Motion on A Straight Line, Dr. Erkal, September 2001(cerkal@utm.edu, 587-7432, www.utm.edu/~cerkal/cahit.htm)

Objective: To introduce description of straight line motion using stop watches (or digital wrist watch with timer). To integrate skills developed in math class (algebraic operations, graphing data, table representation of data, averaging).

Material: Measuring tape or meter stick, bowling ball (can substitute basketball), graph paper. All of these can be obtained in a school environment. Ask the coach to give you a basketball, and have a student bring a bowling ball.

Skills practiced: algebraic operations, graphing, interpretation of graph, tabular representation of data, number line, time measurement.

Experiment:

This experiment requires about 30 feet long corridor. The ground can be carpeted or concrete. Line up students at the side of the corridor about 3 feet apart on each side. Use as many students as you like. Step back from the first student, and roll the ball between the students. When using the bowling ball, practice a few times for safety purpose to make sure that the ball will be stopped by a stopper at the other end. This could be a cardboard box positioned to catch the ball with some rags in it, or if you have a long corridor no stopper may be needed for the ball will come to a stop due to friction. Do not give any curve to the ball.

Have the students start their watched to time when the ball arrives at the first students position. This can be simultaneously done if the first student yells start when ball arrives in front of her/him, and everybody starts their watch. This should be practiced a few times. Once you get the hang of it, begin taking data after the start command, and have each student stop their watch as the ball arrives at their position. Choose another students who is not on the line to go to each students on the line and record their data on a table. This table will provide a time history of the ball’s motion. Repeat the throw several times to obtain several data sets. Note that each time you may or may not throw the ball with the same speed.

Exercise:

Have the students copy the table to their notebook. Ask them to produce a graph of position versus time. Positions will be 3,6,9,12,..in feet, and times will be , for example, 2.0, 3.2, 5.3, ..in seconds. The graph will not be a straight line due to deceleration from the friction. Bowling ball will be less affected by acceleration, giving rise to a more straight line than the basketball. Have them try both ball to compare, and ask them the reason why the two are not similar. The correct answer is not as important as an answer with good scientific thinking. Note that you need to provide graphing paper, and give them enough time to graph. This may be a good opportunity to teach them how to graph.

Student	Position(ft)	Time (s)
John	0	0
Allisa	3	2.0
Kathy	6	5.2
..	9	9.3
..	12	14.9

Questions:

1) Does the data indicate equal distance coverage at equal times? Since students are lined up at equal distances, they can answer this from the data or graph.

2) Which ball slows down faster?

3) What causes the slow down? Here an opportunity to discuss Newton’s first and second laws!