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Work, Power & Energy Class 11 | Everything about work, power & energy | NEET 2023 | Shreyas Sir

Vedantu NEET English・88 minutes read

The session delves into work-energy concepts in physics, covering topics like work calculation, forms of energy, and the work-energy theorem. It also discusses conservative and non-conservative forces, potential and kinetic energy, and the work-kinetic energy theorem. The importance of understanding these principles for problem-solving in exams like NEET is emphasized, along with practical applications and advice on approaching physics problem-solving systematically.

Insights

Work in physics is defined as the transfer of energy through a force causing displacement, with key factors like direction and magnitude influencing whether it is positive, negative, or zero.
The distinction between conservative and non-conservative forces is crucial, as conservative forces have associated potential energy and follow the Law of Conservation of Mechanical Energy, while non-conservative forces do not store energy and have work dependent on the path taken.
Understanding the relationship between conservative forces and potential energy is essential, as work done by conservative forces is always the negative change in potential energy, impacting how energy is stored, transferred, and utilized in physical systems.

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Recent questions

What is work in physics?
The transfer of energy via force producing displacement.

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Summary

00:00

Mastering Work-Energy Chapter in Physics

The session is an all-rounder for students aiming to excel in the work-energy chapter in physics.
The importance of the chapter is highlighted, with its relevance to multiple topics in exams like NEET.
Key areas covered in the session include understanding work calculation, power calculation, different forms of energy, and the work-energy theorem.
Work is defined as the transfer of energy via a force producing displacement, represented by F cos Theta or F dot s.
The unit of work is joule, with the conversion factor to ergs provided.
Work can also be expressed as force in the direction of displacement multiplied by displacement or force multiplied by displacement in the direction of the force.
Work can be positive when force and displacement are in the same direction, negative when opposite or at an obtuse angle, and zero when no force is applied, or force is perpendicular to displacement.
Zero work can also occur when force is perpendicular to displacement, as in circular motion scenarios.
Negative work signifies energy being taken away from the system, while positive work indicates energy being added.
Zero work can result from no force applied, force perpendicular to displacement, or force and displacement being perpendicular in circular motion.

16:58

Calculating Work: Forces, Angles, and Integration

Work is being discussed, focusing on how to calculate work and solve basic questions related to it.
An example is presented with a block of mass 5 kg, a force of 20 Newton at an angle of 60 degrees, and a displacement of 10 meters.
The work done by all forces acting on the block is to be determined.
The weight and normal forces are identified, with the work done by the external force calculated as 173.2 joules.
The work done by gravity and the normal force is found to be zero due to the angles involved.
The total work done by all forces is calculated as 173.2 joules.
The concept of dot product is introduced, explaining how to calculate it and its properties.
A question involving a force of 10i - 20j + 4k Newton and a displacement of 2i + 4j + k meters is presented, with the work done by the force calculated as -72 joules.
The concept of work done by a variable force is explained, emphasizing the calculation of small work and the total work as the sum of small works.
Integration is highlighted as the mathematical method to find the total work done by variable forces, with attention to positive and negative areas in the graph.

33:57

Work Done by Conservative and Non-Conservative Forces

Total work done is the sum of all signed work values, including signs.
The total work done can be equated to the area under the force versus displacement graph.
Forces are classified into conservative and non-conservative categories.
Conservative forces only consider the initial and final positions, not the path taken.
Non-conservative forces have work dependent on the path taken.
For conservative forces, the total work in a closed path is zero.
Examples of conservative forces include gravity, electromagnetic force, and spring force.
Non-conservative forces include friction, viscous force, drag force, normal force, and tension force.
Friction can do positive work when the direction of motion is considered.
In a scenario where work is done from point A to D, the total work done by a conservative force is calculated based on the given work values for different paths.

51:21

Calculating Work and Energy Transfers in Physics

Total work done from A to D is the focus of the question.
Work done from A to B is 20 joules, from B to C is -10 joules, and from C to D is 40 joules.
The total work done is zero, so the work done from D to A is -50 joules.
To find the work done from A to D, it is the negative of the work done from D to A, resulting in +50 joules.
Kinetic energy is the energy of motion, measured in joules, and depends on speed.
Kinetic energy formula is half mv^2 or momentum squared divided by 2m.
Potential energy is the ability to perform work and is measured with respect to a zero reference.
Examples of potential energy include gravitational, elastic, and electrostatic potential energy.
Conservative forces have potential energy associated with them, as they store and restore energy.
Non-conservative forces like friction do not have potential energy as they do not store energy.

01:08:50

Conservative Force and Potential Energy Relationship Explained

The relationship between conservative force and potential energy is that the work done by the conservative force is always the negative change in potential energy.
Understanding this relationship is often challenging for many students.
An example is provided to illustrate the concept: when moving an object up, the work done by gravity is negative, but the potential energy increases.
Conversely, when moving the object down, the work done by gravity is positive, but the potential energy decreases.
The formula for work done is force multiplied by displacement, represented as -dU/dx, where the conservative force is the negative gradient of potential energy.
Gradient refers to the change with respect to distance, while the slope of a U versus X graph indicates the force's nature at different points.
The formula for gravitational potential energy is mgh, where m is mass, g is acceleration due to gravity, and h is height.
The formula for spring potential energy is half kx^2, where k is the spring constant and x is the extension or compression.
A practical question involves finding the force required to extend a spring by 2 meters, determining the initial and final energies, and calculating the work done, which is positive when increasing potential energy.

01:28:01

Energy, Power, and Momentum in Physics

Momentum increases by 100%, leading to a final momentum of 2P.
Final kinetic energy is calculated as 4 times the initial kinetic energy.
The increase in kinetic energy is three times the original, equivalent to a 300% increase.
Power is defined as the rate of doing work, expressed as Force dot product with velocity.
One joule of work done in one second equals one watt of power.
Horsepower is approximately 746 watts.
Power can be positive, negative, or zero based on the relationship between force and velocity.
Gravitational power after time T is calculated as mg squared times T.
Work-Kinetic Energy Theorem states that total work done changes the total kinetic energy of a system.
Law of Conservation of Mechanical Energy asserts that total mechanical energy remains constant if non-conservative forces do not interfere.

01:48:40

"Energy Transformation and Practical Physics Problem-Solving"

The text discusses the concept of kinetic and potential energy, illustrating how the kinetic energy of a moving block can be transformed into potential energy stored in a spring when the block stops.
It highlights the equation for kinetic energy (½ MV²) and potential energy of a spring (½ KX²), emphasizing the relationship between them (MV² = KX²) and deriving the expression for displacement (X = V√(M/K)).
The text advises on the practical application of simpler methods over complex storytelling in physics problem-solving, recommending systematic approaches for cases involving multiple forces and energies, concluding with encouragement for further study and engagement with educational resources.

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