**Category : **6th Class

**Motion and Measurement of Distances**

**Physical Quantities **

The quantities like length, mass, time etc. that can be measured are called physical quantities.

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**Measurement:** It is the comparison of an unknown quantity with a certain fixed quantity of the same kind.

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**Unit of Measurement:** A unit of measurement is a definite magnitude of a physical quantity. For example metre, centimetre, kilometre etc are units of length.

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**Standard Unit:** For the sake of uniformity, scientists all over the world have accepted a system of units called SI system. In the SI System

- The unit of length is metre (m).
- The unit of mass is kilogram (kg).
- The unit of time is second (s).
- Other system used are FPS, CGS and MKS.

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**Multiples and Submultiples of Units**

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**Length **

1 centimetre (cm) =10 millimetres

1 decimetre (dm) = 10 centimetres = 100 mm

1 metre (m) = 100 centimetres

1 metre (m) = 1000 millimetres

1 kilometre (km) = 1000 metres

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**Mass **

1 gram (g) = 1000 milligrams

1 kilogram (kg) = 1000 grams

1 kilogram (kg) = 1000000 milligrams

1 quintal = 100 kilograms

1 metric ton = 1000 kilograms

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**Time**

1 minute = 60 seconds

1 hour = 60 minutes

1 day = 24 hours

1 year = days

1 century =100 years

1 millennium = 1000 years

**Motion**

If an object changes its position with respect to time, the body is said to be in motion. If an object does not change its position with respect to time. The object is said to be at rest.

**Types of Motion**

There are different types of motions. Let us know about them.

**Rectilinear Motion:** Motion in a straight path. For example, ear moving on straight road.

**Circular Motion:** Motion in a circular path. For example, toy train moving on a circular path.

**Periodic Motion:** The motion which repeats itself after a regular interval of time. For example, motion of hands of clock.

**Rotational Motion:** Spinning of an object about a fixed axis. For example, motion giant wheel and rotation of earth.

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