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[Jim]
Hi LB, Once again I have no idea what you are about. I will never get anything out of a third party comment so please tell me what you know here that should be clear to me if I was a smart as you or at least not as dumb as I am.
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I really don't think you have a problem in the smart category - it is merely a matter of training.
[Jim] Velocity is speed with direction.
[ LB] Velocity has two "pieces", speed and direction.
[Jim] Speed being motion with no particular direction.
[ LB] Speed is the non-directional "piece" of velocity.
Motion ALWAYS has a particular direction. But in a lot of situations the direction is not important, or is obvious from the context, etc. So we often just talk of something's speed.
And way too often we use the word velocity when we should use the word speed. WAY TOO OFTEN. As in almost all the time.
Any description of motion that doesn't *explicity* specify the direction is a speed, not a velocity.
75 miles per hour to the north east is a velocity.
75 miles per hour at a compass heading of 45 degrees is a velocity.
75 miles per hour straight up is a velocity.
(You might recognize that the first two are the same velocity.)
95 miles per hour "that way" is a velocity. Don't forget to point.
(But even if you are pointing to the north east, it is not the same velocity as the first two examples above.)
75 miles per hour is a speed.
[Jim] The Earth is accelerated by the Sun but its velocity does not change.
[ LB] Change "velocity" to "speed" and you are correct.
The speed of the Earth in its orbit is always about 30 km/sec (neglecting small variations due to eccintricity), but the direction in which the Earth moves is constantly changing. Because one of the pieces of the Earth's velocity is never constant, the velocity itself is never constant.
This is one of those times when the common practice of referring to a speed as a velocity is way too common. There isn't a convenient, widely understood way to specify a continuously changing direction. The closest we have is for talking about celestial bodies - we say "orbital" velocity to imply the changing direction.
As in - the Earth's orbital velocity is about 30 km/sec.
Since velocity is both pieces, they both have to be constant before you can say velocity is constant.
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Force and acceleration are related by f=ma. If energy particles are zero mass and this is used the mass can be removed and f=a. This is how a photon can be defined? Or this is false?
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Sorry; if mass = 0, then
f = 0 * a
and
f = 0.
I'm pretty sure this is not how photons are defined.
And it leaves another issue hidden - units.
Suppose mass = 1, then
f = 1 * a
and
f = a.
Right?
No, not right. f (still equals) ma. Plugging in numbers doesn't allow you to "remove the mass".
If you don't understand units, you don't understand reality (physics).
If you do a units analysis of each equation, you will see what I'm getting at when you get to "f = a". There is no way to make units of force equal units of acceleration.
Regards,
LB
