Physical science teacher Nicolas Poynter wanted his students not only to learn but also to think for themselves. His solution: a competition to build the fastest car!
To motivate my students and teach them about simple machines, electricity, experimental variables, the laws of motion and the scientific method, I developed a car-racing project, spanning the topics of the entire first block of a standard textbook for ninth-grade (aged 15) physical science in the USA (Hsu, 2005). Teams should compete to construct the fastest car, using the knowledge gained in lessons.
I felt that the usual textbook activities were too constrained: the students followed instructions as if baking a cake. I wanted to make them reason and think. The project was a complete success: not only did the students gain a deep understanding of the subjects, but their enthusiasm was strong and infectious.
Timing: Each section described below takes approximately 80 minutes: 20 minutes for the introduction, and an hour for practical work. Before starting, students should know about measurements, unit conversions and the scientific method.
I taught the class every day of the week, alternating between introductory laboratory activities and the car project: e.g. after two days of simple circuit construction, we would advance to building a circuit from scratch for the project.
Rules: In addition to a gearbox, each car should have a wooden chassis, be powered by two 1.5 volt batteries, and have a switch included in the design of the circuit.
Each group of students needs:
I bought old remote-controlled cars cheaply and then pulled wheels, axles and motors off for parts. However, anything that rolls, such as screen-door castors, can be used as wheels; many cheap, acceptable items can be found in any hardware store or on the Internet.
Students should learn the function of gearing as they construct the gearbox for their car, and understand the parallels with the gears in a real car and bicycle. Newton’s laws and gears should be covered before you start.
From the very beginning of the project, I told them the precise length of the race and the track: 10 m on a waxed floor. It is important that they know the distance they are aiming for: some cars run straight for 5 m and then fly off course.
The students should understand Newton’s second law of motion, and air resistance (drag), and translate what they have learned to the design of their car chassis.
I used a simple NERF gun which fires toy foam bullets with the same force every time. I wrapped sticky tape around some bullets to increase their mass, and had students weigh them (for some reason, it makes all the difference whether they weigh them or the teacher does). Then I asked my students how far the bullets would fly: if a bullet had twice the mass of another, it would go half as far. The law is fundamentally simple, but it is also vital to winning this car race.
I gave students the choice of oak, pine or bamboo, with pine being the lightest. Balsa would also be possible, and is even lighter. I chose bamboo, the densest and least efficient, for the prototype, in order to reward teams that selected the lightest wood, rather than those that simply copied the prototype. Weighing scales were available in the classroom. I guided the students as little as possible, but answered all their questions and ensured that they all had a choice between heavy and light wood.
When the finished cars were weighed several lessons later, I explained why pine was the best choice.
Students should attempt to make their cars lighter by choosing the right wood, cutting pieces away, and sanding their cars into more aerodynamic shapes.
Students should learn about simple electric circuits and electric motors. They should have been instructed on voltage, current and resistance, as well as simple circuits.
Students are again taught about friction. Although air resistance and sliding friction are detrimental to their cars’ speed, some rolling friction is essential for the cars to move forward at all. The goal is for the students to comprehend the balance between beneficial and detrimental friction: a paved highway is better than a gravel road because there is less friction, but if ice coats the road, the cars will not be able to function without using chains to bring back some friction.
The students should choose between tyres that supply a great deal of friction and tyres that supply very little. By pure chance, each group of my students chose a different type of tyre. Unfortunately, their cars also differed in many other ways, so it was impossible to tell precisely which tyres were best. Ideally, a smaller selection of tyres might be used, and they could be tested beforehand under controlled conditions.
For us, it was obvious which tyres did not work, but not so clear which ones worked best. Smooth tyres spun a little whereas tyres with very deep treads grabbed the road too much. Wider tyres seemed to be better, but if they were too wide, their mass became an issue. The students who picked first chose abnormally large tyres with large masses. The cars with these tyres were horribly slow, so the tyres had to be exchanged. Still, the entire class learned something from watching this happen, and such mistakes can help everyone examine the science involved. The teams were allowed to change tyres at any point.
The most common problem was cars that ran off-course and had to have their gearboxes realigned to be square with the chassis. Slow cars often needed the screws of the gearbox to be tightened properly.
A small grub screw locks the gearbox axle into place, spinning it. It readily works itself loose and must be tightened with a hex wrench, included in the gearbox kit.
Now the teams can see how they rate against each other. The fastest car recorded a speed of 2.776 m/s and had a mass of 298 g.
We had battery holders for both AA and D batteries. Two teams chose the larger batteries, incorrectly supposing that they supplied more power. During time trials, their mistake became obvious (their cars had twice the mass and half the speed of the others) and they quickly switched to the lighter AA batteries.
I allowed the fastest cars during time trials to select their lanes first and had textbooks lined up as retaining walls for the cars to stay on track. During the race, contact was allowed between the cars. In fact, it was common for cars to collide.
The two fastest cars during head-to-head racing were not at the top of the list during time trials. This turned out to be the result of a hidden variable – the orientation of the switch. Most teams copied the prototype, so the switch was flipped backwards to turn the motor on, giving the car backward momentum that needed to be overcome. Two cars had the switch reversed, giving them large head starts when pitted against other cars. This did not show in the time trials because cars were timed not from a standstill but when passing the first photogate.
This turned out to be a fantastic opportunity to teach test variables (mass of the car, aerodynamics of the body shape, detrimental friction, tyres, how well the parts were assembled) and controlled variables (chassis material, gearbox, switch, 3 V battery power). The orientation of the switch came as a surprise test variable also to me: we had to analyse data to find out why the cars that won time trials did not win the races. I figured it out, but did not tell my students. To my delight, several teams came to the same conclusion independently, which was the reason for the project in the first place – to get the students thinking.
Through this project, my students not only achieved the academic objectives but also became better problem solvers and learned practical skills that will stay with them for the rest of their lives. Although the project needs a prototype and directions, I would suggest keeping as much variability as possible. In fact, I encouraged my students to deviate from the prototype as long as they stayed within the rules. Although some of their designs were functional disasters, the students were deeply involved in the scientific process.