Magnitud VS. Component of a vector

AI Thread Summary
The discussion clarifies the distinction between the magnitude and component of a vector, particularly in the context of force. It emphasizes that while the magnitude of a vector is always positive, components can have signs that indicate direction, which is crucial for calculations like work done. The example of weight force illustrates how confusion can arise when interpreting F as either magnitude or component, potentially leading to incorrect results. The importance of understanding when to use magnitude versus component is highlighted, especially in integrals involving direction. Overall, clear differentiation between these terms is essential for accurate physics problem-solving.
almarpa
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Hello all.

Sometimes, when reading a physics book, I find it difficult to distinguish between the magnitud of a vector and the component of a vector. For example, take the weight force, with the positive z direction pointing upwards (that is, F= -mg k). Sometimes, people write F=mg referring to its magnitude, but in other cases, people use F= -mg referring to the z component (the only component of the force vector, in this case).

Of course, it is important to distinguish between both cases, because components have sign, but amplitudes are always positive. For example, it could be a seious problem when evaluating the work done by the weight. When performing the integral ∫ Fdz, F is the component, and must have sign (and dz as well, depending on the direction of the movement).But it is easy to think that F is a magnitude, and forget about the sign, so we would get a wrong result.

Can you please tell me what is the general rule to know when F represents the magnitude of a vector, and when F represents component of a vector?

Thank you so much.
 
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Hi,

Basically ##\vec F## is always a vector. But, as you describe, sometimes the direction is so evident that we only need the magnitude. Sloppy, but it happens.
 
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