CONCEPTUAL PHYSICS

Lab Experiment/Activity 1

Graphing Motion

Go! Go! Go!

Purpose

In this experiment, you will plot a graph that represents the motion of an object.

Required Equipment and Supplies

constant velocity toy car (or wind up toy)

Butcher paper or continuous (unperforated) paper towel

access to tape

stopwatch (or watch)

meterstick

Online access

Discussion

Sometimes two quantities are related to each other, and the relationship is easy to see.  Sometimes the relationship is harder to see.  In either case, a graph of the two quantities often reveals the nature of the relationship.  In this experiment, we will plot a graph that represents the motion of a real object.

Procedure

We are going to observe the motion of the toy car.  By keeping tack of its position relative to time, we will be able to make a graph representing its motion.  To do this, we will let the car run along a length of butcher paper.  At one-second intervals, we will mark the position of the car.  This will result in several ordered pairs of data – positions at corresponding times.  We can then plot these ordered pairs to make a graph representing the motion of the car.

Step 1:  Fasten the butcher paper to the top of your table.  It should be as flat as possible, no hills or ripples.  (Note:  If you car goes fast, you may need to do this on the floor.)

Step 2:  If the speed of the toy car is adjustable, set it to the slow setting. 

Step 3:  Aim the car so that it will run the length of your table.  Turn it on and give it a few trial runs to check the alignment.

Step 4: Practice using the stopwatch.  For this experiment, the stopwatch operator needs to call our something like,”Go!” at each one-second interval.  Try it to get a sense of the one-second rhythm. 

Step 5:  Practice the task.

1. Let the car drive across the length of the butcher paper.

2. Soon after it starts, the stopwatch operator will start the stopwatch and say, “Go!”

3. Mark the location of the front or back of the car on the butcher paper every time the watch operator says, “Go!”  For the practice run, simply touch the eraser of the pencil to the butcher paper at the appropriate points.

4. The watch operator continue to call out, “Go!” once each second and the marker continues to practice marking the location of the car until the car reaches the end of the butcher paper or table. 

Step 6:  Perform the task.

1. Let the car drive across the length of the butcher paper.

2. Soon after it starts, the stopwatch operator will start the stopwatch and say, “Go!”

3. Mark the location of the front or back of the car on the butcher paper each time the watch operator says, “Go!”  No marks are to be made on the paper until the car is moving!

Step 7:  Label the marked points.  The first mark is labeled “0”, the second is labeled “1”, the third is “2”, and so on.  These labels represent the time at which the mark was made.

Step 8:  Measure the distances – in centimeters – of each point from the point labeled “0”.  (The “0” point is 0 cm from itself.)  Record the distances on the data table.  Don’t worry if you don’t have as many data points as there are spaces on the table.

Data Table

Step 9:  Make a plot of position vs. time using the online graphing tool. Go to the website http://nces.ed.gov/nceskids/createagraph/default.aspx .In the green box, “Please select a type of graph to begin”, click “Line”. Create your graph. When prompted to Print or Download, choose Download. Save as a pdf or jpeg. Upload it with your Report using the assignment tool.

1. Title the graph  “Position vs. Time.” 
2. Make the horizontal axis time and the vertical axis position.  Label the horizontal axis with the quantity’s symbol and the units of measure: “t (s)”. 
3. Label the vertical axis in a similar manner.  Make a scale on both axes starting at 0 and extending far enough so that all your data will fit within the graph. 

Step 10:  Determine the slope of the line.  Slope is often referred to as “rise over run”.  To determine the slope of your line, proceed as follows:

1. Pick two convenient points on your line.  They should be pretty far from each other.  Convenient points are those that intersect grid lines on the graph paper.
2. Extend a horizontal line to the right of the lower convenient point, and extend a vertical line downward from the upper convenient point until you have a triangle.  Using the graph paper units (not a ruler), the horizontal length is your “run” and the vertical length is your “rise”.
3. Calculate slope = rise/run = __________  cm/s

Summing Up

1.  Suppose a faster car were used in this experiment.  What would have been different about

        a.  the distance between the marks on the butcher paper?

        b.  the number of seconds the car would have spent on the butcher paper before reaching the edge?

        c.  the resulting distance vs. time graph?  (How would the slope have been different?)

2.  Suppose a slower car were used in this experiment.  What would have been different about

        a.  the distance between the marks on the butcher paper? 

        b.  the number of seconds the car would have spent on the butcher paper before reaching the edge?

        c.  the resulting distance vs. time graph?  (How would the slope have been different?)

3.  Suppose the car’s battery ran out during the run so that the car slowly came to a stop.  What would happen to the space between marks as the car slowed down?