[RESOLVED] Speed Calculations

[Duckfather]

Ok, I need a little help here guys, I've got a bit of brain fade

If I have a vehicle that takes 2 seconds to travel 33 feet, how can I calculate the vehicles speed in miles per hour and kilometers per hour from this data?

Thanks in advance

Andy

[Atheist]

Think like this: Convert the distance to the new unit (lets go with kilometers, which will be 33 feet = 0.01 Km)

So your vehicle travels 0.01 Kilometers in 2 seconds.

But you dont' want 2 seconds, you want an hour, which is 3600 seconds. The time value will have to be multiplied by 1800 to get up to 3600 ( 2 * 1800 = 3600), and if we multiply the seconds by 1800, we need to do the same with the distance: 0.01 * 1800 = 18 Km.
The vehicle is traveling at 18 Km/h

[NickThissen]

You can also use the simple formula:

speed = distance / time

If you want the speed in miles per hour, you enter the distance in miles, and the time in hours. If you want it in kilometers per hour, you enter the distance in kilometers and the time in hours.

Since the distance is in feet, you have to convert it to miles or kilometers first.
Since the time is in seconds, you have to convert it to hours first.

After that, use the formula to get the speed.

[jemidiah]

You can remember Nick's formula (if not naturally) by noticing the units involved:

Speed = [a unit of distance / a unit of time] = [km / hour] for example
Distance = [km] for example
Time = [hour] for example

Speed = [km / hour] = [km] / [hour] = Distance / Time

This same sort of trick can be used for a large number of formulas in Physics. It's probably a trick worth remembering.


As for the unit conversion, I love the "railroad" method as the first teacher who taught it to me named it. Here's a quick example.

How many hours is 49124 seconds?

60 seconds = 1 minute, so (1 minute) / (60 seconds) = 1. Similarly (1 hour) / (60 minutes) = 1. From this you can figure out that

1 second = 1 second * 1 = 1 second * (1 minute) / (60 second) = (1/60) minute = (1/60) minute * 1 = (1/60) minute * (1 hour) / (60 minutes) = (1/(60*60)) hour = 1/3600th of an hour ~= .0002777 hours = 1 second. We just converted one second into hours, and got a really small number--which makes sense. It helps to write this out in standard fractional notation, so you can see the things canceling.

Then we can answer the original question easily:
49124 seconds = 49124 seconds * (1/3600 hour) / (1 second) = (49124 / 3600) hours ~= 13.645 hours.


I chose an obtuse example and included lots of steps that can be skipped or done differently. In reality this process would have taken about 3 seconds.

[Duckfather]

Thanks guys. That's a massive help