String Racers

Challenge: To design an elastic-powered string racer that can travel as far and fast as possible along a wire strung across the room.

Parameters: The racer cannot be powered by more than six rubber bands which can be no more than 1/8” width and 3” length. It cannot be constructed from manufactured kits or parts of kits. It must be student-made. It cannot damage or alter the wire in any way, or use it for propulsion. It cannot be longer than 30 centimetres in any dimension, extend above the wire more than 10 cm, or have a mass exceeding 200 grams. It must remain in one piece over the entire race.

Scoring: Your system will be scored based on the distance it travels and the average velocity it moves, as follows:

Score = (D/D_max)²(D/time)

D is the distance your racer moves. Time is how long it takes from start to the moment it comes to a halt. We will allow for up to three tries, and will use your best one for your score.

Rank will be based on score, with highest score earning highest rank.

Physics: Although not perfectly linear due to sag, we will be using this challenge to review all one-dimensional kinematics and dynamics topics from grade 11. I will detail these requirements as we proceed in class.

Extensions: As you wind elastics, you do work and store potential energy in the elastics. The amount of work can be estimated by measuring the input force (which normally increases as you wind) and the circumference of the circles your finger is moving. The elastics shorten as they twist, which complicates your design. Be careful in your propeller design as its efficiency is critical to your success. Newton’s third law helps us understand why the racer moves at all.

Help/Hints: You should try to keep your racer light, to take advantage of Newton’s second law. Friction is the enemy, as usual, so be careful to avoid sources of friction. I suggest tiny beads as bushings around the paperclip where it contacts the body of your racer. Video analysis of early attempts should help you figure out where to improve your racer.

Quiz Topics: One-Dimensional Kinematics, FBD, Newton’s Laws, Measurement, Graphing

Online Text: chapter 2, 3, 4, 5 (one dimension only)