Physics Help Forum Common Misconceptions in Physics

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 Aug 1st 2009, 06:49 PM #1 Senior Member     Join Date: Apr 2008 Posts: 815 Common Misconceptions in Physics The post that follow this one was made by the member Pmb. He deserves all the credit. __________________ Isaac If the problem is too hard just let the Universe solve it.
Aug 1st 2009, 06:49 PM   #2
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Common Misconceptions in Physics

Misconception – Light has no mass.

Correct Physics – Whenever you see someone use the term mass you need to make sure you understand exactly what he or she means by it since the term mass when used unqualified, can refer to several different things. It can refer to proper mass (aka rest mass), inertial mass (aka relativistic mass/mass-energy), passive gravitational mass or active gravitational mass. Only when one is referring to proper mass can it legitimately be said that light has no mass.

Misconception – Einstein proved that gravity is a curvature in spacetime.

Correct Physics – Einstein argued that gravity is the result of viewing nature from a non-inertial frame of reference. He showed that when that doesn’t hold as in the case where tidal forces are present the result is due to spacetime curvature. Other physicists objected to the notion that a gravitational field has a relative existence so they chose to interpret tidal forces (i.e. spacetime curvature) as determining the presence of a gravitational field. Einstein actually objected to this interpretation.

Misconception – You cannot simultaneously measure both the position and the momentum of a particle.

Correct Physics – This is the wrong phrasing of Heisenberg’ s Uncertainty Principle (HUP), which actually states that the position and momentum are not simultaneously determined. This has a very different meaning than what is stated above. Uncertainty is a property of the quantum state and not determined by how measurements are done. No matter how a measurement is taken one cannot change the inherent uncertainty determined by the quantum state of the system.

Misconception – Mass does not depend on velocity.

Correct physics – This is a statement made by people who choose to use the term mass only to refer to what is known as proper mass aka rest mass. All the statement really means is that the proper mass of a particle does not depend on the particle’s speed.

Misconception – Physicists no longer use relativistic mass.

Correct physics – This is an incorrect statement. For example; Gary Oas, the author of the article On the Use and Abuse of Relativistic Mass, did a survey of literature on relativity and concludes that the concept of relativistic mass is still prevalent in the literature on relativity. The survey done by the author included 637 works. Of those 477 relied on the concept. This is 74% of the literature examined! Hardly more than the claim of zero! For a complete analysis one should read the above article and see exactly what the numbers mean. For example Figures 1 and 2 are diagrams illustrating the use over the years. It’s interesting that those textbooks, which are about special and general relativity, tend to use the concept more than books on modern physics that only contain a few chapters on relativity. As figure 1 shows, a greater percentage of relativity texts published in the years 2000-2005 used relativistic mass! I.e. 4 didn’t use it whereas 8 did! The claim “Physicists no longer use relativistic mass.” is a very serious mistake.

The following articles are from the American Journal of Physics. They are examples of the use of relativistic mass in a highly respected physics journal. This particular journal is a teaching journal.

The mass of a gas of massless photons
, H. Kolbenstvedt, Am. J. Phys. 63(1), January 1995

The inertia of stress
, Rodrigo Medina, Am. J. Phys. 74(11), November 2006

Apparatus to measure relativistic mass increase
, John W. Luetzelschwab, Am. J. Phys. 71(9), September 2003

When someone tells you that mass does not depend on speed your caution antenna should go up!

Misconception – Since a light has no mass it isn’t affected by gravity

Correct Physics – Physicists recognize three types of mass according to threes aspects of the concept of mass. They are

(1)Inertial mass – The ratio of a particle’s momentum to its speed
(2)Passive gravitational mass – The mass on which gravity acts
(3)Active gravitational mass – The source of gravity

Inertial mass is properly defined as the quantity m such that the quantity (momentum, $\displaystyle \bold p =$) $\displaystyle \mbox m \bold v$ is conserved. To be precise we say that inertial mass is defined so that momentum is conserved. The equivalence principle, from Einstein’s general theory of relativity, postulates the equality of inertial mass and passive gravitational mass. Since photons have momentum they have inertial mass; since they have inertial mass they have passive gravitational mass; since they have passive gravitational mass they are acted upon by gravity. We can also find the mass of a photon if we know its energy. For a photon $\displaystyle |\bold v|= \mbox c$. Since the energy of a photon is related to its momentum by $\displaystyle \mbox E = \bold p \mbox c$ we obtain $\displaystyle \bold p = \frac{\mbox E}{\mbox c}$. Setting this equal to $\displaystyle \mbox{mc}$ we obtain $\displaystyle \frac{\mbox E}{ \mbox c} = \mbox {mc}$. Solving for $\displaystyle \mbox m$ gives $\displaystyle \mbox m = \frac{\mbox E}{ \mbox c}$

Richard Feynman points all of this out in The Feynman Lectures on Physics – Volume I, by Feynman, Leighton and Sands. From page 7-11

 In the Einstein relativity theory, anything which has energy has mass – mass in the sense that it is attracted gravitationally. Even light, which has an energy, has a “mass.” When a light beam, which has energy in it, comes past the sun there is an attraction on it by the sun. The light does not go straight, but is deflected.

Misconception – Gravity is not a force, it’s a curvature in spacetime.

Correction –Einstein’s general theory of relativity treats the gravitational force on the same footing as inertial forces. First consider the definition of inertial force

Definition
: Suppose a particle is moving at constant velocity in the inertial frame S’. The momentum of the particle will be constant in S. Now consider another frame S which is accelerating relative to S’. The particle i When the motion of the reference system causes the momentum, as measured in the reference system, to be a function of time, i.e. $\displaystyle \bold p = \bold p(t)$, we say that there is an inertial force acting on the particle, the value being $\displaystyle \bold F \equiv \frac{d \bold p}{dt}$. The value of the inertial force will always have the form $\displaystyle \bold F = m\bold g$

An example of an inertial force is the centrifugal force which is the force felt by an object moving in a curved path that acts outwardly away from the center of rotation. Prior to general relativity inertial forces were viewed as being a result of viewing nature from the wrong frame of reference. It was for this reason that they are more often referred to by the fictitious force, pseudo-force or apparent force. Non-inertial forces can be expressed with a non-zero 4-force. It should be noted that Einstein viewed inertial forces as being “real.” Einstein also never interpreted gravity to be a curvature in spacetime either. Laymen often confuse spacetime curvature with the curved path of a particle being deflected in a gravitational field. A more common name for spacetime curvature is gravitational tidal gradient, i.e. tidal force. Loosely speaking, gravitational tidal force is the difference in gravitational force in a gravitational field. For an object to experience a tidal force it must have a finite spatial extension. The gravitational force can act on a single point particle. Point particles are not subject to tidal forces but are affected by inertial forces. It is therefore misleading to say that gravity is not a force but merely a curvature in spacetime.
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Last edited by arbolis; Aug 2nd 2009 at 05:05 PM.

 Nov 14th 2016, 02:56 AM #3 Junior Member   Join Date: Nov 2016 Posts: 1 Great explanation. I haven't heard about it before. Thanks for the wonderful information. Last edited by topsquark; Nov 14th 2016 at 05:03 AM.
 Feb 22nd 2018, 07:58 AM #4 Physics Team   Join Date: Apr 2009 Location: Boston's North Shore Posts: 1,576 Misconception – A particle can be in two places at the same time Correct physics - This statement is blatantly false. All one can say in quantum mechanics is what can be measured. Its not possible for the position of a particle to be measured in more than one place at a time. If the position is not measured then its position is not merely unknown, its undetermined.
 Feb 22nd 2018, 03:21 PM #5 Senior Member   Join Date: Apr 2017 Posts: 506 More importantly 99% of the people on this planet don't know the difference between Weight and Mass!!! Even science programs on History Channel promote this confusion by incorrect use of terms. But let me play devils advocate to get a debate going .... There is only one mass .... rest mass , gravitational mass, inertial mass , it's all the same .... but this mass will increase as velocity increases . Last edited by oz93666; Feb 22nd 2018 at 03:26 PM.
Feb 22nd 2018, 10:24 PM   #6

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 Originally Posted by oz93666 More importantly 99% of the people on this planet don't know the difference between Weight and Mass!!! Even science programs on History Channel promote this confusion by incorrect use of terms. But let me play devils advocate to get a debate going .... There is only one mass .... rest mass , gravitational mass, inertial mass , it's all the same .... but this mass will increase as velocity increases .
gravitational mass vs. inertial mass: The debate goes on. They are known to be virtually the same by measurement but there is no theoretical reason why they should be, so far as I know.

rest mass vs. "this mass will increase as velocity increases": Typically we don't consider mass to depend on the speed any more. The speed dependence is taken to be a part of the momentum 4-vector, meaning that the mass of the particle (object, whatever) is always the same. (Warning: Some actually do keep the speed dependence with the mass. But I believe that the statement I made is valid for most theorists anyway)

-Dan
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Feb 22nd 2018, 11:41 PM   #7
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 Originally Posted by topsquark gravitational mass vs. inertial mass: The debate goes on. They are known to be virtually the same by measurement
Only virtually the same ??? What experiments show a difference??? In Physics experiment is everything ...I don't see what debate there can be unless there is experimental evidence of a discrepancy .

 Originally Posted by topsquark rest mass vs. "this mass will increase as velocity increases": Typically we don't consider mass to depend on the speed any more.
Really ?? I didn't know that !! Are you saying mass doesn't increase with speed???

I was taught that the kilo of sugar in my kitchen reacted to other mass by the well known gravity equation , and accelerated from force by F=Ma ..

And this kilo mass increased by the well known formulae as it's speed increased , becoming infinite at the speed of light .

As far as I know all this still stands.

The reason my kilo of sugar can be more than a kilo is because I've increased its energy and energy is equivalent to mass ...E=mc2

If I take it up stairs it's potential energy has increased so it's more massive.

If I throw it at my wife it's kinetic energy has increased so it's more massive.

And this increase is real mass ... the sugar upstairs will resit acceleration from a force more than the same bag of sugar downstairs.

Last edited by oz93666; Feb 23rd 2018 at 12:56 AM.

Feb 23rd 2018, 01:34 AM   #8
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 gravitational mass vs. inertial mass: The debate goes on. They are known to be virtually the same by measurement but there is no theoretical reason why they should be, so far as I know.
Two of the greatest bringing together of theory from disparate starting points in Physics are

1) The fact that as TS points out the inertial mass and gravitational mass coincide.

2) That Thermodynamic theory deduced from classical considerations coincides with theory deduced from statistical considerations.

Oz93666,

Mass is a funny thing. Have you ever heard of effective mass, particularly in connection with Newton's second law?

Last edited by studiot; Feb 23rd 2018 at 01:37 AM.

Feb 23rd 2018, 02:18 AM   #9
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 Originally Posted by studiot Oz93666, Mass is a funny thing. Have you ever heard of effective mass, particularly in connection with Newton's second law?
Just looked into it ....as usual it's idiot "physicists" trying to complicate and confuse things ... make things more mysterious than they really are ... they have to make a noise so people think they're doing something ....

Mass is Mass , we understand and can predict precisely how it behaves ...nothing new has happened in this regard for 100 years ... so all these different "types" of mass are just hot air (IMHO) ...

Feb 23rd 2018, 04:36 AM   #10
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 Originally Posted by oz93666 Just looked into it ....as usual it's idiot "physicists" trying to complicate and confuse things ... make things more mysterious than they really are ... they have to make a noise so people think they're doing something .... Mass is Mass , we understand and can predict precisely how it behaves ...nothing new has happened in this regard for 100 years ... so all these different "types" of mass are just hot air (IMHO) ...

nothing new has happened in this regard for 100 years ?

Bessel and Green independently proposed the idea almost 200 years ago, and had developed the formal variation to Newton 2 by 1830.

Alain Connes (a mathematician and Fields medallist) writing in 2008 for Cambridge University "On the fine structure of Spacetime" carried it further.

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