Work and Energy Class 9th Notes - Free NCERT Class 9 Science Chapter 11 Notes - Download PDF

NCERT Notes for Class 9 Science Work and Energy: Download PDF

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NCERT Notes for Class 9 Chapter 10

Work

• Not much ‘work’ despite working hard, performing physical or mental jobs is not considered work concerning this chapter.
• The scientific concept of work-

Work is done on an object when it is displaced by the action of a force.

• Work done by a constant force

Work done = force x displacement

W= Fs {F=force acting on the object, s= displacement caused due to F, W= work done on the object}

• Work done by a constant force can be given by the multiplication of the magnitude of the force and the distance moved in the direction of that force. The SI unit of work done is ‘Nm’ (Newton metre) or ‘J’ (joule). When 1 N of force is applied to displace an object by 1m, 1 J of work is said to be done.
• Work done could be either positive (force applied is in the direction of displacement) or negative (force applied is opposite to the direction of object’s displacement).

Energy

• The Sun is the ultimate source of energy for sustaining life on Earth. Energy is defined as the capacity to do work. The object which performs work will lose energy, whereas the object on which work is being performed will gain energy. Transfer of energy happens when an object having energy starts to exert force on the other object. Energy has the same SI unit as of work i.e., joule (J), this is because energy is measured as the capacity to perform work. 1J work requires an input of 1J energy.
• 1 kJ= 1000J
• Forms of energy: Energy can exist in the form of mechanical energy (potential energy + kinetic energy), heat energy, sound energy, chemical energy, and light energy and these forms of energy are interconvertible.

Kinetic energy

• It is the energy possessed by an object due to its motion, it is directly proportional to the mass of an object and is proportional to the square of the object’s velocity.
• Let there be an object of mass ‘m’, travelling with initial velocity ’u’. If a force ‘F’ acts on it to cause a displacement ‘s’, then its final velocity changes to ‘v’, and work done in it is ‘W’

W= Fs

From the equation of motion if a is the constant acceleration on a body;

v²-u²=2as

Rearranging the above equation gives

s=(v²-u²)/2a

Since, F=ma, work done can be written as,

W= ma x (v²-u²)/2a

Simplifying the above equation,

W= m(v²-u²)/2

If the object starts at rest, its initial velocity (u) =0

W= mv²/2

Since all work done on the object results in an increase in its kinetic energy,

Kinetic energy= E_k = mv²/2

where E_k= Kinetic energy of the object

m = mass of the object

v= velocity at which the object is moving

Potential energy

• Potential energy can be defined as the energy possessed by an object by the virtue of its position or configuration.
• The potential energy of an object at a height:

When an object is raised to a greater height, its energy increases. This is because work is done on it as it is being raised against gravity. The gravitational potential energy is the energy present in such an item. The work done in elevating an object from the ground to a point above the ground is defined as the gravitational potential energy of that object.

Displacement of the object is its change in elevation=h, the force required for this action is mg, where m is the mass of that object is g is the acceleration due to gravity.

W= Force x displacement

W= mg x h

W= mgh

Since all work done on the object results in an increase in its potential energy,

Potential energy = E_P = mgh

Law of conservation of energy

• It is only possible to transform energy from one form to another; it cannot be created or destroyed. Before and after the transition, the total energy of the system is the same. When an object of mass ‘m’ falls from height ‘h’ to the ground (h=0), its potential energy E_P = mgh gets converted to kinetic energy E_K = mv²/2, no energy is lost in this process.

mgh + mv²/2 = constant, mechanical energy is constant

Rate of doing work

• Power is defined as the rate of doing work or the rate of transfer of energy.
• Power = work/time

P=w/t

• SI unit of power is Watt (W)
• 1W= 1 Js^-1
• Average power is the total work divided by the total consumed time to perform that work.

Commercial unit of energy

• Since a joule is very small, a larger unit kilowatt-hour (kW h) is used in commercial applications.
• 1 kW h is defined as the energy that is utilized in one hour at the rate of 1000 Js^-1 (or 1 kW).
• 1 kW h = 3.6 x 10⁶ J
• 1 unit of electricity consumed equals 1 kWh

Work and Energy Previous year Question and Answer

Q1: Which of the following are example of mechanical energy -

1.Kinetic energy, 2.chemical energy, 3.electrical energy, 4.heat energy, 5.Potential energy

Answer:

Mechanical energy is the sum of potential energy and kinetic energy. It is the macroscopic energy associated with a system.

Q2: A ball of mass 1 kg is thrown vertically upward with velocity 2 m/s. then what is the total energy of the ball when it reaches the highest point? (Assume potential energy of a body at the ground is zero)

Answer:

- Kinetic Energy $(KE)=1/2$ * mass * velocity^2
- $\mathrm{KE}={0.5}^{\ast }1\mathrm{kg}$ * $(2\mathrm{m}/\mathrm{s}{)}^{\wedge }2$
- KE = 2 Joules
Total energy at any point:
- The total energy of the ball is the sum of its kinetic and potential energy.
- At the highest point, the ball's velocity is zero, so its kinetic energy is zero.
- The total energy at the highest point is equal to the initial kinetic energy, which is 2 Joules.

Therefore, the total energy of the ball at the highest point is 2 Joules.

Q3: If acceleration due to gravity is 10 m/s2, what will be the potential energy of a body of mass 10 kg kept at a height of 2 m? (Assume potential energy of a body at the ground is zero)

Answer:

The potential energy of a body is calculated by using the formula :

Potential energy = m × g × h=10*2*10=200J

NCERT Notes Class 9 Chapter-Wise

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