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Chapter: 11th Physics : UNIT 4 : Work, Energy and Power

Physics : Work, Energy and Power : Book Back Important Questions, Answers, Solutions : Short Questions and Answer

Work, Energy and Power (Physics)

1. Explain how the definition of work in physics is different from general percep-tion.

● Work refers to both physical as well as mental work. In fact, any activity can generally be called as work.

● But in physics, the term work is treated as physical quantity with precise definition.

● Work is said to be done by the force when the force applied on the body displaces it.

2. Write the various types of potential energy. Explain the formulae.

i) The energy possesed by the body due to gravitational force gives rise to gravitational potential energy

u = mgh

Where, m is mass of the body

g is acceleration due to gravity

h is the height of the body above the ground.

ii) The energy due to spring force and other similar forces gives rise to elastic potential energy.

U = 1/2 k(xf2 – xi2)

Where, k is the force constant

xi is the initial position of the spring

xf is the final position of the spring

iii) The energy due to electro static force on charge gives rise to electrostatic potential energy.

U = q1q2 / 4πεor

Where, q1, q2 are the point charges

r is the distance between two point charges

εo is the permitivity of free space

εo = 8.854 × 10-12 C2N-1m-2

3. Write the differences between conservative and Non-conservative forces. Give two examples each.

Conservaive forces

1. Work done is independent of the path

2. Work done in a round trip is zero

3. Total energy remains constant

4. Work done is completely recoverable

5. Force is the negative gradient of potential

6. Examples:

i. Elastic spring force

ii. Electrostatic force

iii. Magnetic force

iv. Gravitational force

Non - Conservaive forces

1. Work done depends upon the path

2. Work done in a round trip is not zero

3. Energy is dissipated as heat energy

4. Work done is not completely recoverable

5. No such relation exists.

6. Examples:

i. Force due to air resistance

ii. Viscous force.

4. Explain the characteristics of elastic and inelastic collision.

Elastic collision

1. Total momentum is conserved

2. Total kinetic energy is conserved

3. Forces involved are conservative forces

4. Mechanical energy is not dissipated

In elastic collision

1. Total momentum is conserved

2. Total kinetic energy is not conserved

3. Forces involved are non-conservative forces

4. Mechanical energy is dissipated into heat, light sound etc.

5. Define the following

a) Coefficient of restitution

b) Power

c) Law of conservation of energy

d) loss of kinetic energy in inelastic collision.

a) Co-efficient of restitution

It is defined as the ratio of velocity of separation (relative veloctiy) after collision to the velocity of approach (relative velocity) before collision.

e = Velocity of separation (after collision) / Velocity of approach (before collision);

e = (v2−v1) / (u1−u2)

b) Power

It is defined as the rate of wrok done (or) energy delivered.

Power (P) = Work done (w) / time taken (t)

P = w / t

c) Law of conservation of energy

It states that energy can neither be created nor be destroyed. It may be transformed from one form to another but total energy of an isolated system remains constant.

d) Loss of kinetic energy in inelastic collision

● In perfect inelastic collision, the loss in kinetic energy during collision is transformed to another form of energy like sound, thermal, heat, light etc.

● Let KEi be the total kinetic enrergy before collision and KEf be the total kinetic energy after collision.

● Total kinetic energy before collision

KE1 = 1/2 m1u12 + 1/2 m2u22     ………….(1)

● Total kinetic energy after collision

KEf = 1/2 (m1+m2)v2           …………………(2)

● Then the loss of kinetic energy, ∆Q = KEi − KEf

∆Q = 1/2 m1u12 + 1/2 m2u22 − 1/2 (m1+m2)v2          ………….(3)

● Substituting equation v = [ m1u1 + m2u2 ] / (m1+m2) in equafion (3) and On simpiytying, we get loss of K.E ∆Q = 1/2 (m1m2 / [m1+m2] ) (u1–u2)2

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