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Old May 5th 2016, 10:30 AM   #1
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Energy and work

Hello, I'm a highschool student just starting to learn about energy and work and I have a trouble visualizing what some of these terms means in the real world. For example Newtonmeters, if I act on a object with 10Nm(Joules) what does that mean? Every meter ive acted on the object with a force of 10 newton? This is very confusing for me.

Also J/second is also very confusing for me to visualise, if an engine has a maximum output of 1000J/s what does that mean in the real world? It can put out 1000 Newton of force per meter every second? That doesnt make sence for me.

Sorry that these are very basic questions, and sorry if I've messed up some terms, English is not my first language
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Old May 5th 2016, 11:23 AM   #2
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Hello Daniel

Sorry that these are very basic questions, and sorry if I've messed up some terms, English is not my first language
No problem, everyone must start somewhere and right here is a very good place to ask questions.


I'm going to change history around to help you.



Energy is the most general idea.
It comes in many forms and these days we measure all forms in the same units - the Joule.
This is very convenient.
You probably have some idea what energy is.
It is what we need to make things happen.
That is to cause specific effects.
And we find that every time we cause particular effect, such as boiling a litre of water in a kettle, the same amount (number of Joules) of energy is required.

I said there are many types of energy

Heat energy
Mechanical energy
Electrical energy

and so on

Historically the first form of energy studied was mechanical energy but I will explain using heat energy as it makes things more obvious.
Heat energy is simpler as there is only one sort of heat energy - so we just say heat.

There are several sorts of Mechanical energy, which is probably why you are confused.

So when you put 1 litre of water from the tap at 10 degrees C in a kettle, and heat it to say 90 degrees C you have to input 33600 Joules of energy from the electric element.

You are converting one form of energy to another

Energy is never created or lost it is always converted from one form to another.

If you input 560 Joules per second it will take you one minute to heat the water.
If you input 1120 J/s it will only take 30 seconds to heat the water.

This brings us to the second important point.

Power is how fast you convert (input) the energy.

The faster you do this the higher the power.
So the power in my second example is double the power in my first.

The total energy is the same in both cases

The unit of power is the Joule per second or the watt.

So the kettle in the first instance has a power rating of just over half a kilowatt (560 watts)
and in the second instace it is just over 1 kilowatt (1120 watts)

You have probably heard of these power ratings on electrical equipment?

I said that mechanical energy comes in several types and if you like my approach we can deal with this in my next post.

Are you with me and have you any questions so far ?
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Old May 5th 2016, 11:35 AM   #3
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Thank you very much, it is alot easier to understand Joules/second now with your example. Though I still don't understand Joules fully. It is Newton multiplied by distance traveled correct? So if I push an object with a force of 10 Newton for 10 meters I've done a work of 100 Newton meters? Actually it is all making sence now. Thanks to your example. So if I'm capable of pushing with 50 Joules/second it will take me 2 seconds to perform it. Thank you, I now have a much clearer picture of it all
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Old May 5th 2016, 11:43 AM   #4
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This is a question in my book that I don't fully understand that is related to this.

An electric car has an effect of 12kW

A) The car drives 36km/h and accelerates then with maximum force, how much is the tensile force (don't knkw I that is the correct term I relied on Google translate) the moment the car accelerates? What happens with the tensile force as the cars speed increases?

B) The car has a maximum speed of 80 km/h. How much is the tensile force then? Why doesn't the car increase in speed?

Though this is not a homework problem I'm not looking for the solution, but rather an explanation that will help me understand the problem and solve it. Thank you for your time
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Old May 5th 2016, 11:59 AM   #5
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OK so on to greater things.

Mechanical energy comes in three forms.

Kinetic energy, KE
which is the energy something has because it is moving.
The faster something moves the more kinetic energy it has and the bigger bump it will give you if it hits you.

Potential energy, PE
is the energy something has because of its position and comes in many forms, gravitational, elastic strain energy.
I will return to these

Work
The first two are about the energy stored in or possessed by a body,
work is a measure of mechanical energy transferred from one body to another.
The energy transferred then becomes the property or possession of the other body.

For example if I pull out an old fashioned spring balance or pull back a bowstring I transfer energy to the balance or bow by doing work on them
In both cases the energy I transfer is stored by the spring or bow as elastic strain energy, which I said is a form of PE.

In order to perform this work I must exert a force on the spring or bow and the work done is calculated as this force times the distance I stretch the spring or bow.
The units are all worked out to mesh in with the standard units of energy
So if I pull with 1 Newton force a distance of 1 metre stretch I will do 1 joule of work.
You can see compared to the kettle heat energies these are very small units so we nomally work in kiloNewtons.
This 1 joule is stores as 1 joule of PE.

How are we doing?

A question for you to think about.

Work is about transfer of energy.
Do you know what energy do I possess to transfer to the bow or spring?
It is not one that I have listed so far.
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Old May 5th 2016, 12:32 PM   #6
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Originally Posted by studiot View Post
OK so on to greater things.

Mechanical energy comes in three forms.

Kinetic energy, KE
which is the energy something has because it is moving.
The faster something moves the more kinetic energy it has and the bigger bump it will give you if it hits you.

Potential energy, PE
is the energy something has because of its position and comes in many forms, gravitational, elastic strain energy.
I will return to these

Work
The first two are about the energy stored in or possessed by a body,
work is a measure of mechanical energy transferred from one body to another.
The energy transferred then becomes the property or possession of the other body.

For example if I pull out an old fashioned spring balance or pull back a bowstring I transfer energy to the balance or bow by doing work on them
In both cases the energy I transfer is stored by the spring or bow as elastic strain energy, which I said is a form of PE.

In order to perform this work I must exert a force on the spring or bow and the work done is calculated as this force times the distance I stretch the spring or bow.
The units are all worked out to mesh in with the standard units of energy
So if I pull with 1 Newton force a distance of 1 metre stretch I will do 1 joule of work.
You can see compared to the kettle heat energies these are very small units so we nomally work in kiloNewtons.
This 1 joule is stores as 1 joule of PE.

How are we doing?

A question for you to think about.

Work is about transfer of energy.
Do you know what energy do I possess to transfer to the bow or spring?*
It is not one that I have listed so far.
*isn't it chemical energy in your muscles from ATP? And could you also please take a look at the question I posted in the lost above your last post
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Old May 5th 2016, 01:00 PM   #7
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Yes it is chemical energy.

A force is a push or a pull.

I think your car question in post 4 is trying to run before you can walk so make sure you understand the basics first.

However I will offer this.

I think you mean 'tractive force' not tension force.
This is the driving force accelerating the car.

Since you are asking these questions I assume you have not yet covered Newton's Laws.

Newton's first law says that it does not require a force to keep something in steady motion.

Newton's second law says that a force is required to accelerate a body and that the force required is the mass times the acceleration

So you (tractive) force = the mass of the car x the acceleration

The tractive force is limited by the power of the engine

However as the speed increases the air resistance (which is a force opposing the motion and reducing the effective tractive effort force) increases even more until they exactly balance.
At this speed the car can go no faster.

I don't think with the information given it is possible to say more than this.
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