Workdone and Energy

Workdone and Energy

By Tashi Phuntsho - Physics Teacher

What is Workdone?

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By Tashi Phuntsho - Physics Teacher

Ans: work, in physics is the measure of energy transfer that occurs when an object is moved over a distance by an external force at least part of which is applied in the direction of the displacement. - Qualitative Definition

By Tashi Phuntsho - Physics Teacher

By Tashi Phuntsho - Physics Teacher

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Work (w) is defined as the product of an external force (F) acting on an object and the displacement (d) the force has caused. Quantitative Definition

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By Tashi Phuntsho - Physics Teacher

By Tashi Phuntsho - Physics Teacher

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Unit of Work done:

Workdone is measured in the unit, called Joule (J) - SI Unit and erg - CGS Unit.

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By Tashi Phuntsho - Physics Teacher

​Equivalent unit of Joule and erg

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By Tashi Phuntsho - Physics Teacher

​Mathematical Definition of Joule and erg

​From the previous slide we can say that:

---1 Joule is amount of work done when a force of 1 Newton displaces a body through a distance of 1m in the direction of the force applied.​

​---1 erg is the amount of work done by a force of one dyne when it's point of application moves one centimeter in direction of action of force

By Tashi Phuntsho - Physics Teacher

​Relation between Joule and erg

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By Tashi Phuntsho - Physics Teacher

​Work done(Special Case)

Where the ratio of adj to hyp gives us the direction between F and d.​

By Tashi Phuntsho - Physics Teacher

Special Case of Workdone​​

1. ​When the direction of Force (F) and Displacement (d) are in same direction.

2. When the direction of Force (F) and displacement (d) are at normal (90 degree)

3. ​When F or d is zero.

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​1. Force (F) and Displacement (d) are in same direction.

In this case the hyp = adj so the ratio is 1 (Maximum)

Hence the wokdone is ​Maximum when the F and d are in the same direction. So

W=F X d X 1 (Maximum Workdone)

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​​2. Force (F) and Displacement (d) are at normal to each other.

In this case the adj = 0, so the ratio is 0 (Zero)

Hence the wokdone is ​Zero when the F and d are in normal to each other.

So W=F X d X 0 = 0 (Zero Workdone)

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​​3. When F= 0.

In this case the adj and hyp can have any value.

But here the magnitude of Force (F) is zero​

Hence the wokdone is ​Zero when the value of F =0, So W=0 X d X any value = 0 (Zero Workdone)

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​​4. When d= 0.

In this case the adj and hyp can have any value.

But here the magnitude of displacement (d) is body didn't move or moved in circular direction

Hence the wokdone is ​Zero when the value of d =0, So W=F X 0 X any value = 0 (Zero Workdone)

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​​5. When Force and Displacement are in opposite direction.

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In this case F and d are in opposite direction, hence the workdone is negative.

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So W=F X d X any negative value = negative workdone

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Multiple Choice

The displacement of the moving body, when moving in a circular path is

• Zero

• One

• Maximum

• Any value

Multiple Choice

The amount of workdone is maximum when the body moves along the direction of application of force, because....

• Force applied is maximum

• Displacement is maximum

• The ratio of adj to hyp is maximum

• None of the above

Multiple Choice

Choose the CORRECT statement about work and energy

• Work and Energy are measured with same unit

• Work and Energy are measured with different unit

Multiple Choice

Dorji is standing with a 50kg bag of rice on his head for 20 sec. Choose the CORRECT statement about the statement

• Workdone by Dorji is Zero because there is no displacement.

• Workdone by Dorji is zero because there is no force applied

• There is workdone by Dorji

​1. Calculate the work done on the body by a force of 500 N if the body moves 7.5 m in the direction of the force.

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2. Find out the magnitude of force acting on a box when 1000 J of work is done to displace it by 5 m.

An object is pushed by a horizontal force of 14 Newton on a rough floor with the friction force of 10 Newton. Determine the net work of move the object as far as 8 meters​.

​Determine the net workdone based on figure below

​BCSE - 2022

​Workdone against the gravity

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​Power in Physics

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​Power in Physics

​Qualitatively Power is the amount of energy transferred or converted per unit time.

​Power (Quantitative Definition)

​BCSE - 2022

​ 2020 BCSE Question

​The efficiency of a machine indicates how well its input energy is converted to useful output energy or work. It is a major factor in the usefulness of a machine and is the fraction or percentage of the output divided by the input. Click here for a Video

​ Efficiency of a Machine

​Input workdone and output workdone

​Efficiency of a machine is always less than 1 (100%) because OR practical machine are not idle:

• ​Parts of the machine are not weightless

• Moving Parts of the machine are not 100% friction-less

• Moving parts are not rigid

​ 2021 BCSE Question

​ 2023 BCSE Question

​ 2022 BCSE Question

​ IG Revision Question

​Four different electric bicycles were driven along the road. The work done by their motors and time taken to do that work is shown in the table given below. Which one of the electric bicycles produces the most power?

​Energy

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​Introductory Video

​Now we will focus only on two types of energy:

1. Potential Energy

2. Kinetic Energy​

​The Kinetic Energy can be calculated using the relation:

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​Law of Conservation of Energy

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​For an introductory video on law of conservation of Energy, Click here

Experimental Evidence of Energy Conservation ​using Swinging Pendulum

Follow the links below (In case of Swinging Pendulum)

https://youtu.be/OTK9JrKC6EY

Experimental Evidence of Energy Conservation ​using Freely Falling Body​

Follow the links below (In case of of freely falling body)

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​https://youtu.be/0RJu06yb2ic

Law of Conservation of Energy​

​The law of conservation of energy states that energy can neither be created nor destroyed, can only be converted from one form of energy to another. This means that a system always has the same amount of energy, unless it's added from the outside.

The law of conservation of energy can be proved/validated theoretically in the following areas
1. In swinging pendulum
2. In free falling body

​1. Swinging Pendulum

​Resistive Force

​Work done Against Resistive Force/Energy Dissipated

BCSE Questions

BCSE Questions

​1. At which position will the stone possess maximum kinetic energy?

2. Name and state the law demonstrated.

BCSE Questions

BCSE Questions

​Energy Conservation Questions

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​Theory Questions on energy conservation.
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