# Motion and Time

## Motion is the change in the position of an object with respect to its surroundings. It is the distance moved by an object or its displacement. The distance moved by an object in a unit time is called its speed. The speed of an object helps to compare whether a body is moving fast or slow. Speed of an object is the ratio of the distance travelled it to the time taken to cover the particular distance. The time that a body takes to cover a particular distance is measured with the help of certain measuring instruments. All the time measuring instruments work on the principle of periodic motion. A simple pendulum represents a periodic motion. The to and fro motion performed by a simple pendulum is called oscillatory motion. The time taken by a pendulum to complete one oscillation is called its time period. Odometer and speedometer record and compare the speed of different moving vehicles. A graph is used to represent a pictorial form of such records. Most commonly used graphs are bar graphs, pie charts and line graphs. Motion of an object is represented in a pictorial form by using a line graph called distance-time graph. It represents the change in the position of an object with respect to time.

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• Q1

To calculate speed, we need

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distance and time.

##### Explanation:
To calculate speed, we need distance and time as, speed = distance covered / time taken

• Q2

The distance - time graph shown below represents

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a car moving with the variable speed.

##### Explanation:

The distance - time graph shown here represents a car moving with variable speed. As time changes by 1 s, the distance covered is not equal and hence it covers unequal distances in equal intervals of time.

• Q3

The standard unit of speed is

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m/s.

##### Explanation:

The standard unit of distance is metre (m) and of time is seconds (s).

Speed = distance covered / time taken

Thus, the standard unit of speed is m/s.

• Q4

A car moves at the speed of 60 km/hr for 15 minutes. The distance covered by the car is

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15 km.

##### Explanation:
Distance = Speed x Time
Time = 15 mins = [15 / 60] hours = [1/4] hours.
Speed=60 km/hr
So, Distance = 60 x [1/4] = 15 km
• Q5

Distance-time graph for a car moving with a constant speed is a

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