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. Let the same bag fall and it arrives at the floor moving fast enough to burst, so whatever you paid has clearly not vanished: it was stored on the way up and handed back on the way down. Physics measures both the paying and the storing in one unit, the joule, and this lesson is where you learn to put numbers into that idea.
You will meet three relationships and nothing more. One prices the energy stored when something is raised, one prices the energy carried by something that is moving, and one tells you how quickly a machine can move energy from one of those places to another. Between them they explain why a hoist motor is rated in kilowatts, why a reservoir on a hilltop is a battery made of water, and why the same lift done twice as fast needs twice the power. Every one of the three is printed in the specification's own equation sheet, so the marks here are never for reciting them: they are for knowing which one the question wants and substituting into it cleanly.
Objectives
- 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:
- 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:
- 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: and
Mind map
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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 crate of mass 40 kg is raised through a vertical height of 2.5 m. The gravitational field strength is 9.8 N/kg. How much gravitational potential energy does it gain? A. 100 J B. 392 J C. 980 J D. 1960 J Answer: C
- 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 ball of mass 0.50 kg travels at 6.0 m/s. What is its kinetic energy? A. 1.5 J B. 3.0 J C. 9.0 J D. 18 J Answer: C
- Which of these is the unit of power? A. The joule B. The newton C. The watt D. The newton per kilogram Answer: C
- A motor transfers energy at a rate of 250 W. How much energy does it transfer in 2.0 minutes? A. 125 J B. 500 J C. 3000 J D. 30 000 J Answer: D
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