simple physics question involving speed and distance...?

Question

how long would it take to cover 21 feet going 20 mph? 27ft going 20? can someone remind me how to do these problems so i can do the rest of them?

Answer 1

First convert mph to feet/s. Then divide distance by speed to get time in seconds.

For conversion, remember that 1 mile = 5280 ft. and 1 hour = 3600 seconds.

Thus, 20 mph = (20) * (5280) / 3600 = 29.3 ft/s

Time to cover 21 ft = (21) / (29.3) = 0.72 s.

You can do the next one similarly.

Answer 2

Sure. Convert mph onto feet per second (1 mile is 5280 feet and 1 hout is 3600 seconds) and divide that into the distance in feet to get the time in seconds.

Doug

Answer 3

Convert known units to usable units: Miles to feet, and hours to seconds.
1 mile = 5280 feet.
20mph = 20 x 5280 = 105,600 feet per hour
Reduce hours to minutes.
105,600 / 60 = 1760 feet per minute.
Reduce minutes to seconds
1760 feet per minute / 60 = 29.333... feet per second.

29.333... feet per 1.0 second = 21 feet per X seconds.

Use quadratic equation to solve for X.

29.333... 21
_______ = __
1.0 ... X

(The above formula may not look right because some of the spaces between the 1.0 and the X seem to be deleted when I post the answer. X is supposed to be directly underneath the 21.)

The equation can be read as," 29.333...feet is to 21 feet as 1 second is to X seconds.
Cross multiply the two known values on opposite sides of the equals sign, and divide by the remaining known value to solve for X.
In this case: Cross multiply 21 x 1, and divide by 29.333... = X
X = .715909090909... seconds to travel 21 feet at 20 mph.

Use the same process for second problem.

Answer 4

Distance = Rate X Time.

(I remember that formula as "DIRT")

Rate is speed.