What is a Newton? Is it correctly understood?

  • #26
ehild
Homework Helper
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Thank you for the reply,
So in terms of your definition of the joule What is the difference between saying
A) "The work done by 1N force"
B) "The work done by 1N force applied at an object when it displaces 1 meter"

My confusion is this: Both quotes seem to involve ONE newton of force" Why would you want a Joule to be a newton of force applied to move some object one meter? Why not just a newton of force?
The difference between A and B is the specified distance. Work is proportional both the force and the distance travelled. The work done by 1 N force can be anything very small or very great, depending on the displacement. B is the definition of the unit of work.
 
  • #27
The difference between A and B is the specified distance. Work is proportional both the force and the distance travelled. The work done by 1 N force can be anything very small or very great, depending on the displacement. B is the definition of the unit of work.
Ahh now i understand :D
So is this correct:


When we measure energy we don’t measure how much energy some object has (absolute energy), we measure how much work one object or system did on some other object or system.

Work=force*distance = N*m = Joules

So if you push on a couch with 10 Newton of force, but you don’t move it, you exterted NO energy onto the system/couch (because distance = zero). So work is force exerted through some distance, and that is what a joule is.

A last thing:
So if you exert 1 Netwon and displace an object 10 meters, the work done on the system is 10Joules. This however seems strange to me because why would the energy of the system be bigger if the object travels longer: Let's say you push and object across ice and it slides 10 meters, then you place it on grass and it travels 0.5meters, did i really do less work on the object? Or was it friction which played some role?
 
  • #28
ehild
Homework Helper
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Energy is a different concept than work.

There are several kinds of energy. The most simple of them is kinetic energy. In case when a body of mass m translates with speed v, its kinetic energy is KE=1/2 mv2.
There is a theorem of Mechanics, the Work-Energy Theorem : The change of kinetic energy of the body is equal to the work done on it by all the forces applied.
Your 10 joules applied on a sledge on ice will increase its kinetic energy by 10 joule. If you apply the same force on the sledge and push it on grass by 10 m distance, it will gain less kinetic energy, as the force of friction opposes your force and the resultant force applied at the sledge would be smaller.
 
  • #29
A.T.
Science Advisor
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Let's say you push and object across ice and it slides 10 meters,
Are you pushing it for 10m, or is it sliding on it's own for 10m?

then you place it on grass and it travels 0.5meters,
Why? What stops you from pushing it for 10m on grass?

did i really do less work on the object?
If you pushed it along less distance with the same force, you did less work

Or was it friction which played some role?
Friction will do negative work on the sliding object, in the rest frame of the surface.
 

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