I need help with part of this physics problem.?

Question

A speeding car is pulling away from a police car. The police car is moving at 30 m/s. The radar gun in the police car emits an electromagnetic wave with a frequency of 20.0 x 109 Hz. The wave reflects from the speeding car and returns to the police car where the frequency is measured to be 475 Hz less than what was emitted. The speed of sound is 343 m/s. How fast is the speeding car going?

This is the question I'm working on. I know that I first have to find the frequency when the wave hits the speeding car, however I'm not sure how to do that. I can't seem to find a way to figure out the frequency when I do not have the speed of the car. What equation should I use to find the frequency? Thanks in advance for any help.

Answer 1

This is a doppler-effect problem...but if you don't remember the formula or don't know how to apply it, think of it this way:

During each second, the radar gun sends out "X" number of wave pulses (X = 20.0 * 10^9). However, during that same second, only "X - 475" pulses make it back to the police car. Where are the missing 475 pulses? They are still between the police car and the speeder. So this must mean that the space, between the police car and the speeder, has _widened_ by the length of 475 wave pulses during that 1 second.

In other words, the speeder is pulling away from the cop, at a rate of 475 "wavelengths" per second.

So now all you have to figure out is, how long is one "wavelength"? That's where the "20.0 x 10^9 Hz" comes in. You get the wavelength by dividing the speed of the wave by the frequency. For radar, the speed of the wave is the speed of _light_ (the speed of _sound_ is irrelevant to this problem; I expect it was thrown in just to throw you off track). The speed of light is 3 * 10^8 m/sec. So the wavelength is: (3 * 10^8 m/sec) / (20.0 * 10^9 Hz) = 0.015 meters.

One wavelength is 0.015 m, so 475 of them are (0.015 m)*(475) = 7.125 meters.

That means the car is pulling away at a rate of 7.125 meters every second (to make room for the "missing" 475 wavelengths that don't make it back to the cop).

Now, that's in addition to the 30 m/sec that the cop is already going. So the actual speed of the speeding car is 37.125 m/sec.

Answer 2

Firstly, when you want to do scientific notation, add a carrot between the "10" and its power "9 in this case". You can find that by hitting Shift+6.

Doppler effect:

Observed = [ v / (v +s) ] * Emitted
Where v = the speed of frequency in medium,
s = velocity of object.

Moving things around nets you:

s = v * (Emitted - Observed) / Observed

It doesn't matter than you don't need each car's speed at first. Physics lets you use the fact that the car is going 30 m/s faster as the velocity. Remember though, that by doing this, you will be answering how much faster than the police. Which means that your answer will be:

velocity = s + 30

Answer 3

Look up the doppler effect in your textbook....ANd use it to figure out the answer!! You will be able to answer it once you know how doppler works....Just like the pitch of a police siren changes whenver it passes you......

You can do it!

Answer 4

you'll be able to reproduction and paste those equations into google and replace mass1, mass2 and radius with the values out of your subject.. a. ((5*9.80 one*radius )^0.5*( a million-(mass1-mass2 )/( 2*mass1 )))^2/ (2*9.80 one) b. ((5*9.80 one*radius )^0.5*(a million-( mass2-mass1 )/ (2*mass2 )))^2/ (2*9.80 one)