Work in Physics: Does it Always Involve Movement?

[adjacent]

Wikipedia defines work as:

In physics, a force is said to do work when it acts on a body so that there is a displacement of the point of application, however small, in the direction of the force. Thus a force does work when it results in movement.

What if in space,where there is no external force acting,Someone applies a force and the object will move on to a infinite displacement forever(Newton's first law).Is work done?

Another question is from that wikipedia:

For example, the centripetal force exerted inwards by a string on a ball in uniform circular motion sideways constrains the ball to circular motion restricting its movement away from the center of the circle. This force does zero work because it is perpendicular to the velocity of the ball.

Work done=Energy transferred . That means in circular motion no energy is transferred to the object ,not even kinetic.Then how can it move?

 

[tiny-tim]

hi adjacent!

What if in space,where there is no external force acting,Someone applies a force and the object will move on to a infinite displacement forever(Newton's first law).Is work done?

work is done is getting the object moving

once it is moving, it carries on in a straight line at the same speed (Newton's first law of motion) without any more force

"its work is done!"

Work done=Energy transferred . That means in circular motion no energy is transferred to the object ,not even kinetic.Then how can it move?

work was done to get it moving

the string keeps it in a circle, but doesn't reduce its speed

no further work is neeeded

 

[adjacent]

hi adjacent!


work is done is getting the object moving

once it is moving, it carries on in a straight line at the same speed (Newton's first law of motion) without any more force

"its work is done!"

How can I calculate the work done then?
W=FS
It moves to an infinite distance,Meaning work done is infinite?

 

[adjacent]

hi adjacent!
...
work was done to get it moving

the string keeps it in a circle, but doesn't reduce its speed

no further work is neeeded

We need centripetal force all the time it is moving in a circle,meaning work is done always?

 

[tiny-tim]

How can I calculate the work done then?
W=FS
It moves to an infinite distance,Meaning work done is infinite?

no, S is the displacement of the point of application of the force

the force is only applied at the start (to get it going)

after that, there is no force (so zero displacement)

We need centripetal force all the time it is moving in a circle,meaning work is done always?

the displacement (of the point of application of the force) is tangential, the force is radial

so the displacement "in the direction of the force" (see wikipedia) is zero

the full formula is the dot product, W = F·S​

 

[adjacent]

no, S is the displacement of the point of application of the force

the force is only applied at the start (to get it going)

after that, there is no force (so zero displacement)

But displacement is defined as the distance traveled in a stated direction.No force is needed.
Or do you mean S is the displacement(in the direction of force) covered when the force is still there?

Again

the displacement (of the point of application of the force) is tangential, the force is radial

so the displacement "in the direction of the force" (see wikipedia) is zero

the full formula is the dot product, W = F·S

We still apply a force and get tired why?If no work is done,then we would not get tired?

 

[tiny-tim]

But displacement is defined as the distance traveled in a stated direction.No force is needed.
Or do you mean S is the displacement(in the direction of force) covered when the force is still there?

yes

the displacement is of the point of application of the force

when the force is no longer applied, it no longer has a point of application!

We still apply a force and get tired why?If no work is done,then we would not get tired?

stand between two doorposts, and push apart

the doorposts don't move, but parts of your body do move, and work is done

now relax your arms … if they were made of elastic, you would get all the energy back, and then do the same again without getting tired

but they're not made of elastic, so the energy is wasted, and if you do it again, you have do work all over again

 

[Octans]

But displacement is defined as the distance traveled in a stated direction.No force is needed.
Or do you mean S is the displacement(in the direction of force) covered when the force is still there?

Think of it this way: work is only done when a force is applied. Imagine you and an object are the only two things in the whole universe. Start with the object at rest (relative to you) and push on it. You apply a force and the object accelerates from rest to some velocity in a certain distance while you push on it. Because you're applying a force and the object is moving, you're doing work. F isn't 0 and s isn't 0 in the W = F \cdot s equation, so W also isn't 0.

But once you let go of the object, it no longer accelerates. It travels at constant velocity and it does have a displacement, however you're no longer applying a force. Now W = 0 \cdot s = 0. So you can see that no work is done even though the object moves.

That was an ideal situation where the only force was your pushing. So that you could see the concept clearly, I neglected an important fact: just by having mass you would be attracting the object gravitationally and the object would be attracting you. So even once you let go of the object and you're not pushing on it, there would still be a gravitational force pulling it back toward you. Does this force do work?

 

[Dale]

We still apply a force and get tired why?If no work is done,then we would not get tired?

We are inefficient, 0% efficiency in some circumstances. You get tired, not because you are doing external work, but because you are wasting lots of energy generating heat by using your muscles inefficiently.