NCERT Class 11 Physics Solutions: Chapter 6 – Work, Energy, and Power Part 7

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Question 6.14:

A molecule in a gas container hits a horizontal wall with speed and angle with the normal, and rebounds with the same speed. Is momentum conserved in the collision? Is the collision elastic or inelastic?

Yes, Collision is elastic

Explanation:

The momentum of the gas molecule remains conserved whether the collision is elastic or inelastic.

The gas molecule moves with a velocity of and strikes the stationary wall of the container, rebounding with the same speed.

It shows that the rebound velocity of the wall remains zero. Hence, the total kinetic energy of the molecule remains conserved during the collision. The given collision is an example of an elastic collision.

Question 6.15:

A pump on the ground floor of a building can pump up water to fill a tank of volume . If the tank is above the ground, and the efficiency of the pump is how much electric power is consumed by the pump?

Volume of the tank,

Time of operation,

Height of the tank,

Efficiency of the pump,

Density of water

Mass of water,

Output power can be obtained as:

For input power, efficiency is given by the relation:

Question 6.16:

Two identical ball bearings in contact with each other and resting on a frictionless table are hit head-on by another ball bearing of the same mass moving initially with a speed. If the collision is elastic, which of the following figure is a possible result after collision?

Explanation:

Case (i):

Total kinetic energy of the system after collision:

Hence, the kinetic energy of the system is not conserved in case (i).

Case (ii):

It can be observed that the total momentum before and after collision in each case is constant.

For an elastic collision, the total kinetic energy of a system remains conserved before and after collision.

For mass of each ball bearing, we can write:

Total kinetic energy of the system before collision:

Hence, the kinetic energy of the system is conserved in case (ii).

Case (iii):

Total kinetic energy of the system after collision:

Hence, the kinetic energy of the system is not conserved in case (iii).