physics PAPER WORK

Lab 3: Measuring Speed

Purpose

To calculate the speed of an object from measurements of distance and time and to compare instantaneous and average speeds

Background

How long does it take you to get to school in the morning? How long would it take you to run a mile? These times depend on the speed you can travel. Speed is calculated from measurements of distance and time. The relationship between distance and time for moving objects is often expressed as average speed, which is calculated by dividing the distance traveled by the time elapsed. Over the course of time, average speed can be calculated to show the general trends in an object’s motion. This value can be quite different from the speed that an object is traveling at from moment to moment, which is called instantaneous speed.

Skills Focus

Graphing, applying concepts, predicting, interpreting data

Procedure

1. Start Virtual Physics and select Measuring Speed from the list of assignments.

The lab will open in the Mechanics laboratory.

2. The laboratory will be set up with a block on a frictionless table. Attached to the block is a plunger used to hit the block. You will measure the length of the table and the time it takes the block to slide across this distance. You will also record the force you used to hit the block and get it sliding.

One block slides across the full length of a table in a short time. Another block takes a long time to slide across the same table. How do the speeds of the two blocks compare?

3. The plunger is initially set to hit the block with a force of 78 N. Start the block sliding by clicking the Force button. When the block gets to the end of the table the experiment will stop automatically. Observe the final position of the block recorded next to r in the data display. Record this distance in Data Table 1. Also record the time it took the block to slide across the table. You will find this time in the Time display in the Detector area. Reset the experiment by clicking the blue Reset button. Repeat the experiment two more times using different force settings for the plunger. Change the plunger force using the Forces tab in the Parameters Palette. Use a weaker hit and a harder hit. Record your data in Data Table 1.

Measuring Speed 7

Data Table 1

Force (N)	Distance of slide (cm)	Time of slide (s)	Average speed (cm/s)

Analyze and Conclude

1. Graphing Plot the time and distance for the initial experiment on the graph. Label the horizontal axis Time (s). Label the vertical axis Distance (cm). You have two data points. The first data point will be (0 s, 0 cm). This is the time and place the slide began. The second data point is the time and total distance you measured. You will need to scale the graph to fit your data.

2. On the same graph, draw a different line starting at point (0 s, 0 cm) for each of the forces you used. Use different colors for each line. The lines you draw show that the block started in the same place and traveled the measured distance over a different amount of time.

8 Measuring Speed

3. Applying Concepts Each line on the graph you just completed should have a different slope. What does the slope of each line tell you about the block as it slid across the table? Think back to what you observed in the different experiments.

4. Predicting What would you predict for the slope of the line if another block took an even shorter amount of time to cross the table?

5. Interpreting Data You can calculate the slope of a line by using the following equation.

slope rise/run

In this experiment the rise is the distance the block traveled. The run is the time it took on the x-axis. From the data on the graph or in Data Table 1, calculate the average speed of the blocks and record the value in Data Table 1. Was the speed constant, or did it change throughout the experiment?

Data Table 2

Distance (cm)	Time (s)	Speed (cm/s)

6. Predicting Reset the experiment again and under Friction in the Palette, change the object material to plastic. This will turn on the table friction, to a value as if the table and object were made of plastic. How will having friction on the table affect the speed of the block?

7. Interpreting Data Open the Lab Book and click on the red Recording button to save the experimental data. Repeat the experiment and record the distance traveled, the time it took, and your calculation of the average speed below. A link will appear in the Lab Book with the position and speed data.

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8. Graphing Plot the time and speed for the experiment with friction on the graph below. Label the horizontal axis Time (s). Label the vertical axis Speed (cm/s). Use the data from the data link in the v_tot column. Plot several data points to get the general shape of the curve. This graph shows how the speed of the object changed over time. Now draw a line to show the average speed you calculated in question 7. You will need to scale the graph to fit your data.

9. Interpreting Data Why are the two lines on the graph different? What is the difference between the average speed you calculated and the instantaneous speed that you used actual data to graph? How did the average speed in this last experiment compare with the previously calculated speeds?

10 Measuring Speed

ISBN 1-269-73240-4

Measuring Speed

Measuring Speed

ISBN 1-269-73240-4

ISBN 1-269-73240-4

Measuring Speed

Measuring Speed

ISBN 1-269-73240-4