Speed

Many objects in the world around us are moving. To compare how fast they are moving, we need to know their speed. You may have observed the speedometer in motor cars which tells us how fast the car is travelling. Usually, the Odometer in a vehicle shows the distance travelled in kilometers and the Speedometer the speed of the journey in kilometers per hour. Look at the speedometers of two vehicles shown here and decide which vehicle is slower.

Car-1                Car-2

Car-1 is slower.

Activity:

Travel on a motor vehicle with your father to market or any other place and observe the changes in speedometer reading. Note the exact time when you started from home and observe speedometer reading carefully and note it and the corresponding time in the table. Some examples are given in the table.

a) Does the speed of the vehicle remain the same throughout the journey? 

b) If not, what can you say about the speed of the vehicle during the journey?

Ans: 

1) We experience in our everyday life that most of the objects in motion do not have the same speed throughout the journey.

2) To express the speed of the object, we consider its average speed.

a) The speed of the vehicle is not the same throughout the journey.

b) If such is the case, we consider its average speed.

How to calculate the average speed?

Average speed = Total distance travelled / Total time taken to travel the distance

Units of speed:

1. Depending on the need and context, speed is measured in different units.

2. Unit of speed in S.I. the system is meter per second (m/s)

3. Another unit commonly used for speed is kilometer per hour (Km/h)

Example:

1 Km / h = 5/18 m/s

1 km = 1000 m

1 h = 3600s

1 km / h = 1000 m / 3600s

= 5/18 m/s

Consider a car driven on a road. A person seated beside the driver recorded the distance travelled after every 10 minutes by noting the distance reading in the Odometer. The distance travelled by car at different instances of time is as follows.

a) What is the total distance travelled by car? 

b) What is the time taken to travel the distance?

c) How do you find the speed of the car?

d)  Is the speed of the car uniform throughout the journey?

Ans: 

a) Total distance travelled by car = 60 km

b) Time taken = 40 min

c) To find the speed of the car in the journey we should calculate the total distance travelled by car and the total time taken to cover that distance.

Total distance travelled by car = 60 km

Time taken = 40 min.

The distance must be either in meters or kilometers and time in seconds or hours. We express the speed either in m/s or in km/h. In this example distance is 60 km and time is 40 min.

1 hour = 60 min

40 min = $$\frac{40}{60}$$ hour $$\frac{2}{3}$$ h

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

= $$\frac{60 km}{\frac{2}{3}h}$$ = $$60 \times \frac{3}{2}$$ km/h.

= 90 km/h.

Thus, the car travelled with an average speed of 90 km/h.

d) From the table, we notice that the car has covered unequal distances in equal intervals of time (10 min), which shows the speed of the car is not uniform. 

The speed of a bus is 72 km/h, whereas the speed of a car is 12.5 m/s. Which vehicle moves faster?

Ans:

The speed of the bus = 72 km/h. 

                               = $$72 \times \frac{5}{18}$$ km/h = 20 m/s.

The speed of the car = 12.5 m/s

As the speed of the bus is more than the speed of the car, the bus moves faster.

Note: To compare two speeds. They must be expressed in the same units. In the above example, the speed of a bus is expressed in km/ h and the speed of a car in m/s, to compare these speeds, one of them must be converted to others.
Speed of a bus = 72 km/h = 20 m/s.