Intro to work | Work & Energy | Physics | Khan Academy

Khan Academy India - English
16 May 201908:36
EducationalLearning
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TLDRThe video script delves into the precise definition of work in physics, contrasting it with its everyday ambiguous usage. It explains that work is calculated as the product of force applied to an object and its displacement, demonstrated through various relatable examples. The importance of both pushing a force and causing movement is highlighted, as neither alone constitutes work. The concept is further clarified with examples of pushing a grocery cart, an astronaut throwing a ball in space, and the force of gravity on a falling barbell. The unit of work, the joule, is introduced as newton meters, setting the foundation for future discussions on energy.

Takeaways
  • 🔍 **Work in Physics**: In physics, work is defined as the product of the force applied to an object and the displacement of that object.
  • 💡 **Necessity of Force and Displacement**: To do work, there must be a force applied to an object and that object must be displaced.
  • ⚙️ **Zero Work Scenarios**: If there is no force (e.g., pushing a wall that doesn't move) or no displacement (e.g., holding a barbell without moving it), no work is done.
  • 🚀 **Example of Work**: Pushing a grocery cart on grass involves work because a force is applied and the cart is displaced.
  • 🧱 **Non-Work Scenario**: Pushing a wall without causing it to move does not constitute work, despite the effort involved.
  • 🌌 **Work in Space**: An astronaut throwing a ball in space does work on the ball due to the force applied during the throw and the ball's subsequent displacement.
  • 🤔 **Post-Throw Consideration**: Once the ball is released and moves due to inertia, no work is being done on it by the astronaut, as there is no longer a force being applied.
  • 🏋️‍♂️ **Holding a Barbell**: A person holding a stationary barbell is not doing work because there is no displacement, even though force is applied to counteract gravity.
  • 🏋️‍♂️ **Lifting a Barbell**: Lifting a barbell involves work because the person applies a force and the barbell is displaced in the process.
  • 📉 **Falling Barbell**: When a barbell is dropped, gravity does work on it as it displaces the barbell towards the ground.
  • ⚖️ **Calculating Work**: The work done can be calculated using the formula: work = force × displacement, with units of newton meters (or joules).
  • 📚 **Unit of Work**: The unit of work is the joule, which is equivalent to a newton meter, named after the scientist James Prescott Joule.
Q & A

  • What is the general definition of 'work' in daily life and how does it compare to its definition in physics?

    -In daily life, 'work' is a loosely defined term that can be interpreted differently. For example, one person may view recording a video as work, while another may see it as just playing on a computer. In contrast, physics seeks precise definitions. According to physics, work is defined as the force applied to an object multiplied by the displacement of that object. This means that for work to be done, a force must be applied to an object, and the object must move in the direction of the force.

  • What are the two necessary conditions for work to be done on an object in physics?

    -The two necessary conditions for work to be done on an object in physics are: 1) A force must be applied to the object, either by pushing or pulling it; and 2) The object must move or displace in the direction of the applied force. If either of these conditions is not met, no work is done.

  • Can someone doing push-ups be considered as doing work on the ground?

    -No, a person doing push-ups is not doing work on the ground. Although they are applying a force, there is no displacement of the ground. The person is moving themselves up and down, but the ground remains stationary. Work is only done when a force causes a displacement of the object on which the force is applied.

  • What is the formula for calculating work done in physics?

    -The formula for calculating work done in physics is given by Work = Force × Displacement, where 'Work' is measured in newton-meters (N·m), which is also known as a joule (J).

  • What is the significance of the unit 'joule' in the context of work?

    -The joule (J) is the standard unit of work in physics. It represents the amount of work done when a force of one newton displaces an object by one meter in the direction of the force. The term 'joule' is named after the scientist James Prescott Joule, who made significant contributions to the field of energy and thermodynamics.

  • How does the concept of work relate to the law of conservation of energy?

    -While the provided script does not directly address the law of conservation of energy, it is implied that work and energy are closely related. Work is a form of energy transfer. When work is done on an object, energy is transferred to that object, which may result in kinetic energy, potential energy, or other forms of energy, depending on the situation. This concept will be explored more thoroughly in future videos that connect work and energy.

  • Why does pushing on a wall without moving it not count as doing work?

    -Pushing on a wall without moving it does not count as doing work because there is no displacement of the wall. Work requires both the application of a force and the displacement of the object in the direction of that force. Since the wall does not move, there is no displacement, and therefore, no work is done.

  • What happens when an astronaut throws a ball in outer space?

    -When an astronaut throws a ball in outer space, work is done on the ball while the astronaut is applying a force to it and it is moving in the direction of that force. However, once the ball is released and continues to move on its own without any further force being applied by the astronaut, no additional work is done by the astronaut on the ball.

  • How is the work done by gravity different from work done by a person?

    -The work done by gravity is a result of the force of gravity acting on an object and causing it to move or displace. This is seen when a barbell is dropped, and gravity pulls it towards the ground. The work in this case is done by gravity, not by the person. In contrast, when a person does work, they are the ones applying the force and causing the displacement of the object, such as pushing a grocery cart.

  • What is the relationship between force, displacement, and work?

    -Work is directly related to both force and displacement. The amount of work done is the product of the force applied to an object and the displacement of that object in the direction of the force. If the force is zero (no pushing or pulling), or if the displacement is zero (no movement), then no work is done. The relationship is mathematically expressed as Work = Force × Displacement.

  • Can you provide an example of a situation where no work is done despite physical effort being expended?

    -An example of a situation where no work is done despite physical effort is expended is a person holding a barbell stationary above their head. Although the person is exerting effort to counteract the weight of the barbell, no work is done because there is no displacement of the barbell. The person is applying a force, but since there is no movement, the work done is zero.

Outlines
00:00
🔧 Defining Work in Physics

This paragraph introduces the concept of work in physics, contrasting it with the everyday, loosely defined understanding of the term. It explains that in physics, work is defined as the product of the force applied to an object and the displacement of that object in the direction of the force. The speaker clarifies that both pushing/pulling and movement (displacement) are necessary to do work. Examples are provided to illustrate the concept, such as pushing a grocery cart on grass, pushing on a wall, and an astronaut throwing a ball in outer space. The paragraph emphasizes the importance of force and displacement in determining whether work is done and concludes with the idea that work is not done when there is no force acting on the object, such as when a ball is thrown and moves without further force from the astronaut.

05:01
📈 Calculating and Understanding Work

This paragraph delves into the calculation of work in physics, using the example of a lady pushing a grocery cart with a force of 30 newtons over a displacement of 2 meters. The speaker explains that work done is calculated as the product of force and displacement, resulting in a value of 60 newton meters in this case. The paragraph introduces the unit of work, the joule (denoted by the capital letter J), which is derived from newton meters. It also touches on the concept that any force, not just human effort, can do work as long as it causes displacement. The speaker mentions that the connection between work and energy will be explored in future videos to provide a deeper understanding of the significance of work being defined as force times displacement.

Mindmap
Keywords
💡Work
In the context of physics, work is defined as the product of the force applied to an object and the displacement of that object in the direction of the force. It is a measure of energy transfer and is central to the video's theme as it differentiates 'work' in everyday language from its scientific definition. For example, pushing a grocery cart on grass counts as work because a force is applied and the cart moves, whereas pushing on a wall without moving it does not.
💡Force
Force is a physical quantity that describes the push or pull upon an object resulting from the interaction between two bodies. In the video, force is a necessary component for work to be done, as it must be applied to an object for work to occur. An example from the script is pushing a grocery cart, where the force is the push applied by the person.
💡Displacement
Displacement refers to the change in position of an object and is a vector quantity that has both magnitude and direction. In the context of the video, displacement is the movement of an object in the direction of the applied force, which, when combined with force, determines if work is done. For instance, when an astronaut throws a ball in space, work is done during the throw due to the displacement of the ball.
💡Physics
Physics is the natural science that involves the study of matter, its motion, and the related energy and forces. The video is centered around the concept of work within the realm of physics, providing precise definitions and examples to distinguish it from colloquial usage. Physics principles are used to explain the conditions under which work is done or not done.
💡Energy
Although not explicitly defined in the video, energy is a fundamental concept in physics related to work. It is the capacity to do work and is often connected to the concept of work in further studies. The video hints at the relationship between work and energy, suggesting that understanding this connection will be explored in future content.
💡Joule
The joule is the unit of work or energy in the International System of Units (SI). It is defined as the work done when a force of one newton displaces an object by one meter in the direction of the force. In the video, the joule is introduced as the unit for work done, with the example given of 60 joules of work done by pushing a grocery cart.
💡Newton
A newton is the SI unit of force. It is defined as the force required to accelerate a one-kilogram mass by one meter per second squared. In the video, newtons are used to quantify the force applied in examples of work, such as the 30 newtons of force used to push a grocery cart.
💡Effort
Effort, in the context of the video, refers to the physical exertion made by an individual, which may or may not result in work in the physics sense. For example, a person pushing on a wall without causing any displacement is exerting effort but not doing work according to physics.
💡Gravity
Gravity is the force that attracts two bodies towards each other, and in the context of the video, it is an example of a force that can do work on an object without direct contact. The video mentions gravity doing work on a falling barbell by displacing it towards the ground.
💡Friction
Friction is a force that resists the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. While not explicitly discussed in the video, friction is a force that can affect work, particularly in the example of pushing a grocery cart on grass where it opposes the motion.
💡Space
Space, as mentioned in the video, refers to the vacuum of outer space where there is no gravity or friction. It is used as a context to illustrate the concept of work without these forces complicating the scenario, such as an astronaut throwing a ball in space.
Highlights

The concept of work in daily life is loosely defined and often disputed.

In physics, work is defined precisely as the product of force and displacement on an object.

For work to be done, both a force must be applied and the object must move.

An example of work in everyday life is pushing a grocery cart on grass.

Pushing on a wall without moving it does not constitute work, as there is no displacement.

In outer space, an astronaut throwing a ball does work while the ball is in hand but not after it is released.

Holding a barbell stationary involves force but no work, as there is no displacement.

When a person moves a barbell, work is done because of the force applied and the displacement.

Dropping a barbell results in work being done by gravity, not by the person.

The formula for work done is the force applied multiplied by the displacement (W = F * s).

The unit of work is the newton meter, also known as a joule (J).

The concept of work connects to energy, which will be explored in future videos.

The precise definition of work in physics helps to quantify physical processes and understand energy transformations.

Understanding work is crucial for solving problems in physics involving forces and motion.

The examples provided illustrate the conditions under which work is done and when it is not.

The video clarifies the misconceptions about work and provides a clear understanding of its physical definition.

The significance of force and displacement in determining work is emphasized in the examples.

The video serves as an educational resource for learning the fundamental concept of work in physics.

Transcripts

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Physics ConceptsWork DefinitionForce and DisplacementReal-life ExamplesJoule UnitPhysics EducationMechanicsEnergy IntroductionScientific PrinciplesPhysics for All