TLDR;
Alright, so in this lecture, Alakh Pandey sir is teaching us about Work, Energy, and Power for Class 9th physics. He explains what work actually means in physics, the formula to calculate it, different types of energy like kinetic and potential, and the law of conservation of energy. He also talks about power and electrical energy, solving a bunch of NCERT questions along the way.
- Work is force times displacement in the direction of the force.
- Energy is the ability to do work and comes in forms like kinetic and potential.
- The law of conservation of energy states energy can't be created or destroyed, only transformed.
Introduction [0:00]
Alakh Pandey sir introduces the chapter on Work, Energy, and Power for Class 9th physics. He promises to explain the concepts in a simple way, solve NCERT back exercise questions, and make sure students understand the chapter thoroughly so they can ace their exams. He also mentions a question in the lecture, and those who answer correctly will have their names announced in the next lecture.
Work [1:15]
Sir explains that in physics, work is defined differently than in everyday life. Just sitting and watching a movie or gossiping with friends isn't considered work in physics. Work is said to be done when a force causes an object to move a certain distance in the direction of the force. So, there needs to be both force and displacement for work to be done. If you're pushing a big rock but it doesn't move, no work is done according to physics. Similarly, if you're standing with a heavy load on your head, you're applying force, but since there's no displacement, no work is done.
Formula To Calculate Work (W) [7:29]
Sir explains the formula for calculating work: Work = Force × Displacement (W = F × S). This formula is accurate when the force and displacement are in the same direction. He also mentions that in Class 11th, students will learn about cases where force and displacement are not in the same direction, involving the angle between them. Work is a scalar quantity, meaning it has no direction. The SI unit of work is joules (J), which is the same as Newton-meter (N⋅m). He also mentions another unit of work called erg, where 1 joule equals 10^7 ergs, though this isn't in the NCERT syllabus.
Sir then solves an NCERT problem where a force of 7 N is applied, causing a displacement of 8 m in the same direction. The work done is calculated as 7 N × 8 m = 56 J.
Sir explains three scenarios:
- When force and displacement are in the same direction, work is positive.
- When force and displacement are perpendicular (90° angle), work is zero.
- When force and displacement are in opposite directions, work is negative.
He explains negative work with the example of stopping a rolling ball. The force applied to stop the ball is in the opposite direction of the ball's motion, resulting in negative work.
Sir then discusses several NCERT examples to determine whether the work done is zero, positive, or negative based on the direction of force and displacement.
He explains with an NCERT example, that if a mass is moved horizontally on a table, the work done by the gravitational force is zero because the gravitational force acts downwards, perpendicular to the horizontal displacement.
Sir explains the concept of a coolie carrying a load on his head. The work done by the gravitational force is zero because the force of gravity acts downwards, while the displacement is horizontal. He also explains that the force exerted by the coolie against gravity is also perpendicular to the displacement, so the work done against gravity is also zero.
Sir explains that when an object is thrown at an angle and returns to the same horizontal level, the work done by gravity is zero because the net displacement is horizontal, perpendicular to the vertical force of gravity.
Sir discusses several examples from NCERT to illustrate when work is done and when it is not. These include a girl swimming, a donkey carrying a load, a windmill lifting water, a plant undergoing photosynthesis, an engine pulling a train, food grains drying in the sun, and a sailboat moving due to wind energy.
Sir explains that the work done by the force of gravity on a satellite moving around the Earth is zero because the gravitational force is always directed towards the center of the Earth, while the satellite's displacement is tangential to its orbit, making them perpendicular.
Sir explains that when a person holds a bundle of hay over their head, no work is done in the physics sense because there is no displacement. However, the person gets tired because their muscles are continuously contracting and relaxing, which uses chemical energy from the food they eat.
Energy [41:10]
Sir explains that energy is the ability to do work, and its SI unit is joules, the same as work. He mentions the second law of thermodynamics, stating that not all energy can be converted into work due to inefficiencies. He lists various types of energy, including kinetic energy, light energy, electric energy, heat energy, sound energy, chemical energy, nuclear energy, and potential energy, noting that kinetic and potential energy will be discussed in detail.
Kinetic Energy (KE) [44:36]
Sir explains that kinetic energy (KE) is the energy possessed by a moving object. He proves that moving objects have the ability to do work by giving examples such as a car hitting a block or a ball striking another ball. The kinetic energy depends on the mass of the object and its speed. The formula for kinetic energy is KE = 1/2 mv^2, where m is the mass in kilograms and v is the velocity in meters per second. Kinetic energy is measured in joules and is a scalar quantity.
Sir solves an NCERT problem where an object of mass 15 kg is moving with a uniform velocity of 4 m/s. The kinetic energy is calculated as 1/2 × 15 kg × (4 m/s)^2 = 120 J.
Deriving Formula Of Kinetic Energy [48:46]
Sir derives the formula for kinetic energy, KE = 1/2 mv^2, using the work-energy theorem. He starts with the assumption that the work done on an object becomes its kinetic energy. He considers an object with initial velocity u, final velocity v, mass m, and displacement s. Using the equation of motion v^2 - u^2 = 2as and the formula for work done W = F × s, he derives the kinetic energy formula.
Work - Energy Theorem [56:54]
Sir explains the relationship between kinetic energy and velocity. Since KE = 1/2 mv^2, kinetic energy depends on the square of the velocity. If velocity doubles, kinetic energy quadruples; if velocity triples, kinetic energy becomes nine times greater; and if velocity is halved, kinetic energy becomes one-fourth.
Sir solves an NCERT problem where the kinetic energy of an object moving at 5 m/s is 25 J. He calculates the new kinetic energy when the velocity is doubled and tripled. He also discusses what happens when both mass and velocity are doubled.
Sir introduces the work-energy theorem, which states that the work done by all forces on an object equals the change in its kinetic energy. He provides the formula: Work = Final Kinetic Energy - Initial Kinetic Energy.
Sir solves a problem where a force acting on a 20 kg mass changes its velocity from 5 m/s to 2 m/s. He calculates the work done using the work-energy theorem, finding it to be -210 J, indicating that the force opposes the motion.
Sir solves an NCERT problem to calculate the work required to stop a car of 1500 kg moving at 72 km/h. He uses the work-energy theorem, converting the speed to m/s and calculating the initial and final kinetic energies. The work done is -300,000 J, indicating that the work is done against the motion.
Sir solves an NCERT problem where an object of mass m is moving with a constant velocity v. He calculates the work required to bring the object to rest using the work-energy theorem, resulting in -1/2 mv^2.
Potential Energy (PE OR U) [1:04:49]
Sir introduces potential energy (PE), which is the energy stored in an object due to its position or configuration. He gives examples such as a compressed spring, a stretched bow, and a stretched catapult. Potential energy is a scalar quantity and is measured in joules. He discusses potential energy due to height above the Earth's surface, given by the formula PE = mgh, where m is mass, g is acceleration due to gravity, and h is height above the Earth's surface.
Sir solves an NCERT problem to find the potential energy of an object of mass 10 kg at a height of 6 meters above the ground, using g = 9.8 m/s^2. The potential energy is calculated as 588 J.
Sir solves an NCERT problem where an object of mass 12 kg has a potential energy of 480 J at a certain height above the ground. He calculates the height using the formula PE = mgh, finding it to be 4 meters.
Deriving Formula Of Potential Energy [1:11:52]
Sir derives the formula for potential energy, PE = mgh. He explains that when an object is lifted from the ground to a height h, work is done against gravity, and this work is stored as potential energy. The force required to lift the object is equal to its weight, mg, and the work done is force times displacement, resulting in PE = mgh.
Sir solves an NCERT problem where a porter lifts luggage of 15 kg from the ground and puts it on his head 1.5 meters above the ground. He calculates the work done by the porter using the formula PE = mgh, finding it to be 225 J.
Law Of Conservation Of Energy [1:16:54]
Sir introduces the law of conservation of energy, which states that energy can neither be created nor destroyed, but can only be transformed from one form to another. He provides examples of energy transformations, such as electric fans converting electrical energy to kinetic energy, electric heaters converting electrical energy to heat energy, and solar panels converting light energy to electrical energy.
Sir solves an NCERT problem where a battery lights a bulb, describing the energy changes involved: chemical energy in the battery is converted to electrical energy, which is then converted to heat energy in the filament, causing it to glow and emit light energy.
Sir explains the conservation of kinetic energy and potential energy for a freely falling object. At the highest point, potential energy is maximum and kinetic energy is zero. As the object falls, potential energy decreases and kinetic energy increases, but the total mechanical energy (potential + kinetic) remains constant. Just before hitting the ground, potential energy is nearly zero and kinetic energy is maximum.
Sir addresses a theoretical question from NCERT about whether the decreasing potential energy of a freely falling object violates the law of conservation of energy. He explains that it does not because the potential energy is converted into kinetic energy.
Sir addresses what happens to the kinetic energy of a freely falling object when it eventually stops on reaching the ground. He explains that the kinetic energy is converted into sound energy, heat energy, and deformation energy upon impact.
Sir solves a problem where a 5 kg ball is dropped from a height of 10 meters. He calculates the initial potential energy and the kinetic energy just before it reaches the ground using the conservation of mechanical energy.
Sir solves a problem where a 5 kg ball is thrown upwards with a speed of 10 m/s. He calculates the initial kinetic energy and the potential energy when it reaches the highest point using the conservation of mechanical energy.
Sir solves an NCERT problem where an object of mass 40 kg is raised to a height of 5 meters above the ground. He calculates the potential energy at that height and the kinetic energy when it is halfway down, using the conservation of mechanical energy.
Sir explains the concept of mechanical energy conservation in a pendulum. At the highest point, potential energy is maximum and kinetic energy is zero. At the lowest point, potential energy is zero and kinetic energy is maximum. He also explains why a pendulum eventually comes to rest due to air resistance and friction, which convert mechanical energy into heat and sound.
Power [1:42:24]
Sir defines power as the rate of doing work or the rate of transfer of energy. The formula for power is Power = Work / Time or Power = Energy / Time. The unit of power is joules per second (J/s), which is also known as watt (W). He also mentions the unit kilowatt (kW), where 1 kW = 1000 W.
Sir solves an NCERT problem where a lamp consumes 1000 joules of electric energy in 10 seconds. He calculates the power as 100 W.
Sir solves an NCERT problem involving two girls climbing a rope. He calculates the power expended by each girl, considering their weights, the height they climb, and the time they take.
Electrical Energy (E) [1:47:28]
Sir explains a special unit for electrical energy called kilowatt-hour (kWh). He derives the formula Energy = Power × Time and explains that if power is measured in kilowatts and time in hours, the unit of energy becomes kilowatt-hour. He explains that the electric meter in homes measures energy in kWh, and one unit on the meter corresponds to 1 kWh of energy consumption.
Sir solves an NCERT problem where an electric heater is rated 1500 watts. He calculates the energy it uses in 10 hours, converting the power to kilowatts and finding the energy consumption to be 15 kWh.
Sir solves an NCERT problem where a household has consumed 250 units of electric energy in a month. He converts this energy to joules, using the conversion factor 1 kWh = 3.6 × 10^6 J.
Sir solves an NCERT problem to find the energy in joules consumed in 10 hours by four devices of power 500 W each. He calculates the energy consumed by one device in kWh and then converts it to joules.
Sir concludes the chapter, announces the names of students who correctly answered the question from the previous lecture, and encourages students to keep studying.
