Simple qn on vectors and scalars

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    Scalars Vectors
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Discussion Overview

The discussion revolves around the classification of physical quantities as scalars or vectors, exploring various examples and mathematical operations involving vectors. Participants examine the implications of formulas and operations, such as dot and cross products, on the nature of these quantities.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants propose that the classification of a physical quantity as a scalar or vector can be determined by the nature of the quantities involved in its formula, citing examples like velocity and density.
  • Others argue that while velocity is a vector due to its dependence on displacement, density is a scalar since both mass and volume are scalars.
  • One participant mentions that with two vectors in 3D, various operations can yield different results, including scalars, pseudovectors, vectors, and second-rank tensors.
  • Questions arise about the definitions and creation of pseudovectors and second-rank tensors, with some participants seeking clarification on these concepts.
  • Concerns are raised regarding the classification of work done, potential energy, and kinetic energy, with discussions on how vector multiplication affects these quantities.
  • Participants discuss the graphical addition of vectors, emphasizing the importance of coordinate systems and component breakdown for accurate calculations.
  • There is curiosity about the appropriate contexts for using dot and cross products in vector operations, with some suggesting that the definitions of quantities often dictate the choice of mathematical operation.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement regarding the classification of physical quantities and the implications of vector operations. The discussion remains unresolved on several points, particularly concerning the nature of work and energy in relation to vectors.

Contextual Notes

Some statements rely on specific definitions and assumptions about vectors and scalars, which may not be universally accepted. The discussion includes unresolved questions about the mathematical operations and their implications for different physical quantities.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of physics and mathematics who are interested in understanding the distinctions between scalars and vectors, as well as the mathematical operations involving them.

gunblaze
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ok...
Is it possible to use the formula to decide whther a physical quantity is a scalar or a vector?

eg:
Velocity=change in displacement/time

Thus, since displacement is a vector,
therefore, velocity is also a vector>?

And for density...
Density=mass/volume

SInce mass and volume are both scalar,
Therefore, density is a scalar>?
 
Last edited:
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gunblaze said:
ok...
Is it possible to use the formula to decide whther a physical quantity is a scalar or a vector?

eg:
Velocity=change in displacement/time

Thus, since displacement is a vector,
therefore, velocity is also a vector>?

And for density...
Density=mass/volume

SInce mass and volume are both scalar,
Therefore, density is a scalar>?

Yes and yes. If the input to a formula is a vectors or vectors then the answer to the formula will be a vector. Unless that is the formula has a dot product, or absolute value. Or in some cases it is possible to take one component of a vector and use that as a scalar answer.
 
With 2 vectors in 3D multiple operations can be made and the result can be
1.A scalar.
2.A pseudovector.
3.A vector.
4.A second rank tensor...

Daniel.
 
Hey Daniel, that's interesting. So, I'm assuming that you arrive at 1 using the dot product, and 3 using the cross product. But what is a pseudovector? How do you make one? And is a second-rank tensor just a matrix? Again, how do you combine the two vectors to make one? Thanks.

I hope the answer will not be too hard for me to understand...
 
Not really.At a vector you reach by vector-space operations...Addition and scalar multiplication.Cross-product is a special case of exterior product and the result is a pseudovector...As for second rank tensor,well it's the tensor/dyadic (ancient name) product...Scalar product (inner product) is just a contracted tensor product.

Daniel.
 
But... how about work done, potential energy and kinetic energy?

eg:
Work done=force*distance
work done=mass *acceleration*distance

Acceleration is a vector but that doesn't mean work done is a vector too rite?
 
Last edited:
"distance" is not a scalar...It is the "displacement vector"...In the definition of work,there is a dot/scalar product which makes the result a scalar...

Daniel.
 
Gunblaze, the answer to your question have to do with vector multiplication, which I am guessing you have not been exposed to.

F = ma (bold quantities are vectors)

This equation is a scalar times a vector. The reason they are called scalars is that they will scale a into a different vector F

Now look at work:

[tex]W = \vec{F} \cdot \vec{d}[\tex]<br /> <br /> This is a multiplication of two vectors, fundamentally different than Newtons Second law. Notice that the answer is a number (a scalar) , so this is a scalar product between vectors. Another example:<br /> <br /> [tex]KE = \frac{1}{2}m(\vec{v})^2 = \frac{1}{2}m\ vec{v} \cdot \vec{v}[\tex]<br /> <br /> So kinetic energy, again a scalar, is a product of v with itself.[/tex][/tex]
 
How do you graphically add vectors? For example Vector A is equal to a force of .980 N at a compass angle of 35 degrees and Vector B has a force of 1.96 N at 165 degrees.
 
  • #10
If u've given not only modulus,but also angles,then those angles are defined wrt certain axis of coordinates.Then u'll simply add their components...

Daniel.
 
  • #11
kdawg090 said:
How do you graphically add vectors? For example Vector A is equal to a force of .980 N at a compass angle of 35 degrees and Vector B has a force of 1.96 N at 165 degrees.
Well, the strategy for 2D is this. Choose a coordinate system, with one of the axes preferably along one of the vectors. Find the angles that the vecotrs make with the axes, and break into x and y components. Find the moduli of the components by multiplying with [tex]sin[/tex] or [tex]cos[/tex] and give (+) or (-) signs to the components, then add them as scalars to get the net x and y components.
To get the modulus of the resulting vector use the Pythagorean theorem, while to get the angle use the [tex]arctan[/tex].
 
  • #12
I'm curious as to when you use each method between 2 vectors. I've just been following equations up until now, but i would like to understand why, and for what reason (and to what end) i use the cross product between vectors, and when the dot. In what situations does one use each method??
 
  • #13
Depends of what he wishes to write.Usually,quantities are DEFINED using these matematical operations (with vectors),and therefore your last question loses meaning.


Daniel.
 

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