## Thursday, August 18, 2011

### CBSE_ NCERT_7th Class Study notes for Chapter Motion _ Rest and Time

Motion and Rest
A body is said to be in the state of rest when it remains in the same position with respect to time.
e.g.: The position of trees around the building is not changing with respect to the building. Then the trees are at rest.
A body is said to be in the state of motion, when it continuously changes its position with respect to time
e.g.:  When we are cycling or running, we are changing position with respect to trees and buildings. we are said to be moving

DISTANCE AND DISPLACEMENT

Distance is the length of the path covered by a body. It has no sense of direction so it is a scalar quantity. Its SI unit of measurement is  meter or m

The distance between the two places is not the same; it depends upon the path chosen.

Displacement: The shortest distance, or distance travelled along a straight line, is known as displacement.
If we draw a semicircle of radius 10cm. Then,
Displacement  =  r + r = 20cm
Distance          =  2pr/2 = 10 x 22/7 cm = 31.4cm

UNIFORM MOTION AND NON UNIFORM MOTION
If an object covers equal distances in equal intervals of time, it is said to be in
uniform motion.
If an object covers unequal distance in equal intervals of time, it is said to be in
non-uniform motion.

MEASURING THE RATE OF MOTION

Speed :It is the distance travelled in one second (or) rate of distance travelled.

A car starts from Samastipur  and reaches  Patna  in 6 hour. A bus takes 8 hour to travel the same distance. Which has moved faster? Why?

The car travels faster than the bus, because it covers the distance in a short time.

When a body covers a distance in a short time, it is said to be fast. If it takes more time to cover the distance, it is said to be slow.

Hence,  Speed is the quantity used to say whether the motion is slow or fast.

Speed = Total Distance travelled/ Time taken
Speed has no sense of  the direction of motion so it is a scalar quantity

SI Units: Speed is measured in m/s (or) m/s  It can also be expressed in km/hour (or) kmh-1
Velocity : The speed with direction is known as velocity.
So to measure the velocity, you should  consider displacement instead of distance.
Velocity is the displacement made in one second (or) rate of change of displacement.
Rate of change means, change per second.
Velocity = Displacement/Time
It S I Unit is also m/s

Uniform Velocity: Equal displacement covered by a body in equal intervals of time is known as uniform velocity.

Acceleration is the change in velocity of an object per second or rate of change of velocity.
Acceleration = Change in velocity/Time taken. The unit of acceleration is m/s2 or ms-2

There are two cases of of change of velocity

If the velocity of the body increases with time, the acceleration is positive, and the kind of motion is called accelerated motion.

If the velocity of the body decreases with time, the acceleration is negative (retardation), and the motion is called decelerated motion.

Uniform Acceleration: If an object travels in a straight line and its velocity increases or decreases by equal amount in equal intervals of time, then the acceleration of the object
is uniform.

We can derive a formula for acceleration.
a= [v – u] / t
Where   u - initial velocity, v - final velocity      t – time

Distance - time graph

We can easily understand the relation between time and distance by using a graph.

Taking a suitable scale, a graph is drawn bytaking time along the x axis and distance along the y axis. The graph is known as distance – time graph.

In the graph for uniform speed is a straight line and curved for Non uniform speed.

CBSE Class 7 Science Chapter wise notes and test papers

Science deals with nature and various phenomena, which is related to our daily life. In class 7, students learn the basic concepts of physics, chemistry and biology, which build a strong foundation for higher education and the tough competition