Work Power energy class 11

Question 1

How will the momentum of a body change, if its kinetic energy is doubled ?
(H.P.S.S.C.E. 2008)

Answer 1

Ans. If E is kinetic energy of the body, then
p=V2mE
It follows that if the kinetic energy of the body is doubled, then its momentum will become /2 times.

Question 2

“Chemical, gravitational, and nuclear energies are nothing but potential energies for different types of forces in nature”. Explain this statement clearly with examples.

Answer 2

Ans. A system has potential energy, when the objects constituting the system are held at certain distance against the force between them. Chemical energy results from the chemical bonding between the atoms. Gravitational energy results, when the objects are held at some separation against the gravitational force of attraction. Nuclear energy is the result of the nuclear force between the nucleons.

Question 3

When is potential energy of a body said to be positive or negative?

Answer 3

Ans. The potential energy of a body is said to be positive, if they are held at some distance against the force of repulsion.
On the other hand, the potential energy is said to be negative, if the bodies are held against the force of attraction.

Question 4

A spring is cut into two equal halves. What is the spring constant of each portion?

Answer 4

Suppose that when a spring having spring constant k is applied a deforming force, its length increases by x. Then,
F=-k x
(i)
Here, F is the restoring force set up in the spring and it is equal and opposite to the applied deforming force.
When the spring is cut into two halves and the same deforming force is applied to each half, the increase in length will be x/2. If k’ is spring constant of the each portion of the spring, then
F=-K’ (x/2)
… (in)
From the equations (i) and (ji), it follows that
k’=2k

Question 5

Distinguish between a head-on and an oblique collision.

Answer 5

If the colliding objects move along the straight line joining their centres, the collision is said to be a head-on collision.
If the colliding objects do not move along the straight line joining their centres, the collision is said to be an oblique collision.

Question 6

Throwing mud on the wall is an example of perfectly inelastic collision. Explain.

Answer 6

Ans. A perfect inelastic collision is that in which there is a total loss of kinetic energy of the system. It will happen so, if the two colliding objects stick together and do not move at all.
Hence, throwing of mud on the wall is an example of perfectly inelastic collision.

Question 7

Explain, how fast moving neutrons can be quickly slowed down by passing through water or heavy water.

Answer 7

When an incident particle in motion strikes against a stationary target particle, the transfer of energy is maximum, when the two have the same mass*. In water or heavy water, mass of a hydrogen atom is almost the same as that of the neutron. Therefore, when fast neutrons pass through water or heavy water, they transfer most of their energy to the hydrogen atom and are slowed down.

Question 8

What type of energy is stored in the spring of a watch?

Answer 8

Potential energy

Question 9

What is spring constant of a spring? Give its SI unit.

Answer 9

The spring constant of a spring is numerically equal to the restoring force set up in the spring per unit extension.
In SI, the unit of spring constant is N m-1.

Question 10

Which spring has greater value of spring constant
-a delicate or a stout spring?

Answer 10

A stout spring.

Question 11

A spark is produced, when two stones are struck against each other. Why?

Answer 11

Work done in striking the two stones gets converted into heat energy, which appears as spark.

Question 12

Can kinetic energy be negative?

Answer 12

No

Question 13

What is gravitational unit of work?

Answer 13

In SI,
Gravitational unit of work is kg m

Question 14

What is the work done by earth’s gravitational force in keeping the moon in its orbit for its one revolution?

Answer 14

Zero. It is because the gravitational force (conservative force) is always perpendicular to the displacement.

Question 15

1.16. What is the amount of work done by
(a) a weight-lifter is holding a weight of 100 kg on his shoulders for 40 s ?
(b) the earth’s gravitational force in keeping the moon in its orbit (assumed perfectly circular)?
plane?
(c) a locomotive against gravity, if it is travelling on a level (dan electron moving with half the speed of light in empty space free of electromagnetic fields and far from all matter?

Answer 15

Ans. (a) Zero (b) Zero (c) Zero (d) Zero.

Question 16

Give one example each of the following:
(i) Zero work
(H.P.S.S.C.E. 2003)
(it) Negative work.
(H.P.S.S.C.E. 2005, 2003)

Answer 16

Ans. (i) Work done by a man carrying load on his head and walking along horizontal is zero.
(i) When a body is raised above the surface of the earth, work done by gravity is negative.

Question 17

A man rowing a boat upstream is at rest with respect to the bank. Is he doing work?

Answer 17

Ans. The man applies force for rowing the boat upstream.
Since he is at rest w.r.t. the bank, the displacement is zero.
Likewise, the work done is also zero.