Matric Notes Physics 9th Ch 6 Work and Energy Long Questions
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Q3) 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 Swork 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 Fx and Fy as;
Fx = Fcosθ
Fy = Fsinθ
In this case, only x-component of force i.e. Fx causes the body to move. Thus work done is only due to Fx and not Fy. Hence
W = FxS
= (Fcosθ)S
= FScosθ
Q.4) 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 vf = v as shown in the figure;
This work done ‘W’ appears as the kinetic energy KE 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.
Q5) 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: “The energy posses by a body due to the virtue of its position or configuration in a force field is called potential energy”. It is a scalar quantity and its SI unit is Joule.
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 = EG.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
EG.P.E = mg x h
Q6) 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.
Q7) 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.
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.
Q9) 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.
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