- #1
pc2-brazil
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Good evening,
(NOTE: This post got a little long; I hope someone has enough patience to read it.)
I self-teach physics to myself, but I don't have a complete book (I only have "Volume 2: Gravity, Waves and Thermodynamics" of "Physics" by Paul Tipler). Volume 1 discusses work and energy, but I don't have it.
So, it becomes much more difficult to understand little details that would otherwise simply be read in a book.
Another thing: my knowledge of calculus is also self-taught, so it is still basic.
One of those details is the sign of the work, that is, what signs should be put in the definition of work, depending on the situation, so that the final result has the correct sign according to whether the work is resistent or motive. I understand work as being dW = Fds, where F is the component of force acting in the direction of displacement.
Do physics books usually discuss the sign of work?
For example, in the case of the force of gravity. Its absolute value is given by:
[tex]F = \frac{GMm}{r^2}[/tex].
But every site that discusses the work of the gravitational force gives this expression:
[tex]W = \int^{r_2}_{r_1} -\frac{GMm}{r^2} dr = \frac{GMm}{r_2} - \frac{GMm}{r_1}[/tex]
Where does the first minus sign come from? I can't accept that it is from the definition of gravitational force, because its definition in absolute value is the one I gave above, and it doesn't have a minus sign.
What I can accept is that the minus sign arises when the body is moving away from the source of the gravitational field; since the force that acts in the moving body is opposite to the orientation of motion, then you have to put a minus sign in the force to indicate that the work will be resistent, negative. However, in the case of a body approaching the source of gravitational field, the gravitational force acts in the same orientation of motion; in this case, since the work is motive, positive, shouldn't it be dW = +Fds?
After thinking about it for a long time, I came to the following conclusions:
(I will represent work done by gravitational force along a radial displacement as dW = Fdr, where F is the absolute value of the gravitational force as I gave above, which, of course, always acts in the radial direction. Also, I will assume that POSITIVE displacement is to move away from the source of gravitational field, and this is an obvious choice, because, if r2 > r1, the displacement r2 - r1 will be positive):
Is this a correct way to determine the sign of work? Or is there other way?
Thank you in advance.
(NOTE: This post got a little long; I hope someone has enough patience to read it.)
I self-teach physics to myself, but I don't have a complete book (I only have "Volume 2: Gravity, Waves and Thermodynamics" of "Physics" by Paul Tipler). Volume 1 discusses work and energy, but I don't have it.
So, it becomes much more difficult to understand little details that would otherwise simply be read in a book.
Another thing: my knowledge of calculus is also self-taught, so it is still basic.
One of those details is the sign of the work, that is, what signs should be put in the definition of work, depending on the situation, so that the final result has the correct sign according to whether the work is resistent or motive. I understand work as being dW = Fds, where F is the component of force acting in the direction of displacement.
Do physics books usually discuss the sign of work?
For example, in the case of the force of gravity. Its absolute value is given by:
[tex]F = \frac{GMm}{r^2}[/tex].
But every site that discusses the work of the gravitational force gives this expression:
[tex]W = \int^{r_2}_{r_1} -\frac{GMm}{r^2} dr = \frac{GMm}{r_2} - \frac{GMm}{r_1}[/tex]
Where does the first minus sign come from? I can't accept that it is from the definition of gravitational force, because its definition in absolute value is the one I gave above, and it doesn't have a minus sign.
What I can accept is that the minus sign arises when the body is moving away from the source of the gravitational field; since the force that acts in the moving body is opposite to the orientation of motion, then you have to put a minus sign in the force to indicate that the work will be resistent, negative. However, in the case of a body approaching the source of gravitational field, the gravitational force acts in the same orientation of motion; in this case, since the work is motive, positive, shouldn't it be dW = +Fds?
After thinking about it for a long time, I came to the following conclusions:
(I will represent work done by gravitational force along a radial displacement as dW = Fdr, where F is the absolute value of the gravitational force as I gave above, which, of course, always acts in the radial direction. Also, I will assume that POSITIVE displacement is to move away from the source of gravitational field, and this is an obvious choice, because, if r2 > r1, the displacement r2 - r1 will be positive):
- If the displacement is positive ([tex]r_2 - r_1 > 0[/tex]) and force acts in the orientation of motion, dW = +Fdr => W > 0
- If the displacement is positive ([tex]r_2 - r_1 > 0[/tex]) and force acts opposite to the orientation of motion, then dW = -Fdr => W < 0
- If the displacement is negative ([tex]r_2 - r_1 < 0[/tex]) and force acts in the same orientation of motion, then dW = -Fdr (because the displacement is negative, the force, having the same orientation, has to be negative too; no sign needs to be put in the displacement, because, since it is already negative and minus * minus = +, it will make the final result positive) => W > 0
- If the displacement is negative ([tex]r_2 - r_1 < 0[/tex]) and force acts opposite to the orientation of motion, dW = +Fdr (since force is opposite to displacement and displacement is negative, then force is positive) => W < 0
Is this a correct way to determine the sign of work? Or is there other way?
Thank you in advance.
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