# NCERT Class 11 Physics Solutions: Chapter 6 – Work, Energy, and Power Part 10

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Question 6.23:

A family uses of power.

(a) Direct solar energy is incident on the horizontal surface at an average rate of per square meter. If of this energy, can be converted to useful electrical energy, how large an area is needed to supply?

(b) Compare this area to that of the roof of a typical house.

(a) Area is needed to supply:

Explanation:

Power used by the family,

Solar energy received per square meter

Efficiency of conversion from solar to electricity energy

Area required generating the desired electricity

As per the information given in the question, we have:

(b) Compare this area to that of the roof of a typical house:

The area of a solar plate required to generate of electricity is almost equivalent to the area of the roof of a building having dimensions.

Question 6.24:

A bullet of mass and horizontal speed strikes a block of wood of mass 0.4 kg and instantly comes to rest with respect to the block. The block is suspended from the ceiling by means of thin wires. Calculate the height to which the block rises. Also, estimate the amount of heat produced in the block.

Mass of the bullet,

Initial speed of the bullet,

Mass of the wooden block,

Initial speed of the wooden block,

Final speed of the system of the bullet and the block

Applying the law of conservation of momentum:

For the system of the bullet and the wooden block:

Mass of the system,

Velocity of the system

Height up to which the system rises

Applying the law of conservation of energy to this system:

Potential energy at the highest point = Kinetic energy at the lowest point

The wooden block will rise to a height of

Heat produced Kinetic energy of the bullet Kinetic energy of the system