Punjab 9th Physics Ch 5 Work Energy and Power Short Questions

W = F × S

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 = ½mv²

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 = W/t = 5/10 = 0.5 watt

P₂ = W/t = 3/5 = 0.6 watt

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.

Work is done when a force acts on a body and the body moves in the direction of the force.

Work = Force × Displacement in the direction of force

W = F × S

SI Unit of Work: Joule (J)

1 Joule = 1 Newton × 1 meter = 1 Nm

The potential energy of a body of mass m raised through a height h is given by:

P.E = mgh

Where:
• m = mass of the body
• g = acceleration due to gravity
• h = height through which the body is raised

Kinetic energy is the energy possessed by a body due to its motion.

Expression: K.E = ½mv²

Where:
• m = mass of the body
• v = velocity of the body

Derivation: When a force F acts on a body of mass m initially at rest, it produces acceleration a. The work done by the force is converted into kinetic energy.

W = F × S = ma × S

Using v² = u² + 2aS, with u = 0:

S = v²/2a

W = ma × (v²/2a) = ½mv²

Therefore, K.E = ½mv²

Efficiency is the ratio of useful output energy (or work) to the total input energy (or work), expressed as a percentage.

Efficiency = (Useful Output / Total Input) × 100%

A system cannot have 100% efficiency because:
• Some energy is always lost as heat due to friction
• Energy is lost in overcoming air resistance
• Sound energy is produced during operation
• Some energy is used to overcome internal resistance of the system

These losses mean that useful output is always less than total input.

Power is the rate at which work is done or energy is transferred.

Power = Work done / Time taken

P = W/t

SI Unit of Power: Watt (W)

1 Watt = 1 Joule/second = 1 J/s

Larger units:
• Kilowatt (kW) = 1000 W
• Megawatt (MW) = 10⁶ W
• Horsepower (hp) = 746 W

Renewable Energy Sources	Non-Renewable Energy Sources
Can be replenished naturally in a short time	Cannot be replenished or take millions of years to form
Environmentally friendly and produce little pollution	Cause environmental pollution and greenhouse gas emissions
Examples: Solar, Wind, Hydroelectric, Geothermal, Biomass	Examples: Coal, Oil, Natural Gas, Nuclear fuel (Uranium)
Sustainable for long-term use	Finite resources that will eventually deplete
Often have high initial setup costs but low operating costs	Lower initial costs but subject to price fluctuations

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.

Formula: E = mc²

Where E = energy, m = mass, c = speed of light

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. The accident caused long-term environmental contamination and health problems in the affected regions.

• Fossil fuels: Expensive and cause pollution that harms human health
• Hydroelectric energy: Cheapest and pollution free but may cause water logging by raising the water table
• Solar, wind and tidal energy: Pollution free though they have high initial costs
• Nuclear energy: Cheaper and can easily meet growing energy demands but produces radioactive waste

Each energy source has trade-offs between cost, environmental impact, and reliability.

An ideal machine is a machine with its output equal to input and efficiency of 100%. In an ideal machine, there are no energy losses due to friction, heat, or other factors. However, ideal machines do not exist in practice; all real machines have some energy losses.

The expression of K.E is given by:

K.E = ½mv²

K.E can never be negative because it is calculated by squaring velocity, resulting in a positive value. Mass is always positive, and velocity squared is always positive, so kinetic energy is always positive or zero (when the object is at rest).

As we know that kinetic energy is given by:

K.E = ½mv²

For the object with velocity v:
K.E₁ = ½mv² … (1)

For the object with mass 2m and velocity ½v:
K.E₂ = ½(2m)(½v)² = ½(2m)(v²/4) = ¼mv² … (2)

From comparing equation (1) and equation (2):
K.E₁ > K.E₂

So, the object moving with velocity v has greater kinetic energy.

The formula for K.E is:

K.E = ½mv²

A car moving on a curved road at constant speed has no change in its speed. Since kinetic energy depends only on mass and speed (not direction), its K.E will remain the same even though its velocity (which includes direction) is changing.

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. For example, a 1 N object raised 1 meter above the ground has 1 J of gravitational potential energy.

Tyre bursts on motorways are often caused by:
• Overheating due to high speed and friction
• Under inflation causing excessive flexing of tyre walls
• Tyre wear and aging weakening the structure
• Road debris like sharp objects puncturing the tyre
• Excessive speed generating heat beyond tyre limits
• Overloading the vehicle beyond tyre capacity

The kinetic energy of cricket ball is transferred into:
• Sound energy (the breaking sound)
• Thermal energy (heat from friction)
• Potential energy (deformation of glass)
• Deformation energy (breaking the windowpane)

The total energy is conserved but changes form from kinetic to various other forms.

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 = F × S

Since displacement S = 0 (with respect to shore),

W = F × 0 = 0

However, the man is doing work against the water current, but with respect to the shore, no displacement means no work is done.

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 cycle. However, for wood to be truly renewable:
• Trees must be replanted at the same rate they are harvested
• Sustainable forestry practices must be followed
• The time for regrowth should be reasonable compared to consumption rate

When managed properly, wood can provide a sustainable source of heat energy.

Kinetic Energy: The energy possessed by a body due to its motion is called kinetic energy.

SI Unit: Joule (J)

Determination: Kinetic energy is determined using the formula:

K.E = ½mv²

Where:
• m = mass of the body (in kg)
• v = velocity of the body (in m/s)

The kinetic energy depends on:
1. Mass of the object: Greater mass = greater K.E
2. Velocity of the object: Greater velocity = much greater K.E (since velocity is squared)

Example: A 2 kg object moving at 3 m/s has:
K.E = ½ × 2 × (3)² = 9 J

Law of Conservation of Energy: Energy cannot be created or destroyed; it can only be transformed from one form to another. The total energy of an isolated system remains constant.

Example – Freely Falling Body:
Consider a body of mass ‘m’ at height ‘h’ above the ground.

At Point A (top):
• P.E = mgh
• K.E = 0 (body at rest)
• Total Energy = mgh

At Point B (midway, height h/2):
• P.E = mg(h/2)
• K.E = ½mv² = mg(h/2) [by v² = 2g(h/2)]
• Total Energy = mg(h/2) + mg(h/2) = mgh

At Point C (ground level):
• P.E = 0
• K.E = ½mv² = mgh [by v² = 2gh]
• Total Energy = mgh

In all cases, Total Energy = mgh (constant), proving conservation of energy.

Renewable Energy Sources	Non-Renewable Energy Sources
Definition: Sources that can be replenished naturally in a short period	Definition: Sources that cannot be replenished or take millions of years to form
Environmental Impact: Generally clean and eco-friendly	Environmental Impact: Cause pollution and greenhouse gas emissions
Availability: Unlimited and sustainable	Availability: Finite and will eventually deplete
Cost: High initial cost, low operating cost	Cost: Lower initial cost, but subject to price volatility

**Examples of Renewable Energy:**
1. Solar Energy: Energy from sunlight using solar panels
2. Wind Energy: Energy from wind using wind turbines
3. Hydroelectric Energy: Energy from flowing water using dams

**Examples of Non-Renewable Energy:**
1. Coal: Fossil fuel formed from ancient plant matter
2. Petroleum/Oil: Fossil fuel used for fuel and plastics
3. Natural Gas: Fossil fuel used for heating and electricity

Efficiency of a Machine: Efficiency is the ratio of useful output energy (or work) to the total input energy (or work), expressed as a percentage.

Formula:
Efficiency = (Useful Output Work / Total Input Work) × 100%

Or

Efficiency = (Output Power / Input Power) × 100%

Why Efficiency Has a Limit:
No machine can have 100% efficiency because:
1. Friction: Energy is lost overcoming friction between moving parts
2. Heat Loss: Some energy is converted to unwanted heat
3. Sound Energy: Machines produce sound, which is wasted energy
4. Air Resistance: Energy is lost overcoming air resistance
5. Internal Resistance: Electrical machines lose energy in wires

These unavoidable losses mean useful output is always less than input, limiting efficiency to less than 100%.

**(i) Hydroelectric Power Generation:**

POTENTIAL ENERGY OF STORED WATER (Input)
↓
KINETIC ENERGY OF FLOWING WATER
↓
WORK DONE in running the turbine (Mechanical Energy)
↓
ELECTRIC ENERGY FROM GENERATOR (Output)

Energy losses occur due to:
• Friction in pipes and turbine → Heat energy
• Sound from machinery → Noise energy

**(ii) Electricity from Fossil Fuels:**

Chemical Energy in Fossil Fuels (Input)
↓
HEAT ENERGY produced by burning fuel
↓
MECHANICAL ENERGY: High pressure steam turns turbine
↓
ELECTRIC ENERGY from generator (Output)

Energy conversion process:
1. Burning fuel: Chemical → Heat energy
2. Steam production: Heat → Potential energy of steam
3. Turbine rotation: Potential → Mechanical energy
4. Generator: Mechanical → Electrical energy

Energy losses occur at each stage as heat, sound, and friction.