# Measurement Of Speed And Its Units

Every day we encounter a number of scenarios which gives us a concept of ‘fast’ and ‘slow’ motion. Let us consider the motion of a car and an airplane. We observe that the car travels slower than the plane. In turn, a bicycle travels slower than the car. Or say, the car travels faster than the bicycle. How do we compare their motion? How do we calculate exactly how fast the plane and the car travel? There is often a need to measure how much distance a body can cover in a given period of time. This is how the term speed was coined, which is the measurement of the distance covered per unit time.

### Measuring Speed:

We know that speed is given by the formula:

$Speed = \frac{Distance}{Time}$

It can be inferred from the above formula that to find out the speed of a body, we need to keep track of the distance covered by it and the time taken for the journey. Now, let us take up an activity.

### Requirements: A wool ball, a measuring tape, and a stopwatch:

• Mark a starting point A and draw a line some distance apart (say, a meter)
• Rest the wool ball at A and hold its loose end from the tip of your finger
• Roll the ball while still holding onto its tip. Start the stopwatch as soon as you roll it
• As soon as the ball reaches the line, stop the stopwatch
• Mark the point on the wool ball where it intersects the line
• With the measuring tape, measure the distance between the loose end and the point marked on the wool

You now have the distance traveled by the wool ball and time taken by it to cover that distance on the stopwatch. Using the formula, speed = distance/time, you can measure the speed. A speedometer is used to measure the speed of a vehicle.

Following is the table explaining speed related concepts:

A Speedometer

#### Units of Time and Speed:

The SI unit for the measurement of distance and time are meter and second respectively. So the SI unit for speed is meter/second.

However, for measuring larger amounts of time we have units like minutes and hours. Similarly, we often use kilometers and miles for larger distances. Depending on the usage, we use several units for speed. Some of them are:

• meters per second (m s−1 or m/s)
• kilometers per hour (km/h)
• miles per hour (mi/h or mph)
• feet per second (fps or ft/s)

Some important conversions between them:

• 1 m/s = 3.6 km/h
• 1 mph = 1.6 km/h
• 1 cm/s = 0.6 m/min
• 1 mph = 0.45 m/s