Exact Velocity Car

Challenge:  To design a motorized car that can move across the floor at a constant velocity.

Parameters:  Mr. J will provide you with a battery pack (4 AAA 1.2V rechargeable cells) and a toy DC motor.  The battery pack cannot be built into your car as it must be returned at the end of each class.  Your vehicle should move at a constant velocity (steady speed in a straight line) between 25 cm/s and 75 cm/s.  You choose the velocity and let the judges know before you can use the track.  You cannot touch or steer your device after it leaves the starting line.  The finish line will be 500 cm away.  Your car should fit a 15 cm by 15 cm footprint area.  The car must be started from rest and must travel freely at least 10 cm before the leading edge arrives at the starting line.  A team of three will time the trip, and the average time will be used.  You will get up to three attempts but cannot change your stated velocity between attempts!

Scoring: For scoring we will take average of the three timers, for the best of your three attempts.  That will be compared against the stated velocity to find percent difference. 

% difference = 100% • | (500 cm / time - Stated velocity) / Stated velocity |

The lowest score will receive the highest rank. 

Physics:  The floor is not perfectly smooth or flat, so it is important that your motor has a large mechanical advantage (pulleys or reducing gears), and/or some rotational inertia (flywheel).  In order to maintain a constant velocity, forces acting against the motion of your car must be exactly balanced by the forward friction force on the drive wheel(s). 

Extensions:  The velocity at one moment in time is called instantaneous, while that calculated from displacement and time interval is called average.  If your vehicle moves at constant velocity the whole time, the two velocities will always be the same!

The physics of rechargeable batteries, and the physics of friction, are very complicated, but will affect your results!

Help/Hints:  Use video analysis to get accurate velocity data.  Use lots of light!  Does your vehicle’s motor turn the same rate at the beginning of a run as it does at the end?

You can measure the voltage and current to your motor, which may be different if the cells you are using are running out of charge as opposed to fresh out of the charger.  This may be a factor to consider before the competition.

Quiz Topics:  Kinematics in one dimension

Constant velocity, average velocity, instantaneous velocity.

Force Concept, Free-body diagrams, Newton’s First Law

Simple Circuitry, motor basics

Online Text:  2.1 - 2.3, 4.1, 4.2 (concepts only), 9.3, 19.1-19.4, 20.2 (just for info)