Class 11 work energy and power notes
Answers
by Neepur Garg April 22, 20198 min read
Class Notes
84
SHARES
Work
When a force acts on an object and the object actually moves in the direction of force, then the work is said to be done by the force.
Work done by the force is equal to the product of the force and the displacement of the object in the direction of force.
If under a constant force F the object displaced through a distance s, then work done by the force
W = F * s = F s cos θ
where a is the smaller angle between F and s.
Work is a scalar quantity, Its S1 unit is joule and CGS unit is erg.
∴ 1 joule = 107 erg
Its dimensional formula is [ML2T-2].
Work done by a force is zero, if
(a) body is not displaced actually, i.e., s = 0
(b) body is displaced perpendicular to the direction of force, i.e.,
θ = 90°
Work done by a force is positive if angle between F and s is acute angle.
Work done by a force is negative if angle between F and s is obtuse angle.
Work done by a constant force depends only on the initial and final Positions and not on the actual path followed between initial and final positions.
Work done in different conditions
(i) Work done by a variable force is given by
W = ∫ F * ds
It is equal to the area under the force-displacement graph along with proper sign.
CBSE Class 11 Physics Notes Work, Power and Energy
Work done = Area ABCDA
(ii) Work done in displacing any body under the action of a number of forces is equal to the work done by the resultant force.
(iii) In equilibrium (static or dynamic), the resultant force is zero therefore resultant work done is zero.
(iv) If work done by a force during a rough trip of a system is zero, then the force is conservative, otherwise it is called non-conservative force.
Gravitational force, electrostatic force, magnetic force, etc are conservative forces. All the central forces are conservative forces.
Frictional force, viscous force, etc are non-conservative forces.
(v) Work done by the force of gravity on a particle of mass m is given by W = mgh
where g is acceleration due to gravity and h is height through particle one displaced.
(vi) Work done in compressing or stretching a spring is given by
W = 1 / 2 kx2
where k is spring constant and x is displacement from mean position.
(vii) When on end of a spring is attached to a fixed vertical support and a block attached to the free end moves on a horizontal
table from x = x1 to x = x2 then W = 1 / 2 k (x2x2 – x2x1)
(viii) Work done by the couple for an angular displacement θ is given by W = i * θ
where i is the torque of the couple.
power
The time rate of work done by a body is called its power.
Power = Rate of doing work = Work done / Time taken
If under a constant force F a body is displaced through a distance s in time t, the power
p = W / t = F * s / t
But s / t = v ; uniform velocity with which body is displaced.
∴ P = F * v = F v cos θ
where θ is the smaller angle between F and v.
power is a scalar quantity. Its S1 unit is watt and its dimensional formula is [ML2T-3].
Its other units are kilowatt and horse power,
1 kilowatt = 1000 watt
1 horse power = 746 watt
Energy
Energy of a body is its capacity of doing work.
It is a scalar quantity.
Its S1 unit is joule and CGS unit is erg. Its dimensional formula is [ML3T-3].
There are several types of energies, such as mechanical energy (kinetic energy and potential energy), chemical energy, light energy, heat energy, sound energy, nuclear energy, electric energy etc.
Mechanical Energy
The sum of kinetic and potential energies at any point remains constant throughout the motion. It does not depend upon time. This is known as law of conservation of mechanical energy.
Mechanical energy is of two types:
1. Kinetic Energy
The energy possessed by any object by virtue of its motion is called its kinetic energy.
Kinetic energy of an object is given by
k = 1 / 2 mv2 = p2 / 2m
where m = mass of the object, U = velocity of the object and p = mv = momentum of the object.
2. Potential Energy
The energy possessed by any object by virtue of its position or configuration is called its potential energy.
There are three important types of potential energies: