# Homework Help: How Long Does Is take for a car to go from 0-60 MPH

1. Feb 13, 2013

### lorenamtimas

1. The problem statement, all variables and given/known data

A car accelerates at 9.8m/s^2 How long does it take to go from 0-60 MPH?

2. Relevant equations

I don't know which ones may be relevant, this is where I'm stumped.

3. The attempt at a solution

I somehow got 2.77 seconds as an answer but I'm not sure how I got it. I am new to physics; Conceptual Physics is my very first Physics class and I'm having an awfully difficult time understanding. Thank You.

2. Feb 13, 2013

### voko

Perhaps you could recall how you got and explain to us why you did this or that. And we will help if you get confused along the way.

3. Feb 15, 2013

### lorenamtimas

Well this is what I did:

I converted 9.8m to miles and got this 0.00609mi
I then divided this # by 10,000 to get as close to 60 miles as I could get, which resulted in 60.9mi
I then divided 10,000 by 3600 (the amount of seconds in as hour) which gave me 2.77seconds.
I'm still not clear as to why I made these choices but they seemed to make sense to me for some reason.

4. Feb 15, 2013

### voko

Here is what you did.

You converted $9.8 \frac {m} {s^2}$ to $0.00609 \frac {mile} {s^2}$, which is a little unusual but technically correct.

Then you found that $\displaystyle \frac { 60 \frac {mile} {hour} } {0.00609 \frac {mile} {s^2}} \cong 10000 \frac {s} {hour} s$.

Finally, because $\frac {s} {hour} = \frac 1 {3600}$, you got $10000 \frac {s} {hour} s = \frac {1000} {3600} s$ = 2.78 s.

All of this is correct, but, as I said, is unusual. The unusuality is that you did not convert all units to one system, but kept mixed units throughout. Typically one converts all the different units to one system, then computes, and then, if required, converts to other units.

Here is how I would do it.

Convert 60 MPH to m/s. Because one mile is 1609.34 m, and 1 hour is 3600 seconds, 60 MPH = 60 * 1609.4/3600 m/s = 26.82 m/s. Now we have the velocity and the acceleration in one system of units, SI (meters and seconds), and 26.82 m/s / 9.8 m/s^2 = 2.74 s.