In Suggestion for Improvements we discussed a few things that might add to the quality of this website. One suggestion I made was a page called Misconceptions in Physics. I started working on this last night. I created two of them so far. I'm posting them here for discussion.
Common Misconceptions in Physics
Misconception – Since a light has no mass it isn’t affected by gravity[/FONT]
Correct Physics – Physicists recognize three types of mass according to threes aspects of the concept of mass. They are as follows
(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, p = ) mv 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 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 have its energy. For a photon |v|= c. Since the energy of a photon is related to its momentum by E = pc we obtain p = E/c. Setting this equal to mc we obtain E/c = mc. Solving for m gives m = E / c2.
Richard Feynman points 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: When the motion of the reference system causes the momentum, as measured in the reference system, to be a function of time, i.e. p = p(t), we say that there is an inertial force acting on the particle. The value of the inertial force is F = dp/dt.
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-forceor 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.