#### How a car speedometer works

Speed is the measurement of distance over time. But a car speedometer doesn’t actually measure how fast you travel from Point A to Point B. Car speedos usually work by measuring rotation of the car’s driveshaft, axle or wheel. They then use some basic maths to extrapolate that rotation and determine how fast you are travelling. It’s a very similar concept to a bicycle speedometer.

However, if the diameter of the wheel/tyre alters, the extrapolation calculation will be incorrect. For example, the diameter will increase if you put new tyres on the car (more tread, which wears down over thousands of miles) or increase the tyre pressure. This means that, for each revolution of the wheel, the car is travelling further, meaning your speed is greater.

If the diameter decreases (eg – worn tyres, less air in the tyres, a different brand of tyre with slightly different dimensions, more load in the car weighing it down and compressing the tyres), then the car will be travelling a shorter distance for each revolution of the wheel, therefore you will be going slower.

**Student Furnished Resources**

- Stopwatch (cellphone)

**Instructor Furnished Resources**

- Flagging
- Tape measure
- Vehicle

The following procedure list may not be all inclusive…

- Determine Roles
- Timer – Starts and Stops a stopwatch / Records Time
- Starter – Signals Timer when to Start stopwatch
- Finisher – Signals Timer when to Stop stopwatch
- Digital Documentation – Records video, takes photographs and in any other way documents the lab.

- Create Course
- 100 Meter course (predetermined by instructor)

- Starter locates to the point on the course that the vehicle will enter.

Finisher locates to the point on the course that the vehicle will exit.

Timer locates to a point where they have clear line of sight with both the starter and finisher. - Driver accelerates to predetermined speed, enters course, maintains speed while on course, exits course and then resets for next trial.
- When the vehicle passes the start position the Starter signals the Timer to start their stopwatch.
- When the vehicle passes the finish position the Finisher signals the Timer to stop the stopwatch.
- The Timer records the amount of time that elapsed while the vehicle was on the course.

The following data table needs to be filled out. The third column contains the data from the actual experiment. The fourth column is determined by dividing the distance covered by the time elapsed. The fifth column requires dimensional analysis to convert m/s to mph.

When the car is travelling at | The Car Covered |
In this many seconds |
This means it was travelling (m/s) | Which converts to (mph) |

5 mph | 100 m | |||

10 mph | 100 m | |||

15 mph | 100 m | |||

20 mph | 100 m | |||

25 mph | 100 m |

You will be creating a line graph which shows Time Elapsed (dependent variable) vs Miles per Hour (independent variable).

Your graph will have 2 lines.

- One line will represent the theoretical. You will need to determine how much time it SHOULD take for a car to cover 100 meters given a certain speed.
- One line will represent the observed

You should review the rules I created for making a graph

- The first paragraph will contain the following
- Describe the purpose of the investigation
- Describe new terms which were relevant to understanding

- The second paragraph will contain the following
- Restate the hypothesis and the reason for the prediction
- Summarize the lab procedures
- Explain the setup / Control Variables

- The third paragraph will contain the following
- Describe the outcome of the experiment and how it relates to the hypothesis.
- Refer to Data / Graphs
- Be specific
- What questions does this lab present?