Forces And Energy
Overview
Lift a bag of rice onto a shelf and you have just paid a bill. The bag was low down and now it is high up, and the difference between those two situations cost you something real: a force from your arms, moved through a distance. Physics has a single word for what you paid, and it is the same word whether the payer is a muscle, an engine, a stretched spring or a waterfall. That word is work, and the receipt it prints is measured in joules.
This lesson turns that idea into five equations you can put numbers into. You will learn what work really means and why holding something heavy above your head does none of it, how a stretched spring and a raised mass both hold energy in waiting, why a car travelling at twice the speed carries four times the kinetic energy and needs four times the distance to stop, and how power separates a machine that does a job quickly from one that does the same job slowly. Every equation you meet here is printed in the specification's own equation sheet, so the marks are never for reciting them: they are for knowing which one the question wants.
Objectives
- Work is done when a force causes an object to move through a distance. The relationship between work done, W, force, F, and distance, d, moved in the direction of the force is: W = F ×d
- Energy is transferred when work is done. Work done against frictional forces causes energy transfer by heating. Students should be able to discuss the transfer of kinetic energy in particular situations, for example shuttle re-entry into the atmosphere or meteorites burning up in the atmosphere and braking systems on vehicles.
- The amount of elastic potential energy stored in a stretched spring (assuming the limit of proportionality has not been exceeded) can be calculated using the equation: Ee = ½ × k × e 2
- An object gains gravitational potential energy when it is raised vertically because work is done against the gravitational force. The relationship between gravitational potential energy, Ep , mass, m, gravitational field strength, g, and height, h, is: Ep = m× g × h
- The kinetic energy of a moving object depends on its mass and its velocity. The relationship between kinetic energy, Ek , mass, m and velocity, v, is: Ek = ½ × m × v 2 Students should understand that when the mass of an object is doubled, if it is travelling at the same speed it will have twice the kinetic energy. They should understand that an object travelling at twice the speed of another object with the same mass will have four times the kinetic energy and should be able to apply this idea in the context of road safety.
- Power is the rate at which energy is transferred or the rate at which work is done. The relationship between power, P, work done, W, or energy transferred, E, and time, t, is: P = E / t and P = W / t
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Lesson Note
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Lesson Evaluation
Congratulations on completing the lesson on Forces And Energy. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- A horizontal force of 20 N pushes a box 3.0 m across a floor in the direction of the force. How much work is done on the box? A. 6.7 J B. 17 J C. 23 J D. 60 J Answer: D
- A machine transfers 3000 J of energy in 15 s. What is its output power? A. 0.005 W B. 200 W C. 2000 W D. 45 000 W Answer: B
- A car has kinetic energy Ek when it travels at 10 m/s. What is its kinetic energy when it travels at 30 m/s? A. 3 x Ek B. 6 x Ek C. 9 x Ek D. 30 x Ek Answer: C
- A spring with a spring constant of 200 N/m is stretched by 0.10 m, within its limit of proportionality. How much elastic potential energy is stored in it? A. 1.0 J B. 2.0 J C. 10 J D. 20 J Answer: A
- A sledge slides across level ground and gradually comes to rest. What happens to its kinetic energy? A. It is destroyed by the friction B. It is transferred to the surroundings by heating C. It is stored as gravitational potential energy D. It is converted into a force Answer: B
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Recommended Books
Further learning from open sources
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Textbook section
Work: The Scientific Definition
The scientific definition of work differs in some ways from its everyday meaning. Certain things we think of as hard work, such as writing an exam or carrying a heavy load on level ground, are not work as defined by a scientist. The scientific definition of work reveals its relationship to energy—wh
"College Physics: Work: The Scientific Definition" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/7-1-work-the-scientific-definition. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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Textbook section
Kinetic Energy and the Work-Energy Theorem
What happens to the work done on a system? Energy is transferred into the system, but in what form? Does it remain in the system or move on? The answers depend on the situation. For example, if the lawn mower in Figure 7.2 (a) is pushed just hard enough to keep it going at a constant speed, then ene
"College Physics: Kinetic Energy and the Work-Energy Theorem" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/7-2-kinetic-energy-and-the-work-energy-theorem. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
Published by other organisations under open licences, credited as they ask. Links open their websites.
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