What do you think is a safe distance between the car you are riding in and the car in front of you? Would you be able to stop in time to avoid an obstacle? What are some of the factors that influence stopping distance.

• Design and conduct a series of investigations to determime how changing an object’s mass and forces acting on that object (tension, friction, etc.) affect its motion.                        

MATERIALS

        IBM-compatible computer w/ULI                        Logger Pro 2.0

        Photogate Timer w/ Super Pulley                        Support for Photogate        

2.0-m track with car guides and end stop                Wooden cart (no wheels)

Index card                                                Meterstick (teacher provided)

Part A – Techniques for Measuring Speed and Stopping Distance        

Part A1:  Introduction to the Photogate Timer

Part A2:  Using the Photogate Timer to Determine the Speed of an Object

PROCEDURE:

1. Connect the photogate into port DG1 of the ULI.
2. Place an index card in the slot on the wooden car.  Secure with a small piece of tape.
3. Attach the photogate to a stand and position the timer beside the track at the ‘Start line’.  Adjust the height of the photogate so that the index card will intercept the beam.

4. Open the folder “Investigating Car Speed vs. Stopping Distance”, then open the file,

 “Car Speed Data”.

5. When you are ready to collect data, select “COLLECT”.
6. Slowly slide the car through the photogate timer. 
7. Record the time in the space below under “Slow Run” below.
8. Repeat the procedure except this time slide the car at a faster rate through the photogate.  Record the time under “Fast Run”.

DATA:

Slow Run = _________s                        Fast Run = __________ s

What additional data would be needed to determine the speed of the car as it moved through the photogate? ________________________  Obtain this data and record in the space below.

Additional Data:

CALCULATIONS:

Calculate the average speed of the car for each run.  Show your equations, calculations, and units.

Average Speed of Slow Run:                                Average Speed of Fast Run:

Part A3:  Measuring Speed vs. Stopping Distance

Suppose you push the car along the track and release it BEFORE the index cart intercepts the photogate beam.  If the car is pushed hard enough, it will pass through the beam and come to a stop along the track.  Try this now.

Imagine that the speed of the car as it passes through the beam is similar to the speed of a real car just before a driver slams on his/her brakes.  This speed is called the ‘initial speed’ and the distance the car travels after braking is called the ‘stopping distance’.  In this case, the wheels of the car have been removed so that the apparatus models a car during a skid.

Describe a procedure to measure the stopping distance of the car after it passes through the photogate timer.  (Specifically define where the measurements should begin and end).

Part B - Student Designed Experiment

Now that you have practiced using the photogate to determine the initial speed of a wooden car and developed a procedure for measuring the stopping distance, it is time to design an experiment that will answer the following question:

How does the speed of a car affect its stopping distance?

Background Information  What have you learned from reference materials that leads you to believe that stopping distance is related to car speed?

Hypothesis  Write a hypothesis that will be useful in guiding your investigation.  Remember, a hypothesis is a tentative solution to your question.  It states the results you expect to get when you conduct the experiment.  State you hypothesis both graphically and in words below:

        d

                                                     v

Materials and Apparatus You will be provided with the materials used in Part A of the experiment.  Are there any additional materials you will need?  Who will provide them?  

Are there any changes to the apparatus used in Part A?  If so, make a labeled drawing to illustrate your design.

        

Design Your Investigation  List the steps you will follow in conducting your experiment.  

Variables        What is your independent variable?        _________________________

                What is your dependent variable?        _________________________

                What are your constants?                _________________________________

Data and Calculations  Design a table for recording your data.  Show any formulas or calculations that are used in solving your problem.  Your data table should include the number of trials you are planning to conduct.

Graph  Attach a graph of Stopping distance vs. Speed.  Your instructor will tell you if the graph is to be done on graph paper or using a computer program.

Analysis and Conclusions  Does your data support your hypothesis?  Explain your answer with evidence from your data and observations. Be sure to use the terms ‘speed’ and ‘acceleration’ in your explanation.

Possible Errors   Is is possible that you made errors in your experimental design, in setting up your experiment, or in collecting and recording data?  Please discuss these errors and explain how you would conduct the experiment if it were done again.

Other Questions  The answers obtained from an experiment often lead scientists to ask additional questions about the topic.  State at least one question about speed, acceleration, or force that arises from your investigation?

Class Results:

Record the conclusions of each of the other teams in your class.

Active Physics Transportation Chapter 1                 Name: _______________________

Activity Three:  Stopping Your Car

Post Lab Questions:

I.  Distance traveled by car during response time

You have previously studied the effects of response time on the distance traveled by a car before braking.  As a review what do you think is the effect of doubling the speed on the distance traveled during response time?  Explain.

                                                                                  

        I.                                                      II.

II.  Distance Traveled by Car During Braking

1. Select two values of speed from your graph, with one value just double the other.  When the speed of the car doubles, what happens to the stopping distance required if a driver slams on the brakes? (Provide specific evidence.)

2. Compare two stopping distances for which the second speed is three times as fast.  What is the effect of tripling the speed on the distance traveled during stopping? (Provide specific evidence).

3.          Your data can be used to explain the advantage of driving at slower speeds.

Use your data to explain what will happen to the required stopping distance if you decrease your speed by one-half?