video, we tracked the car's movements by placing dots on the car's position throughout the video. By tracking the car the app was able to create a graph of the car's velocity using the formula velocity equals change in position over change in time. Using the velocity graph, we later calculated acceleration using the formula acceleration equals change in velocity over change in time.

#### Video of Car Used for Analysis

Our video was shot at this angle so the car would be visible as it traveled across the floor for about 9 seconds. Also we shot it so that the CD was facing us so we could add points in the middle of CD as it rolled along. The meter stick was on the ground to provide a scale for the video, determining how many pixels on the screen was equivalent to one meter. Finally, by hitting the graph button, the video physics app calculates velocity based on the dots and creates the appropriate graph.

#### Velocity

This graph displays our car's velocity from the video above. The x axis is velocity, which is basically the car's speed, and the y axis is time in seconds. As the video starts, the car begins to speed up at around 2.5 seconds in, and reaches its maximum velocity of about 1 meter per second 5 seconds in. Besides a small increase in velocity after 5 seconds of the car rolling, the cars velocity begins to decrease. This is because the cars kinetic energy lessens, causing the cars speed to decrease as the test advances past 5 seconds.

#### Acceleration

To calculate the acceleration of the car we began by looking at he acceleration formula which is change in velocity over change in time. In order to plug in the numbers for velocity and time we chose two points from the velocity graph shown above. The two points we chose were 0.55 meters per second at 3 seconds and 1.05 meters per second at 5 seconds. After plugging these points into the equation we came to the solution that the car's acceleration equals 0.25 meters pers seconds squared. The reason why seconds has an exponent is because you're dividing velocity by time which requires you to multiply meters over seconds by one over seconds, causing the seconds to be squared.

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