Why Does the Pitch of an Emergency Siren Change as It Passes By?

[gttjohn]

hi there question 1
why does the pitch of a siron on an emergency car change as it comes and then goes.
2 I've tryied to understand e=mc2 with some difficulty but what i did understand is that the faster an object travels the more energy it needs the more energy it has the more mass it has getting to the speed of light could this work in reverse , like with a massive explotion ie big bang massive speed= massive mass=black hole ..sorry I am a metal fabricator i dot have any degrees in physics I am sorry if theese are stupid questions but I am really interested .3
why do objects look smaller the further they are away I've asked my friends but they think I am crazy asking questions like thees and not talking about football ,thanks guys/girls.
laymans terms pls sorry if you can

 

[berkeman]

Welcome to the PF!

Answer to 1) The Doppler Effect http://en.wikipedia.org/wiki/Doppler_effect

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[Nabeshin]

Again, welcome!

2.

E=mc² is used to describe the amount of energy mass has just by virtue of its having mass. That is, it does not consider the amount of energy something has because it is moving. If you wanted this, you need to consider the total energy formula:
$$E_{tot}=\frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}$$

I can't really make heads or tails out of your question regarding "running this in reverse" or anything, but maybe what I posted above should clear this up?

 

[DaveC426913]

3) Excellent question! Some things are so second nature that we don't even think about them, let alone think about how to describe them. This is such an example.

Light rays from objects travel in straight lines. Things that are farther away from you will subtend a smaller angle (i.e they will span less of your view). We interpret this as appearing "smaller".

See attached diagram.

 

[gttjohn]

thankyou very much for the welcome guys
ok the vision question makes real sense to me
the doplar effect took some reading and understanding but i think i have the hang of it its like the sound waves plus the speed of the siren traveling to you are say the speed of sound + the viechle speed but once they pass you its minus the viechle speed so the waves take longer to reach you and the pitch changes am i right .i know the black hole thing i asked is a bit far out maby ill do a lot of reading before i try to ask thet question properly again ..thank you for taking the time to get back to me