# NCERT Class 11 Physics Solutions: Chapter 6 – Work, Energy, and Power Part 9

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**Question 6.21:**

The blades of a windmill sweep out a circle of area A.

(a) If the wind flows at a velocity v perpendicular to the circle, what is the mass of the air passing through it in time ?

(b) What is the kinetic energy of the air?

(c) Assume that the windmill converts of the wind’s energy into electrical energy, and that and the density of air is . What is the electrical power produced?

**Answer:**

Area of the circle swept by the windmill

Velocity of the wind

Density of air

a) If the wind flows at a velocity v perpendicular to the circle, what is the mass of the air passing through it in time:

Volume of the wind flowing through the windmill per sec

Mass of the wind flowing through the windmill per sec

Mass , of the wind flowing through the windmill in time

(b) What is the kinetic energy of the air:

Kinetic energy of air

c) Assume that the windmill converts of the wind’s energy into electrical energy, and that and the density of air is . What is the electrical power produced:

Area of the circle swept by the windmill

Velocity of the wind

Density of air,

Electric energy produced of the wind energy

Kinetic energy of air

**Question 6.22:**

A person trying to lose weight (dieter) lifts a mass, one thousand times, to a height of each time. Assume that the potential energy lost each time she lowers the mass is dissipated.

(a) How much work does she do against the gravitational force?

(b) Fat supplies of energy per which is converted to mechanical energy with a efficiency rate. How much fat will the dieter use up?

**Answer:**

(a)work does she do against the gravitational force:

Mass of the weight

Height to which the person lifts the weight,

Number of times the weight is lifted,

Work done against gravitational force:

(b) Fat supplies of energy per which is converted to mechanical energy with a efficiency rate. Fat will the dieter use up:

Energy equivalent of

Efficiency rate

Mechanical energy supplied by the person’s body:

Equivalent mass of fat lost by the dieter: