Positive & negative work | Work & Energy | Physics | Khan Academy

Khan Academy India - English
20 May 201908:05
EducationalLearning
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TLDRThis video script explores the concept of work in physics, contrasting it with everyday understanding. It explains that work is calculated as the product of force and displacement, and can be positive or negative depending on their directional relationship. Positive work occurs when force and displacement align, as in pushing a cart, while negative work happens when they oppose, such as Superman stopping a car. The script uses relatable examples like catching a ball and the effect of gravity on a thrown ball to illustrate these principles, emphasizing the difference between physical work and common usage.

Takeaways
  • 😀 In everyday life, the minimum amount of work you can do is zero, but in physics, work can be less than zero.
  • 🔧 Work in physics is calculated as the product of force and displacement of an object.
  • ➡️ Work is positive when the force and displacement are in the same direction.
  • ↔️ Work is negative when the force and displacement are in opposite directions.
  • 🛒 An example of positive work is pushing a grocery cart in the direction of the push.
  • 🦸 Superman does negative work when stopping a car by applying a force opposite to the car's motion.
  • 🤔 Catching a ball involves doing negative work because the force applied opposes the ball's motion.
  • 🌐 Gravity does negative work on a ball during its upward motion because the force of gravity is downward while the ball moves upward.
  • 📉 Gravity does positive work when a ball falls because the force and displacement are in the same downward direction.
  • 🔄 The concept of positive and negative work in physics is related to whether an object speeds up or slows down.
  • 💡 Understanding the relationship between work, force, and displacement will become clearer when connected to the concept of energy.
Q & A

  • What is the minimum amount of work you can do in day-to-day life?

    -In day-to-day life, the minimum amount of work you can do is zero, which is achieved by doing nothing, such as lying around without any physical activity.

  • Is it possible to do less than zero work in physics?

    -Yes, in physics, it is possible to do work that is less than zero, which is referred to as negative work.

  • How is work calculated in physics?

    -In physics, work is calculated as the product of the force acting on an object and the displacement of that object (W = F * s).

  • Under what condition does work become positive?

    -Work becomes positive when the force and displacement are in the same direction.

  • When does work become negative?

    -Work becomes negative when the force and displacement are in opposite directions.

  • What is an example of a situation where positive work is done?

    -An example of positive work is when a person pushes a grocery cart to the right, and the cart moves to the right, indicating that the force and displacement are in the same direction.

  • Can you give an example of negative work?

    -An example of negative work is when Superman tries to stop a car moving to the left by applying a force to the right, resulting in the force and displacement being in opposite directions.

  • What does it mean when it is said that Superman did 'less work than someone standing and watching'?

    -It means that in the context of physics, Superman doing negative work (slowing down the car) is considered 'less' than the zero work done by a person who is not exerting any force or causing any displacement.

  • When catching a ball, is the work done positive or negative?

    -When catching a ball, the work done is negative because the force applied to stop the ball (to the left) is in the opposite direction of the ball's motion (to the right).

  • Is the work done by gravity on a ball during its upward motion positive or negative?

    -The work done by gravity on a ball during its upward motion is negative because gravity acts downward while the ball moves upward, opposite directions.

  • Why is work defined as positive when the force and displacement are in the same direction?

    -Work is defined as positive in such cases because objects tend to speed up when the force and displacement are in the same direction, indicating an increase in kinetic energy.

  • How does the concept of work relate to the everyday understanding of the term?

    -In everyday life, work typically refers to any task or activity that requires effort. In physics, however, work has a specific definition involving force and displacement and can even be negative, which is a different concept from the everyday understanding.

Outlines
00:00
🔧 Understanding Work in Physics

This paragraph introduces the concept of work in physics, contrasting it with everyday understanding. It explains that work is calculated as the product of force and displacement, and can be either positive or negative depending on the direction of these two factors. Positive work occurs when force and displacement are in the same direction, as illustrated by a person pushing a grocery cart. Conversely, negative work is done when the force and displacement are in opposite directions, exemplified by Superman stopping a car. The paragraph also challenges the intuitive notion that doing nothing equates to doing zero work, by demonstrating that negative work can be considered as 'doing less than nothing' in the context of physics.

05:01
📉 Positive and Negative Work: Examples and Implications

The second paragraph delves deeper into the concept of positive and negative work with additional examples. It clarifies that catching a ball involves negative work because the force exerted by the catcher opposes the ball's motion. The paragraph also discusses the work done by gravity on a ball thrown upwards, which is negative since gravity pulls down while the ball moves up. Conversely, when a ball falls, gravity does positive work because the force and displacement are in the same direction, causing the ball to accelerate. The explanation emphasizes the relationship between the direction of force and displacement and their effects on the speed of an object, setting the stage for a deeper understanding of work and energy in future discussions.

Mindmap
Keywords
💡Work
In the context of physics, 'work' is defined as the product of the force applied on an object and the displacement of the object in the direction of the force. It is a scalar quantity and can be positive or negative, depending on the direction of the force relative to the displacement. In the video, work is the central theme, with examples given to illustrate positive and negative work, such as pushing a grocery cart (positive work) and Superman stopping a car (negative work).
💡Force
Force is any interaction that, when unopposed, will change the motion of an object. In the video, force is essential to the concept of work, as it is one of the components in the formula for calculating work (W = F * s). The direction of the force in relation to the object's displacement determines whether the work done is positive or negative.
💡Displacement
Displacement refers to the change in position of an object. It is a vector quantity, meaning it has both magnitude and direction. In the script, displacement is crucial for determining the type of work done, as it must be considered alongside the direction of the applied force.
💡Positive Work
Positive work occurs when the force applied to an object and the object's displacement are in the same direction. This results in the object speeding up. In the video, an example of positive work is given where a lady pushes a grocery cart to the right, in the same direction as the cart's movement.
💡Negative Work
Negative work is done when the force applied to an object is in the opposite direction of the object's displacement, which typically results in the object slowing down. The video uses the example of Superman stopping a car, where his force is applied to the right while the car moves to the left, to illustrate negative work.
💡Zero Work
Zero work is said to be done when there is no displacement in the direction of the applied force, which is likened to doing nothing in everyday life. However, the video points out that in physics, work can also be negative, indicating a unique aspect of work in a physical context.
💡Superman
Superman is used as a character in the video to illustrate the concept of negative work. When he applies force to stop a moving car, he is doing negative work because his force opposes the car's motion, demonstrating the idea that even a hero can do 'less than nothing' in terms of work in a physical sense.
💡Catch
The act of catching is used in the video to discuss work done on a moving object, specifically a ball. When catching a ball, the force exerted by the hand is in the opposite direction of the ball's motion, resulting in negative work, as the ball's speed decreases upon being caught.
💡Gravity
Gravity is the force that attracts a body towards the center of the earth, or towards any other physical body. In the video, gravity is shown to do negative work when a ball is thrown upwards (since gravity pulls down while the ball moves up) and positive work when a ball falls down (as the direction of gravity and the ball's motion are the same).
💡Direction
Direction is a fundamental aspect of vectors, including force and displacement. The video emphasizes the importance of direction in determining whether work is positive or negative. For example, if the direction of the force and displacement are opposite, the work is negative, as seen when catching a ball or when gravity acts on an upwardly thrown ball.
💡Energy
Although not explicitly detailed in the transcript, the concept of energy is closely related to work. The video hints at a future connection between work and energy, suggesting that understanding this relationship will provide further clarity on why work can be positive or negative. Energy is the capacity to do work, and the transfer of energy is what underlies the work done in various scenarios presented in the video.
Highlights

In physics, it's possible to do work less than zero, contrary to everyday understanding where the minimum work is zero.

Work in physics is calculated as the force acting on an object multiplied by its displacement.

Work done can be positive or negative, depending on the direction of force relative to the displacement.

Positive work occurs when force and displacement are in the same direction.

Negative work occurs when force and displacement are in opposite directions, making work less than zero.

An example of positive work is a person pushing a grocery cart in the direction of the push.

Superman stopping a car is an example of negative work, as the force of his push is opposite to the car's motion.

Catching a thrown ball involves negative work, as the force applied opposes the ball's motion.

Gravity does negative work during the upward motion of a thrown ball because it acts in the opposite direction.

When a ball is dropped, gravity does positive work as it acts in the same direction as the ball's displacement.

The concept of positive and negative work is tied to the effect on an object's speed, with positive work increasing speed and negative work decreasing it.

Understanding the sign of work will become clearer when connected to the concept of energy.

The video explains the directional nature of work in physics, distinguishing it from the everyday concept of work.

The video uses relatable examples to illustrate the calculation and significance of positive and negative work.

The explanation of work in physics emphasizes the importance of the relationship between force and displacement direction.

The video aims to clarify misconceptions about work by comparing it with everyday experiences and physical principles.

The concept of negative work challenges the everyday notion of work and introduces a unique perspective in physics.

The video concludes by reinforcing the idea that work's positivity or negativity is determined by its effect on an object's speed.

Transcripts

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PhysicsWorkEnergyForceDisplacementSupermanBallGravityDirectionSpeed