16 January 2014

Time and Speed : NCERT Notes

Time and Speed : Science

Moving Slowly

An object is said to be moving slowly if it covers less distance in a given time.

Moving Fast

An object is said to be moving fast if it covers more distance in a given time.
Time and Speed, CBSE Notes, NCERT Notes, Class 7, Science


The distance covered by an object in a unit time is defined as speed. Objects are said to be in fast or slow motion depending upon the speed of their motion. Speed varies from object to object, and it can be animate or inanimate. Speed is calculated using the expression
The basic unit for time is second, and it can be calculated in minutes and hours, too, depending on the need. The basic unit for distance is metre. So, the unit for speed is metres per second (m/s).
Large speeds are measured in kilometre per hour (km/h). The symbols for units are written in singular form only.
For example, the speed of the cheetah is 112 km/h, and Speed of giant tortoise = 0.27 km/h.

Average Speed

The average speed of a moving object is defined as the total distance covered by it divided by the total time taken. 


An instrument used in vehicles to show speed. Not all moving vehicles have a speedometer. The speedometer has a needle that indicates the speed.
For example, a bicycle does not have a speedometer, whereas a car has one.


An odometer is a device on vehicles to track the distance covered.

Uniform Motion

Objects that move in a straight line and maintain the same speed throughout the distance covered are said to be in uniform motion.

Non-Uniform Motion

Objects that move in straight line and whose speed varies are said to be in non-uniform motion.


A metal ball is considered to have completed one oscillation, if the pendulum moves from its mean position 'Y' and goes to 'X' to 'Z' and back to its mean position 'Y'.


There are certain events in nature that can help us track time. For example, the phases of the moon indicate the time of the month. There are man-made structures that were constructed to measure time. For example, the Jantar Mantar in Jaipur, which is basically a sun clock, is used to measure time. Water clocks and sand clocks were also used hundreds of years ago to measure time. All mechanical clocks used oscillatory motion to measure time.
The time period of a pendulum is the time taken for one full oscillation. The working of a pendulum clock is based on the time period of its pendulum. Winding clocks and wrist watches developed from modifications to the pendulum clock. A quartz clock is more accurate than other clocks because its oscillations are regulated by a quartz crystal.

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