How do you find the mph by dividing length by time?

Question

My project requires a radar gun, but its like $100, and I'm not spending that much on a project. So theres a way to do it by thinking...math. But, I'm not sure how to do it by math. All i know is to divide the length of the platform is going on, by the time it taked to get across. The thing is, is that I will be dividing time by seconds, and by centimeters. Would the time be in decimals? and how would doing the problem look like, Example= say the length is 112 centimeters (about 32 inches) and the time the object takes to get down is 8 seconds. so would it look like this?
______
8 / 112 ? or would the 8 or 112 be in decimals?

Thanks a lot!

Answer 1

do you really need mph?

Just convert the distance to miles (32 inches = 0.000505 miles).
convert time to hours ( 8 seconds = 0.00222 hours)
divide the two .000505/.00222 = .227 mph

1 in = 0.00001579 miles
1 second = 0.00027778 hours

Whereas, 112 cm / 8 sec = 14 cm/sec [length / time]

Answer 2

You need to convert the units. For your example, convert 112 centimeters to miles and 8 seconds to hours. Hence, using some rounding:

112 cm / 100 (cm/m) = 1.12 m
0.32 m / 0.3048 (m/ft) = 3.67 feet
3.67 feet / 5280 (ft/mi) = 0.000695 miles

8 sec / 3600 (sec/hour) = 0.00222 hours

divide length by distance to get velocity:

0.000695 miles / 0.00222 hours = 0.313 mph

Answer 3

Divide your distance by the time to arrive at the speed.
112cm/8sec=14cm/sec
Then divide by 2.54 cm/in : 14/2.54=5.512 in/sec
Next, multiply by 3600 sec/hr : 5.512*3600=19843.2 in/hr
Finally, divide by 63,360 in/mile : 19843/63360=.3132 mph
Hope this helps.