# Common Misconceptions in Physics II

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#### topsquark

Forum Staff
Let's get a fresh start here. This is a place to discuss various subtleties in Physics that may be common or generally accepted but turn out to be wrong.

-Dan

#### neila9876

Rule of parallelogram

Assume a photon moves in a reference frame, the angle with x axis is ß. See the attached picture.
Then, the component of light speed along x axis is:
A.Cx = c*cos ß
B.Cx = c
C.No one knows.
Thank you.

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#### ChipB

PHF Helper
1. Centrifugal forces exist.
2. Kilograms are a measure of weight.
3. A car's horsepower determines how fast it can accelerate.
4. If an object of mass m has its velocity increased by Delta V, it's kinetic energy is increased by 1/2 m (Delta V)^2.

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#### neila9876

charge vs spin

What's exactly "electric"? Some people consider that charge is something as seeds scattering inside watermelon, and so when the particle spin, the so called "spin magnetic moment" occurs.

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#### neila9876

field vs interaction

Some people consider that fields can interact.
In fact, it's matters that interact. Field is only a kind of artificial method of representation.

#### topsquark

Forum Staff
What's exactly "electric"? Some people consider that charge is something as seeds scattering inside watermelon, and so when the particle spin, the so called "spin magnetic moment" occurs.
In Physics the term "electric" is used when we either have an electric field present or when there are electric charges being moved around ie. a current.

When you say that "charge is something as seeds scattering inside [a] watermelon" I get the picture of a bunch of particles inside the watermelon zooming around. That's not what happens. The electrons (staying or not staying with a particular molecule) are represented as a probability wave inside the watermelon.

An object's spin magnetic moment is proportional to the torque created by the magnetic dipole moment in a magnetic field. On the Quantum scale we refer to either the orbital angular momentum or the spin "angular momentum" of a particle. There is a distinction here: An electron may have an orbital angular momentum but it also has spin angular momentum that has nothing to do with the orbital angular momentum. The reason the two concepts are needed is because the term "spin" in the spin magnetic moment is not due to the angular momentum of the electron, which are from different causes. The spin of a particle is not an angular momentum though it can often be used that way. Thus the spin magnetic momentum doesn't really have anything to do with how the particle is torqued by an applied magnetic field.

-Dan

#### topsquark

Forum Staff
Some people consider that fields can interact.
In fact, it's matters that interact. Field is only a kind of artificial method of representation.
It looks like we crossed posted.

The concept of a field was introduced in the 1800s. (I just came up with those dates... I didn't bother to check when.) Most forces can be thought of either fields encountering particles or by fields interacting with other fields. The idea of fields (Classical or QM) is one of the most fruitful constructs in all of Physics and I can't overstress how important this is.

You say "it's matter that interacts." May I ask just how the two particles (or matter, or whatever) actually scatter from each other? The traditional Physics answer is that particles interact via the properties of the field. For example, two electrons scattering off each other cannot be described as two electrons "hitting each other" it's due to the electric fields of the two electrons interacting. Physically colliding electrons are forbidden by the Uncertainty Principle.

-Dan

#### neila9876

seeds vs watermelon

@dragon:
"When you say that "charge is something as seeds scattering inside [a] watermelon" I get the picture of a bunch of particles inside the watermelon zooming around. That's not what happens."
.......
It's somepeople say that, not me...my concept of charge is "presentaion of interaction" rather than something substantial. It's not the topic here.

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#### neila9876

field vs interaction

@dragon:
"You say "it's matter that interacts." May I ask just how the two particles (or matter, or whatever) actually scatter from each other? The traditional Physics answer is that particles interact via the properties of the field. For example, two electrons scattering off each other cannot be described as two electrons "hitting each other" it's due to the electric fields of the two electrons interacting. "
...........
That's true I say that.
Imagine only one rabbit remains in cosmos at last (of course it seems to be Dandan), will field exist?

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