If an object goes from 70 mph to 10 mph in 5 seconds, what is its acceleration

  -12 mph/second . . . or . . . -17.6 ft/s²

Step-by-step explanation:

Acceleration is the change in velocity divided by the change in time.

  a = (10 mph -70 mph)/(5 s) = -60/5 mph/s

  a = -12 mph/s

That is, each second, the velocity decreases by 12 miles per hour.

This can be translated to ft/s² by translating miles per hour to feet per second. The factor for doing that is 1 mph = 22/15 ft/s. So, ...

  a = -12 mph/s = (-12 mph/s)·(22/15 ft/s)/(1 mph)

  a = -17.6 ft/s²

Additional way to represent the result

The acceleration due to gravity is about 32 ft/s², so this acceleration can be expressed in terms of "g".

  (-17.6 ft/s²)·(g/(32 ft/s²)) = -0.55g

That is, the magnitude of the deceleration is a little more than half a gravity unit. A 150 pound person would experience a decelerating force of about 82.5 pounds.

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step-by-step explanation:

step-by-step explanation:

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