1 significant digit but the answer doesn't make sense

[john merrick]

Homework Statement:

A train on a straight track sounds a 400 Hz horn when passing a parked car on
the road. The train is moving at 20 m/s and the temperature is 10°C. Find the
apparent frequency of the horn for a person sitting inside the car when the train
is approaching and when the train is moving away

Relevant Equations:

no equation

my problem is not how you get the answer... 425.16Hz for approaching. it has to be do the right amount of significant digits.

so it should be 1 significant digit which would mean 400hz. which would make no sense since i started with 400hz.

i'm assuming they should have written the question with a decimal after one of the numbers so you would be looking at 2 or 3 significant digits in your final answer.

 

Just call it 425Hz, or if you really want 430Hz. I don't think anyone will really care too much about it!

 

[john merrick]

i got no problem with that but they are marking on the correct significant digits. so you just can't but whatever you want.

 

[phinds]

i got no problem with that but they are marking on the correct significant digits. so you just can't but whatever you want.

I think most likely you are supposed to start w/ the assumption that the 400 implies 400.0, but I could be wrong. If that's not the case, then all bets are off since you have no way of knowing whether either of the 0's in the 400 are significant or not.

 

I feel it's a bit harsh to deduct points for incorrect significant figures, considering they don't tell you anything about the uncertainties in those measurements.

If you take it at face value, then I guess you just have to assume that $f_0 = (400 \pm 1) \text{Hz}$ and $v = (20 \pm 1) \mathrm{ms^{-1}}$. What was the error on the value you used for the speed of sound?

Propagate those through quickly, and check what the error on $f$ is, and print a suitable number of significant figures.