Class IX:
Science
Chapter:
Work and Energy
Chapter
notes
Key
Learning:
1.
Work is done when force acting on a body produces
displacement in it.
2.
Work done = Force × displacement in the direction of force
3.
Work is a scalar quantity.
4.
The SI unit of work is joule (j).
5.
Work done is positive if the angle between force and
displacement is acute.
6.
Work done is negative if the angle between force and
displacement is obtuse.
7.
Work done on an object by a force would be zero if
the displacement of the object is zero.
8.
When a body moves along a circular path, the force
acts along the radius of the circular path and the motion of the body is along
the tangential direction. Therefore, the angle between the direction of motion
and the force is 90°. Hence, no work is done on a body when it moves in a
circular path.
9.
An object having a capability to do work is said to
possess energy.
10.
The energy possessed by a body by virtue of its motion
is called kinetic energy.
11.
The energy possessed by a body by virtue of its
position or change in configuration is called potential energy.
12.
Power is defined as the rate at which work is done.
13.
The change of one form of energy into another is
called transformation of energy.
14.
Law of conservation of energy states that energy can
neither be created nor be destroyed but can be transformed from one form to
another.
15. Energy
exists in nature in several forms such as kinetic energy, potential energy,
heat energy, chemical energy etc. The sum of the kinetic and potential energies
of an object is called its mechanical energy.
16. The energy used in one hour at the rate of
1kW is called 1 kW h.
Top
Formulae:
1. The amount of work done, W = FS cos θ
2. If
θ = 90°, W = FS cos 90°
=
0, as cos 90° = 0
4.
Kinetic energy of an object of mass m and moving
with velocity v
K. E. = 1 mv2 2
4.
Gravitational potential energy of an object of mass
m at height h
P. E.
= m g h
5.
Law of conservation of energy
1 mv2 + m g h = constant
2
6. Power (P) = Energy spent (E)
Time taken
(t)
7.
1 watt = 1 joule/second or 1 W = 1 J / s
8.
1 kilowatt = 1000 watts
9.
1 kW = 1000 W
10.
1 kW = 1000 J / s
11.
1 KWh = 3.6 x 106 J
12.
1 hp = 746 W
= 0.746 KW
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