Module 4

What Causes the Cart's Motion?

Student Guide

Ted Corley, Miriam Fuhrman and Susan Wyckoff

We are accustomed to motion when we drive a car or ride a bike. Sometimes motion is constant and sometimes it speeds up. What causes these two different kinds of motion? How do we distinguish the difference between them? What do their motion graphs look like. What kind of motion does the fan cart have when the fan is turned "on" and when it is in the "off" position?

1. To investigate the motion of the fan cart and to match the distance vs. time and velocity vs. time graphs that were part of your homework.
2. To hypothesize what it is that causes the cart's motion.

The cart fan has a switch with three positions low, off, and high. If a styrofoam plate is attached so that the sonic pulse from the motion detector can be reflected from the plate, then the data you collect will be improved. What does the fan do to cause the cart to move? The fan cart is designed to be almost "frictionless." How can you test whether or not the cart is really frictionless? The other materials on your table can be used in a variety of ways. Feel free to explore.

1. Discuss with your group the results of both parts of your homework . With your group predict the velocity vs. time graphs for plots 1-10 in the homework.
2. Use the fan cart to match each of the graphs in plots 1-10 of the homework. Keep a careful record of your experiments, and describe what you did in each experiment to make the fan match plots 1-10.
3. Write a d(t) equation for each of the distance vs. time graphs you obtained to match the plots 1-10 you did for homework.
4. With your group prepare a white board presentation to answer the following questions about one graph. Your instructor will assign which graph of plots 1-10 your group should discuss. Include answers to the following questions in your white board presentation:
   • Identify each of the variables in your equation and write the units for each.
   • Identify the physical variables that cause or effect the motion of the cart.
   • Describe how the graph of d(t) and v(t) are related.
   • If you found two ways for the fan cart to match a single plot, describe your experiments in each case
   • Explain your d(t) equation.

1. If you are driving up a hill in San Francisco and come to a stop sign, describe what has to happen for you to keep your car from rolling in either direction.
   
2. At the top of the San Francisco hill, what action must you take to proceed?
   
3. Suppose you took a powerful portable fan on board your sail boat on a windless afternoon. In which direction should you aim the "wind" from the fan to cause the boat to move: toward or away from the sail? Explain your reasoning.
   
4. If you were to add weights to the fan cart in this experiment, would it accelerate faster or more slowly? Explain your reasoning.
   
5. A space ship is moving through space with its engine "on". Suddenly the pilot shuts the space ship engines "off". Sketch a distance vs. time graph for the space craft.
   
6. Racing cars going for flat-surface speed records often use parachutes at the end of their racing runs. Sketch the distance-time and velocity-time graphs you would predict for a racing car whose parachute has just opened. Does the racing car behave like the cart in any of your investigations? If so, if the cart acted like the racing car, what acted like the parachute? If not, what is different about this motion?

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Last modified 12 Aug 1997
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