Question

class 9 work and energy. i could'nt understand anything. Kindly list important points.
- Dont spam pls.
- Only main points (at least important headings)

Answer 1

Energy

We already have an idea that energy is associated closely with work and we have defined energy of a body as the capacity of the body to do work. So, an object having a capability to do work is said to possess energy. 
Definition: - The capacity of an object to do work is called energy of the object.The object which does the work loses energy and the object on which the work is done gains energy.An object that possesses energy can exert a force on another object. When this happens, energy is transferred from the former to the latter.The energy possessed by an object is measured in terms of its capacity of doing work.The unit of energy is, therefore, the same as that of work, that is, joule (J). 1 J is the energy required to do 1 joule of work.There are various forms of energy available to us for example mechanical energy (potential energy + kinetic energy), heat energy, chemical energy, electrical energy and light energy.

Kinetic Energy

Kinetic energy is the energy possessed by the body by virtue of its motionBody moving with greater velocity would posses greater K.E in comparison of the body moving with slower velocityKinetic energy of a body moving with a certain velocity is equal to the work done on it to make it acquire that velocity.Consider a body of mass m moving under the influence of constant force F. From Newton’s second law of motion
[
Where a is the acceleration of the bodyIf due to this acceleration a, velocity of the body increases from to during the displacement d then from equation of motion with constant acceleration we have
[
Or,
Using this acceleration in Newton's second law of motion

We know that Fd is the work done by the force F in moving body through distance d.
In above equation, quantity on the right hand side is called the kinetic energy of the bodyThus kinetic energy possessed by an object of mass m and moving with uniform velocity v is
If the mass of the body is doubled, its kinetic energy also gets doubled and if mass of the body is halved its kinetic energy also gets halved.If the velocity of the body is doubled its kinetic energy becomes four times and if the velocity of the body id halved, then its kinetic energy becomes one fourth.

Potential Energy

Potential energy is the energy stored in the body or a system by virtue of its position in field of force or by its configuration.Force acting on a body or system can change its Potential Energy.Few examples of bodies possessing PE are given below
i) Stretched or compressed coiled spring
ii) Water stored up at a height in the Dam possess PE
iii) Any object placed above the height H from the surface of the earth posses PEPotential energy is denoted by letter U.All bodies fall towards the earth with a constant acceleration known as acceleration due to gravity.Consider a body of mass m placed at height h above the surface of the earth as shown below in the figure.

The force acting on this body is the gravitational pull of the earth m×g which acts in the downwards direction.To lift the body above the surface of the earth, we have to do work against the force of gravity. Now, Work Done= Force × distance = m×g×h
Where,
m= mass of the body;
g= acceleration due to gravity;
h= height of the body above the reference point, say the surface of earth.
Since work done on the object is equal to an energy equal toi s gained by the object.
Here from the above equation we see that potential energy of any object at a height depends on the ground level or the zero level you choose.
One important point to note hare is that work done by gravity depends on the difference in vertical heights of the initial and final positions of the object and not on the path along which the object is moved.To understand this point consider the figure given below

Now this figure shows the block is raised from position A to position B through two different paths. If [ is the height then in both the situations work done on the object is [

Answer 2

Heya Akshu,

WORK:-

》Work done is when force is produced.

》Work done by a force on a body depends on 2 factors:-

1) Magnitude of the force
2) Distance through which the body moves.

Work done:-
》Work done in moving body is equal to product of force exerted on the body and the displacement moved by the body in the direction of force.
Work done (W.D) = Force(F) × Displacement(s)

Unit of work:-
》The unit of work is Newton metre (Nm).
S.I unit of work is Joule (J)

》When a force of 1 Newton moves a body through a distance of 1 metre in its own direction, then the work done is known as 1Joule.

Work done against gravity:-
》Work done against gravity is the amount of work done is equal to the product of weight of the body and the vertical distance through which the body is lifted.

Positive, Negative & Zero work:-

1) Postive > Work done is said to be positive when a force acts in the direction of motion of the body.

2) Negative > Work done is said to be negative when a force acts opposite to the direction of the motion of the body.

3) Zero > Work done is said to be zero when a force acts at right angle, circular path and no force acts to direction of the motion of the body.

ENERGY:-

Energy is the ability to do work. Whenever work is done, an equal amount of energy is consumed.

Unit:-
SI unit = Joule.

The energy to do 1Joule of work is called 1Joule of energy.

Kinetic energy:-

The energy of a body due to its motion is called kinetic energy.

Expression for Kinetic energy:

The kinetic energy (K.E) of a moving body is measured by the amount of work it can do before coming to rest.

Consider a body of mass "m" moving with initial velocity "u". When a force "F" is applied on it, which displaces the object through a distance "s" & it's velocity changes to "v" and it produces an acceleration "a".

We know that

v^2 - u^2 = 2as
s = v^2 - u^2/2a

W.D = F × s

[Newton's II law of motion, F = ma]

W.D =ma × v^2 - u^2/2a (or)
W.D = 1/2 m (v^2 - u^2)

When the object starts from rest, u = 0
W.D = 1/2mv^2

This work done is stored in the form of K.E
K.E = 1/2mv^2

1) If the mass of a body is doubled, K.E also gets doubled.
2) If the mass of a body is halved, K.E also gets halved.
3) If the velocity of a body is doubled, then it's K.E becomes 4x.
4) If the velocity of a body is halved, then it's K.E becomes 1/4th.

Potential Energy:-
The energy of a body due to its position or change in the shape is known as potential energy.

The energy of a body due it's position above the ground is called gravitational potential energy

The energy of a body due to a change in it's shape/size is called elastic potential energy.

Expression for Potential energy:-
Consider an object of mass "m". Let it be raised through a height of "h" from the ground. A force "F" is applied to raise it and it is equal to the weight of the object "mg".

The work done in the abject against gravity
W.D = F × s
W.D = mg × h
W.D = mgh

Thus work done is stored in the form of potential energy in the object.

P.E = mgh.

Law of conservation of energy:-
》The energy can neither be created nor can be destroyed.
(or)
》The total amount of energy remains the same although it may get converted from one form to other form.

Transformation of energy:-
The change of one form of energy into another form of energy is known as transformation of energy.

Power:-
Power is defined as the rate of doing work.
Power is scalar quantity
SI unit = watt.
》Power (P) = Work done (W.D)/Time taken (t)
》Power (P) = Energy consumed (E)/Time taken (t)

1 watt is the work done at the rate of 1 joule per second.
1 watt is the energy consumed at the 1 joule per second.

Commercial unit of energy:- (1 kWh)
1 kWh is the energy used in the one hour at the rate of 1000J/s

One kWh is the amount of electrical energy consumed when an electrical appliances having a power rating of 1kilowatt is used for one hour.

I guess I have covered all the topics.

Hope my answer helps you :)

Regards,
Shobana
^_^

Further clarification can be clarified by prasanna xD.