questions, answers, Conceptual Questions, Comprehensive Questions, and, Numerical Questions Mardan board (KPK ) Physics Class 9 Chapter 6 Work and Energy.

## Conceptual Questions Physics Class 9 Chapter 6 Work and Energy

**Q.1) Can a centripetal force ever do work on an object? Explain.**

**Answer:**

No, centripetal force does not do work because the force acting on the object and instantaneous displacement of the object are always perpendicular to each other as shown in the figure. So;

W = FS cosϴ = FS cos90° = FS(0) = 0 J

The centripetal force allows the object to follow the circular trajectory. The centripetal force is directed towards the center while the tangential velocity of the object is perpendicular to the direction of action of force at every point. Hence no work can be done by it.

**Q.2) What happens to the kinetic energy of a bullet when it penetrates into a sandbag?**

**Answer:**

When a bullet of mass ‘m’ is moving with velocity ‘v’ it possesses kinetic energy given by,

KE = 1/2 mv^{2}

When this bullet hits the sandbag, it penetrates into the sandbag and comes to rest after penetrating into it for some distance. When it comes to rest, the final velocity is 0 m/s.

The kinetic energy possessed by the bullet as given in the above equation is exhausted mostly in the work done against the resistance offered by the sandbag while penetrating into it. Some part of the kinetic energy may also have lost as heat and sound.

**Q.3) A meteor enters into earth’s atmosphere and burns. What happens to its kinetic energy?**

**Answer:**

A meteor has both potential and kinetic energy before entering the atmosphere of the Earth. As it enters and falls towards the surface of the Earth, frictional heating occurs, caused by the collision with the molecules in the atmosphere which reduces its velocity. Hence the kinetic energy decreases, which is converted to heat energy. Potential energy is continuously decreasing, being converted to kinetic energy. There is also mass loss due to ablation.

**Q.4) Two bullets are fired at the same time with the same kinetic energy. If one bullet has twice the mass of the other, which has the greater speed and by what factor? Which can do most work?**

**Answer:**

The bullet that has less mass will have greater speed than the bullet of greater mass.It can be proved as follow;

Let mass of first bullet be m_{1 }then the mass of the other bullet will be m_{2}= 2m_{1}. Thus

Thus it can be seen from the above equation that velocity v_{1} of the bullet with mass m1 is √2v2 i.e. √2 times greater than the velocity v2 of the bullet of mass m2.

Since the kinetic energy of both the bullets is the same so, the work done by each of them will be equal.

**Q.5) Can an object have different amounts of gravitational potential energy if it remains at the same elevation?**

**Answer:**

No, an object can not have different amounts of gravitational potential energy as long as it stays on the earth. Gravitational potential energy is given by the formula;

G.P.E = mgh

As given the elevation remains the same it means ‘h’ is constant. The value of ‘g’ would be constant for the same elevation. Also the mass of the object is constant. Hence the gravitational potential energy remains the same.

Physics Class 9 Chapter 5 Gravitation questions, answers Mardan board

**Q.6) Why do roads leading to the top of a mountain wind back and forth?**

**Answer:**

Roads leading to the top of mountain wind back and forth because in this way less power is required is to move to the top than that of a single steep path.

According to the definition of power;

It is clear from the equation (i) that the greater the height, the greater would be the power required. If roads are made steeper, like in case of the inclined plane, the vertical height ‘h’ will be much larger due to this more power is required to reach to the top of mountains.

While for the roads that wind back and forth the steepness of the path is decreased by dividing it into the number of curved paths i.e now the vertical height ‘h’ will be less for each path and so less power is required to reach to the top.

## Comprehensive Questions Physics Class 9 Chapter 6 Work and Energy

**Q.1) Define work and explain how work is calculated if force is applied at an angle.**

**Answer:**__Work:__

*“Work is said to be done when a force acting on an object displaces it in the direction of force”.*

Work is given by the formula;

Work = **Force **× **displacement**

W = **F S**work is a scalar quantity and its unit is joule (J) 1 J = 1 N.m__Calculation of work when force is applied at an angle:__

Sometimes force and displacement do not have the same direction as shown in fig. Here the force ‘**F’** is making an angle ‘**θ’** with the displacement.

Resolving ‘**F**‘ into perpendicular components **F**_{x} and **F**_{y} as;

F_{x} = Fcosθ

F_{y} = Fsinθ

In this case, only x-component of force i.e. **F**_{x} causes the body to move. Thus work done is only due to **F**_{x} and not **F**_{y}. Hence

W = F_{x}S

= (Fcosθ)S

= FScosθ

**Q.2) Define kinetic energy. Derive the expression used for kinetic energy.**

**Answer:**__Kinetic energy:__

* “The energy possessed by a body due to the virtue of its motion is called kinetic energy”*.

It is denoted by “K.E ”. It is a scalar quantity

**Derivation:**

Consider a situation in which all of the work done on the cart transfers only kinetic energy to the cart.

Consider a cart that is initially at rest. The cart moves the displacement ‘

**S**‘ because of the horizontal force ‘

**F**‘ applied to it. It achieves a final velocity of v

_{f }= v as shown in the figure;

This work done ‘W’ appears as the kinetic energy K_{E }such that

This equation shows the kinetic energy of a moving object with its mass and velocity . It demonstrate the work kinetic energy theorem which states that the work done on an object is equal to change in energy.

Read more: Physics Class 9 Chapter 4 Turning Effect of Forces Mardan board

**Q.3) What is potential energy? Prove that the potential energy of a body of mass ‘m’ at a height ‘h’ above the surface of earth is given by mgh.**

**Answer:Potential energy:**

**It is a scalar quantity and its SI unit is Joule**

*“The energy posses by a body due to the virtue of its position or configuration in a force field is called potential energy”.*

*.***Proof:**

Consider an object of mass ‘m’ being lifted vertically by a force ‘F’ to ‘h’ as shown in the figure.

The work done by the force F is given by the equation;

W = E_{G.P.E} = F x S …….. (i)

Since the force, in this case, is equal to its weight. So,

F = w = mg …………… (ii)

Here the distance moved is the height ‘h’

Thus S=h …………… (iii)

Putting equation (ii) and (iii) in equation (i), we get

**E _{G.P.E} = mg x h**

**Q.4) State the law of conservation of energy.**

**Answer:**__Law of Conservation of Energy__* “Energy can neither be created nor destroyed in any process, It can be converted form one form to another, but total but only can be changed from one form to another form.”*__Examples of the law of conservation of energy__

1. The chemical energy stored in the food is converted into heat energy as a result of digestion in the body. This heat energy keeps our body warm and enables us to do the work.

2. In dams, the stored potential energy of water due to height is converted into kinetic energy to run the turbine for producing electrical energy.

**Q.5) Explain briefly major sources of energy. Such fossil fuels, wind, solar, biomass, nuclear and thermal energy.**

**Answer:**__Fossil Fuels:__

Fossil fuels are the remains of million-years-old plant life-now coal-or aquatic animal life-now gasoline and natural gas.

1. Coal:

Coal is the most abundant fossil fuel in the world with an estimated reserve o one million metric tons. but burning coal results in significant atmospheric pollution.

2. Oil:

Crude oil is refined into many different energy products such as gasoline, jet fuel, and heating oil. Despite the limited reserves of oil in the world, it is a preferred source over coal because oil produces more energy than the same amount of coal.

3. Natural gas:

Natural gas is often a by-product of oil, it is a mixture of gases-the most common of which is methane. The advantage that natural gas has is that it is easy to transport.

Fossil fuels are consumed in more than 80% of the world’s demand for energy. However, the waste gases produced in the consumption are polluting the atmosphere.

Wind Energy:

The kinetic energy of the wind is currently used in many parts of the world to generate electricity. It is an eco-friendly source of energy but requires a very large open space.

Solar Energy:

The energy from direct sunlight can be used to produce electricity. Today, solar cells are used to power everything from calculators and watches to small cities. It is very eco-friendly. However, a significant land area is required to produce a large amount of electricity.

Bio-mass:

Bio means “life” and bioenergy is the energy from living things. The term biomass refers to the material from which we get bioenergy. Biomass is produced when the sun’s solar energy is converted into plant matter(carbohydrates) by the process of photosynthesis.

Read more: Physics Class 9 Chapter 3 Dynamics questions, answers Mardan board__Nuclear Energy:__

Nuclear energy is the energy obtained from nuclear fission reactions. When extremely large atoms split into two or more pieces, an enormous amount of energy is released in the form of radiation or heat. The heat is used to boil water that is eventually used to generate electricity.__Thermal Energy:__

Thermal energy is the energy recovered from the Earth’s core. The thermal energy contained within the Earth’s core results from the energy trapped almost five billion years ago during the formation of the planet. This energy is used to generate electricity.

**Q.7) Define and explain power?**

**Answer:**__Power:__

*“The time rate of doing work is known as power.”*

It measures how fast work is done or how fast energy is being converted from one form to another form* . *Like work, power is a scalar quantity.

__Mathematical Expression:__Mathematically, it can also be defined as the ratio of work and time.

**Unit:**

The S.I unit of power is the watt (W) which is defined as;

“The power of an object is 1 watt if it does work at the rate of one joule per sec”

In the British system, the unit of power is the foot-pound per second (ft.lb/s). However, for practical purposes, a large unit is often used which is **horsepower (hp).**** 1 hp = 746 W = 550 ft.lb/s****Explanation:**

Power not only explains how much work is done on the body in order to displace it through some distance or to some height but it also gives information about how much time is required in doing that work. Thus unlike work, it makes a reference to the passage of time, too i.e. it also describes how quickly the work is done on the object or system.

Read more: Physics Class 9 Chapter 3 Dynamics questions, answers Mardan board