Question

Summarize the chapter work and energy in two pages in notebook

Class9​

Answer 1

Answer:

Sign Conventions for Work Done

when both the force and the displacement are in the same direction, positive work is done.

W = F x s

when force acts in a direction opposite to the direction of displacement, the work done is negative.

W= − F x s

Angle between force and displacement is 180o.

If force and displacement are inclined at an angle less than 180o, then work done is given as:

W= Fs cosθ

If force and displacement act at an angle of 90° then work done is zero.Necessary Conditions for Work to be done

Two conditions need to be satisfied for work to be done:

Force should act on the object.

Object must be displaced.

Energy

The capacity of a body to do work is called the energy of the body.

Unit of energy = Joules

1KJ = 1000 J

Forms of Energy

The various forms of energy are potential energy, kinetic energy, heat energy, chemical energy, electrical energy and light energy.Kinetic Energy

It is the energy possessed by a body due to its motion. Kinetic energy of an object increases with its speed.

Kinetic energy of body moving with a certain velocity = work done on it to make it acquire that velocity

Derivation

Let an object of mass m, starts from rest and attains a uniform velocity v, after a force F is applied on it. Let during this period the object be be displaced by distance s.Potential Energy

The energy possessed by a body due to its position or shape is called its potential energy.

For Example:

Water stored in a dam has large amount of potential energy due to its height above the ground.

A stretched rubber band possesses potential energy due to its distorted shape.

Types of Potential Energy

On the basis of position and change in shape of object, potential energy is of two type:

1. Gravitational Potential Energy:

It is the energy possessed by a body due to it position above the ground.

2. Elastic Potential Energy:

It is the energy possessed by a body due to its change in shape.

Expression for Potential Energy

The potential energy (Ep) is equal to the work done over an object of mass ‘m’ to raise it by a height ‘h’.

Thus, Ep = mgh, where g = acceleration due to gravity.

Law of Conservation of Energy

It states that energy can neither be created nor destroyed, but it can be transformed from one form to another.

The total energy before and after the transformation remains the same.

Power

PowerThe time rate of doing work is defined as power (P).

PowerThe time rate of doing work is defined as power (P).Power= work/time

PowerThe time rate of doing work is defined as power (P).Power= work/timeUnit of power

PowerThe time rate of doing work is defined as power (P).Power= work/timeUnit of powersI unit of Power is Joule per second or Js1.

PowerThe time rate of doing work is defined as power (P).Power= work/timeUnit of powersI unit of Power is Joule per second or Js1.1 watt is the power when 1J of work is done in 1s.

PowerThe time rate of doing work is defined as power (P).Power= work/timeUnit of powersI unit of Power is Joule per second or Js1.1 watt is the power when 1J of work is done in 1s.The bigger unit of power is Kilowatt and represented by kW.

Commercial unit of energy

Commercial unit of energy Commercial unit of energy is kilo watt hour (kWh)

Commercial unit of energy Commercial unit of energy is kilo watt hour (kWh) The unit kilowatt-hour means one kilowatt of power supplied for one hour.

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Answer 2

Question:⤵

Summarize the chapter work and energy in two pages in notebook. Class9

Answer:⤵

Work And Energy Class 9 CBSE Notes -

Work Done

• Work done on an object is defined as the product of the magnitude of the force acting on the body and the displacement in the direction of the force. W = F.s
• If a force acting on a body causes no displacement, the work done is 0. For example, pushing a wall.

Energy

• Energy is defined as the ability to do work. Its unit is the same as that of work.
• SI unit of energy or work = Joule (Nm) or Kgm2s−2.
• Energy has different forms: Light, heat, chemical, electrical or mechanical.
• Mechanical energy is the sum of:

(i) Kinetic energy (K.E)

(ii) Potential energy (P.E)

Kinetic Energy

• Objects in motion possess energy and can do work. This energy is called Kinetic Energy.
• F = ma.
• Also W = F.s
• ⇒ From the 2nd equation of motion v2−u2=2as,⇒ we get s = v2−u22a ⇒Substituting equation for work done by a moving body,⇒ we get W =m.a * v2−u22a

Or

• ⇒ Kinetic Energy = K.E= 12 mv2 (taking initial velocity u=0)
• When two identical bodies are in motion, the body with a higher velocity has more K.E.

Work-energy theorem

• The work-energy theorem states that the net work done by a moving body can be calculated by finding the change in KE.
• ⇒ W net = KE final − KE initial
• ⇒ Wnet= 12 m[v2−u2]

• Factors affecting kinetic energy

1. Mass
2. Velocity
3. Momentum
4. Potential Energy
5. Energy can get stored in an object when work is done on it.

• For example, stretching a rubber string. The energy that is possessed by a body by virtue of its configuration or change in position is known as Potential Energy.

Types of Potential Energy.

• The potential energy of an object at a height.
• When an object is raised to a certain height, work is done against gravity to change its position.
• This energy is stored as Potential Energy.
• ⇒W = F.s
• ⇒F = ma
• In the case of increasing the height, F = mg
• Therefore , W (P.E) = mgh
• ⇒ ΔPE=mg(h final−h initial)

Law of Conservation of Energy

• Law of conservation of energy states that energy can neither be created nor destroyed, but can be transferred from one form to another.
• The total energy before and after the transformation remains constant.
• Total energy = KE + PE
• For example: consider a ball falling freely from a height.
• At height h, it has only PE = mgh.
• By the time it is about to hit the ground, it has a velocity and therefore has KE= 12 mv2.
• Therefore, energy gets transferred from PE to KE, while the total energy remains the same.

Power

• The rate of doing work or the rate of transfer of energy is called power.
• It is denoted by P
• ⇒ P = Wt
• SI unit is Watt (Js−1).
• Average power = Total energy consumed/Total time taken.
• The commercial unit of power is kWh i.e. energy used in 1 hour at 1000 Joules/second.
• 1kWh=3.6×106J

Explanation:⤵

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