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Step 8: Go for Speed or Distance?

Picture of Go for Speed or Distance?

The basic car design works well and it can still teach students about momentum, energy transformation, and inertia. However, if you want to specialize the car for either speed or distance, then read on:

Designing for Speed

  • The car takes time to reach maximum speed because of inertia. Inertia is the tendency for an object to resist change in motion. In other words, it’s how much energy is required to move an object.
  • Heavier, more massive objects have more inertia and require more energy to start moving. In this case, any excess weight requires extra energy to move before the car can reach top speed.
  • Therefore, to make a speedy car, it must be as light as possible.
  • To that end, you can change out the idle wheels with small wheels (picture 1), or experiment with reducing some of the materials used in the frame. The drive wheels must remain large; small wheels have too little inertia, and will spin in place when the car is released instead of gripping the floor and propelling the car forward.
  • Furthermore, you can experiment with using more than 1 rubber band. More rubber bands will increase the overall amount of energy that can be stored and converted to kinetic movement, and it will increase the amount of energy released in the same period of time as just 1 rubber band. In other words, 2+ rubber bands can accelerate the car faster, however if there are too many, the torque (rotating force) will be so great that it will cause the wheels to spin in place before they can gain traction, or you may even destroy the frame.

Designing for Distance

  • The reason the car doesn't go on forever is because of friction. Friction is when two objects rub against each other and slow down, usually by converting kinetic energy to heat.
  • The car generates friction mainly in the form of the spinning dowel rubbing the inside of the straws.
  • Assuming there's no excessive friction, like a bit of hot glue stuck inside the straw, then to get the car to go farther it needs to use the stored energy more effectively. Adding more mass will do this.
  • Why? Adding more mass will generate more momentum, which is the tendency for an object to keep going in its current trajectory. For example, adding washers to the drive wheels increases its overall mass, and its potential to generate momentum (picture 2).
  • The additional momentum is harder to slow down and stop, so the small amount of unavoidable friction does not slow down the car as much as before. Therefore, the extra momentum "carries" the car a farther distance.
  • However, if it's heavier, it'll take longer to accelerate, and because momentum = speed x mass, the car must reach a high speed to generate that extra momentum.
  • To achieve greater velocity, you can use more than 1 rubber band (picture 2). Because the drive wheels are heavier, it will generate more traction and will be less likely to slip due to the extra torque.
  • Importantly, the additional mass must be added to the wheels. If it's added to the frame, then the friction between the dowels and the inside of the straws will increase. This is because the weight of the frame is resting on the dowels, so by adding mass to the frame, it will weigh heavier on the dowels and increase the friction. This will actually cause the car to perform worse than without any extra weight.

In either case, if you're facilitating this project with students, encourage experimentation and observational learning. Even if their initial ideas don't work as well as the basic example, those failures will create opportunities to learn about the physics principles at hand.