Unit 5: Work, Energy and Power — Short Questions

W = FS
As S = 0, for body at rest
So W = F × 0
W = 0

Hence, work done on object that remains at rest when a force is applied is zero.

The K.E of moving body is given by the expression
K.E = 1/2 mv²

It means that K.E of body moving with same velocity depends upon mass of the body. Greater is the mass, more is K.E and vice-versa.

Hence, K.E of car is greater as compared to motor cycle due to larger mass of car.

The power is calculated by formula:
P = w/t

So, power of both forces are:
P1 = w1/t1 = 5/10 = 0.5watt
P2 = w2/t2 = 3/5 = 0.6watt

Hence force F2 will deliver greater power as clear from the above relations.

The potential energy of an object is given by expression:
P.E = mgh

This expression shows potential energy is independent of speed of object, so if a women runs up the same stairs with double velocity, then there will be no change in gain her potential energy.

See Q No. 1

See Q No. 5.

See Q No. 4.

See Q No. 11.

See Q No. 10.

See Q No. 8.

A stretched bow stores potential energy in the form of elastic potential energy when the bowstring is pulled back. When the string is released, this stored potential energy is converted into kinetic energy, transferring to the arrow and propelling it forward. Some bows can store enough energy to shoot an arrow as far as 1 km away.

According to Einstein's theory of relativity matter and energy are interchangeable under certain conditions. The loss of some mass in nuclear reactions can transform into energy and similarly energy can be converted into material particles. This leads to the conservation of mass and energy rather than the conservation of each separately.

One of the main benefits of geothermal energy is its low cost for heating. It is significantly cheaper than burning oil to power electric heaters. For example, in Iceland, more than 85% of people use geothermal energy to warm their homes, and the cost of heating is only one-third of the cost of using oil. Geothermal energy is also used in countries like Japan, Russia, Italy, New Zealand, and the USA.

The radioactive fallout from the 1986 Chernobyl nuclear accident in Russia affected people, livestock, and crops. Although only 31 people died from direct exposure, about 600,000 people were significantly exposed to the fallout.

Fossil fuels are expensive and cause pollution that harms human health. Hydroelectric energy is the cheapest and pollution free but it may cause water logging by raising the water table. Solar wind and tidal energy are pollution free though they have high initial costs. Nuclear energy is cheaper and can easily meet growing energy demands.

An ideal machine is a machine with its output equal to input and efficiency of 100%.

The expression of K.E is given by:
K.E = 1/2 mv²
K.E can never be negative because it is calculated by squaring velocity, resulting in a positive value.

As we know that kinetic energy is given by:
K.E = 1/2 mv²
For the object with velocity v:
K.E = 1/2 mv² _{_____}(1)
For the object with mass 2m and velocity 1/2v:
K.E' = 1/2 (2m) (v/2)²
K.E' = 1/2 mv² _{_____}(2)
From comparing equation (1) and equation (2)
K.E > K.E'
So, the object moving velocity v has greater kinetic energy.

The formula for K.E is:
K.E = 1/2 mv²
A car is moving on a curved road at constant speed has no change in it speed. So, it K.E will remain same.

The object is at rest and has the potential to do work but it is not currently doing any work. It means that an object having 1 J of potential energy can perform 1 J of work.

Tyre bursts on motorways are often caused by overheating, under inflation, tyre wear, road debris, excessive speed, and overloading.

The kinetic energy of cricket ball is transferred into sound energy, thermal energy, potential energy and deformation energy as it smashes the windowpanes.

w = FScos(theta) = F × 0cos(theta) = 0
When the man is at rest with respect to the shore of upstream, he is not doing any work because work can be calculated by formula.
w = FS
w = F × 0 = 0

Initial State (Top of First Hill)

• Potential Energy (PE) is maximum due to the cyclist's height.
• Kinetic Energy (KE) is zero since the cyclist is not moving. Downhill Motion
• As the cyclist rolls downhill, PE decreases.
• KE increases as the cyclist gains speed. Bottom of the Hill
• PE is minimum (near zero).
• KE is maximum due to the cyclist's speed. Upward Motion (Next Hill)
• As the cyclist climbs the next hill, KE decreases.
• PE increases as the cyclist gains height. Final State (Top of Next Hill)
• PE is maximum again.
• KE is zero since the cyclist stops at the top. In this process, the total mechanical energy (PE + KE) remains conserved, but the forms of energy are converted from PE to KE and back to PE again.

Wood is a renewable energy source as trees can be replanted and regrown, maintaining a sustainable.

See Q No.4

See Q No.6

See Q No.8

See Q No.11

(i) Hydroelectric power generations
POWER STATION
POTENTIAL ENERGY OF STORED WATER (input) → KINETIC ENERGY OF FLOWING WATER → HEAT ENERGY (due to friction) (output), WORK DONE (In running the turbine), SOUND ENERGY (NOISE) (output) → ELECTRIC ENERGY (FROM ELECTRIC GENERATOR) (output)

(ii) Electricity from fossil fuels:
chemical energy stored in fossil fuels (input) → Heat energy is used to produce steam is high pressure steam in a boiler → Potential energy of steam is used to turn a turbine → Mechanical energy from turbines is used to run an electric generator → Electricity output from an electric generator

Burning Process | Changing P.E. into Mechanical K.E. | Transferring Mechanical energy | Mechanical energy into Electric energy

Several energy conversion processes are involved in producing electricity.