# Doppler Effect of police car

. While standing at a crosswalk, you hear a frequency of 630 Hz from an approaching police car. After the police car passes, its frequency is 430 Hz. What is the velocity of the police car? (speed of sound in air = 343 m/s)

Hootenanny
Staff Emeritus
Gold Member
What do you know?

This sounds very much like a homework problem. Tell us what you have done to solve it so far and we can help to point you in the right direction.

At least you seem to know that the problem involves the Doppler effect.

berkeman
Mentor
On a related question, "As the police car approaches you, your heart rate is 75 beats per minute. As the police car passes you, your hear rate falls to 65 bpm. Why is this?"

Hootenanny
Staff Emeritus
Gold Member
berkeman said:
On a related question, "As the police car approaches you, your heart rate is 75 beats per minute. As the police car passes you, your hear rate falls to 65 bpm. Why is this?"
Hmm I don't know, physiological processes would be my guess. Loud noise stimulates release of adrenaline from the adrenal glands..?

berkeman
Mentor
Hootenanny said:
Hmm I don't know, physiological processes would be my guess. Loud noise stimulates release of adrenaline from the adrenal glands..?
Or maybe we were worried that the police car was going to stop

After all, that would ruin the experiment. Plus, we may not get to submit our paper on time....

Hootenanny
Staff Emeritus
Gold Member
berkeman said:
Or maybe we were worried that the police car was going to stop

After all, that would ruin the experiment. Plus, we may not get to submit our paper on time....
Hehe, speaking of which, I was driving to the gym a couple of weeks back and I notice a police car behind me. It followed me for literally ten minutes, next thing the flashing blue lights come on and sirens start wailing! My heart nearly jumped out of my chest. Anyway, I pulled over and the car just sped past . All worked up over nothing.

Do you reckon our friend aechols1 is coming back?

berkeman
Mentor
Hootenanny said:
Hehe, speaking of which, I was driving to the gym a couple of weeks back and I notice a police car behind me. It followed me for literally ten minutes, next thing the flashing blue lights come on and sirens start wailing! My heart nearly jumped out of my chest. Anyway, I pulled over and the car just sped past . All worked up over nothing.

Do you reckon our friend aechols1 is coming back?
That's too funny. :rofl: I don't know about the OP -- probably they'll check back Sunday night to finish off the assignment? Have a good weekend, Hoot.

Hootenanny
Staff Emeritus