Work

[Nivvedan]

Work = Force x Displacement. Right?

Let’s consider a frictionless space. Here I apply 100N force to a body and it moves with a constant velocity to another point 10 m from the starting position. Then work done = 100 x 10 = 1000J.

If I apply a force of 10N to the same body then too it moves with a constant velocity and reaches the same point, it takes more time but definitely reaches it. Since Work is independent of time, work done now is 10 x 10 = 100J. But how come, to move a same body first 1000J work was done and then now only 100J work is done?

Some people claim work is done only when there is a change in energy, But the formula is Force x Displacement. If a body at rest is accelerated by a force some people say that the work done is exqual to the change in kinetic energy of the body. But the formula is F x s. It is independent of the kinetic energy. It only depends on the displacement.

A force can cause a body to move with a constant velocity, thereby the body will go on and on and on. So the displacement is infinite. So the work done must also be infinite. I thought about it and concluded that only the power of two forces is different but the work done by the two forces is infinite. It’s altogether confusing to me.

Can you please tell me if I have got something wrong and explain this to me?

[Klaynos]

Work is quite a convoluted topic. With lots of different applications, the general deffinition can often be used which is Work = change in energy.

 

A good place to start with reading about work is wikipedia...

 

http://en.wikipedia.org/wiki/Mechanical_work

[insane_alien]

Nivvedan, you have to consdider that the object you used 100N on will have 900J more kinetic energy when it reaches point B. than the object that only got 10N

 

and if an object is moving at constant rate and direction then there is no net force acting upon it. therefore no work is done.

[swansont]

Work = Force x Displacement. Right?

Let’s consider a frictionless space. Here I apply 100N force to a body and it moves with a constant velocity to another point 10 m from the starting position.

 

Contradictory statements. Can't have a nonzero net force and constant velocity.

[Mr Skeptic]

Work = Force x Displacement. Right?

 

Somewhat right. Work is the integral of force with respect to displacement. The integral is just a fancy infinite sum with the displacement infinitesimally small. If the force is constant, then it simplifies to force times displacement. If the force is constant then changes to a different constant, you have to calculate the work for each segment that the force is constant and add them together.

 

 

A force can cause a body to move with a constant velocity, thereby the body will go on and on and on. So the displacement is infinite. So the work done must also be infinite. I thought about it and concluded that only the power of two forces is different but the work done by the two forces is infinite. It’s altogether confusing to me.

Can you please tell me if I have got something wrong and explain this to me?

 

When you stop applying a force, you have Work = 0 * displacement = 0. Though as in your example, both objects get to the same places, one is moving faster -- it has more kinetic energy. No additional work is done on the objects as they keep moving with no force applied.