- Books Name
- CBSE Class 7 Science Book

- Publication
- Param Publication

- Course
- CBSE Class 7

- Subject
- Science

**Measuring Speed :**

After knowing the measurement of time and distance we can measure speed of a given object.

If we roll a ball on the ground and note the distance covered in given time we can easily measure the speed of the object.

For example, let a ball covers 10 meter in 5 sec. than the speed will be

There are many devices by which speed can be measured directly one of them is speedometer, which is fitted on the top of a scooter, or a motor cycle, car etc.

There is another device, which measures the distance moved by the vehicle called an odometer.

**LET US RECAPITULATE**

**1. Measurement of time:**

(i) For measuring the time, our ancestors used sundials, water clocks and sand clocks.

(ii) The watches (or clocks) used today make use of some periodic motion. One of the most well-known periodic motions is that of a simple pendulum, e.g., Wall clock, table clock, Digital clock etc.

**2. Simple Pendulum: **

(i) A simple pendulum consists of a small metallic ball called bob, suspended from a rigid stand by a thread.

(ii) When the bob of the pendulum is released after taking it slightly to one side, it begins to move to and fro. The to and fro motion of a simple pendulum is an example of an oscillatory or a periodic motion.

(iii) One oscillation: The pendulum has said to have completed one oscillation when its bob moves from one extreme position to the another extreme position and comes back to first extreme position.

(iv) Time period: Time taken by the pendulum to complete one oscillation is called its time period.

**3. Units of time and speed :**

(i) The basic unit of time is second. Its symbol is's'.

(ii) The basic unit of speed is m/s.

However, it could be expressed in other units also such as m/min or km/h.

4. Speedometer: It is a device to record the speed (of vehicle). It records the speed directly in km/h.

5. Odometer: It is a device to measure the distance moved by the vehicle.

6. Graphical representation of data: We can represent the data in pictorial form by three ways:

(i) Bar graph (ii) Pie chart (iii) Line graph

7. Motion of an object can be presented in pictorial form by their distance-time graph. The distance-time graph for the motion of an object moving with a constant speed is a straight line.

8. Speed of objects help us to decide which one is moving faster than the other.

9. A pendulum of about 25 cm length takes exactly one second to complete one oscillation.

10. The average speed of a Fox - 42 mph, Lion - 50 mph, Horse - 55 mph, Cheetah - 70-80 mph

(mph- miles per hour)

**KEY WORDS**

**1. Bar graph:** A graph in which quantities are represented in form of bars of different heights.

**2. Graphs: **It is mathematical pictorial representation of the relation between two inter-dependent physical quantities.

**3. Non-uniform motion: **If a body covers unequal distances in equal intervals of time or equal distances in unequal intervals of time, then it is said to be in a non-uniform motion.

**4. Oscillatory motion: **The to and fro motion of a simple pendulum about its mean position is called oscillatory motion.

**5. Speed: **It is defined as the distance travelled per unit time. Thus

**6. Time period:** Time taken by the pendulum for one oscillation is called time period.

**7. Uniform motion: **It can be defined as the motion in which a body travels equal distances in equal intervals of time.

**8. Unit of time: **The S.I. unit of time is second.