RBSE Class 9 Science Notes Chapter 11 Work and Energy

These comprehensive RBSE Class 9 Science Notes Chapter 11 Work and Energy will give a brief overview of all the concepts.

RBSE Class 9 Science Chapter 11 Notes Work and Energy

→ Work generally means any kind of mental or physical activities but scientifically, we say that work has been done on an object when some force is being applied on the object and the object moves through a certain distance in the direction of the applied force.

→ Work is said to be done when

• an object at rest starts moving
• a moving object comes to rest
• shape of an object changes
• velocity of an object changes.

→ Following two conditions must be satisfied for mechanical work to be done :

• A force should act on an object.
• Body should be displaced.

For example – Pushing a car from rest, lifting a book from one place to another, etc. (A person holding a bag does no work on it as there is no displacement).

→ Work Done by a Constant Force :
Work done by the force is defined as the product of force and displacement.
∴ Work done (W) = Force (F) × displacement (s)
It is a scalar quantity. Its SI unit is N-m or Joule.
Work is said to be 1 joulę when a force of 1 Newton acting on an object displaces it by 1m along the line of action of the force.

→ In many cases, the movement of the object is some angle to the direction of applied force. In this situation work done is given by the product of displacement and the component of force along the direction of displacement.

∴ W = F cosθ × s
⇒ W = Fs cosθ

→ If force acts in the direction of displacement, then work is said to be positive (here, θ = 0° and cos 0° = 1). If force acts in the opposite direction of displacement, then work is said to be negative (here, θ = 180° and cos 180° = -1).

→ No work is done by a force acting at right angle to the direction of displacemnt of the object (here, θ = 90° and cos 90° = 0).

→ Energy : Anything having a capability to do work is said to possess energy. The amount of energy possessed by a body is equal to the amount of work it can do. So, the unit of energy is same as that of work (joule).
Larger unit of energy :
1 kilojoule = 1 kJ = 1000J = 10³J If an object does some work it loses its energy and if some work is done on the object it gains energy.

→ Main forms of energy are : (i) Kinetic energy (ii) Potential energy (iii) Chemical energy (iv) Electrical energy (v) Heat energy (vi) Sound energy (vii) Light energy (viii) Nuclear energy.

→ Sum of kinetic energy and potential energy of an object is known as mechanical energy. (Mechanical energy = Kinetic energy + Potential energy)

→ Kinetic Energy : The energy possessed by an object by virtue of its motion is called the kinetic energy. It is directly proportional to mass and the square of velocity. Examples : Fast moving vehicles, flowing water, blowing wind, moving bullet, all possess kinetic energy.

→ Expression for Kinetic Energy: Let us consider an object of mass m lying at rest on a surfact. Let a constant force F be applied on the object so that it can attain a velocity v after displacement s in the direction of force.

If body starts moving from rest, then u = 0 .
∴ W = mv² = K.E.
It is a scalar quantity.

→ Potential Energy : It is the energy possessed by an object by virtue of its position or configuration or change thereof. Examples : Water stored in the overhead tank, stone lying on the top of a hill, a stretched string, all possess potential energy.

→ Gravitational potential energy is the work done in raising an object from ground to a point against gravity.

→ Elastic potential energy is the energy due to configuration (change in shape and size) of an object is called its elastic potential energy.

→ Expression for Potential Energy at a Height :
Consider an object of mass m, let it be raised through a height h.
Force applied on the object,
F = m × g (upward)

∴ Work done against gravity in raising the object through a height h above the ground to upwards.

Work, W = F × h
∴ W = m × g × h = P.E.
Mechanical energy = K.E. + P.E.
= \(\frac{1}{2}\)mu² + mgh

→ Energy Transformations : The process of changing energy from one form to another form is known as energy transformation. For example – (i) In a gas stove, the gas is burnt in the combustion process, the chemical energy of gas is transformed into heat energy which is used to cook food. (ii) During photosynthesis, light energy is transformed into chemical energy. (iii) In bulb, electrical energy is transformed into heat energy and then finally into light energy.

→ Law of Conservation of Energy : According to this law, the energy can neither be created nor be destroyed, but it can only be converted from one form to another. The total energy of an isolated system must remain constant before and after transformation.

→ Conservation of Energy during free fall of a Body : Consider a ball of mass m at height h from the ground. At height h, it has potential energy = mgh and kinetic energy at height h is zero. As ball falls downwards, height h decreases, so the potential energy also decreases and kinetic energy increases. When the ball reaches to ground, potential energy becomes zero and kinetic energy = \(\frac{1}{2}\)mv².

Power : It is defined as the rate of doing work or as the rate of transfer of energy.

→ SI unit of power is W (watt) or J/s.
1 kW = 1000 W
1 W = 1000 Js^-1

→ Commercial unit of Power : We use kilowatt-hour (kWh) to measure power commercially. It is energy consumed kW in 1 hour.
1 kWh = 1000 Wh = 1000 W × 1h = 1000 W × (60 × 60)s
= 3600000 J
1 kWh = 3.6 × 10⁶ J = 1 unit