That's Right! Good job!

If a Jeep brakes to a stop, from 70 mph to zero, in 186 feet, what is its acceleration?

a = - 8.64 m/s^{2}That's Right! Good job!

Seventy miles per hour to zero in 186 feet? Watch out for the units! Let's begin by changing everything to SI units.

v

_{i}= 70 (^{mi}/_{h}) [^{1.61 km}/_{mi}][^{1000 m}/_{km}][^{h}/_{3600 s}]v

_{i}= 31.3 m/sx - x

_{i}= 186 ft [^{0.305 m}/_{ft}]x - x

_{i}= 56.7 mNow we are ready to use

v

^{2}= v_{i}^{2}+ 2 a (x - x_{i})0 = (31.3 m/s)

^{2}+ 2 a (56.7 m)a = - [979.7 m

^{2}/s^{2}] / (113.4 m)a = - 8.64 m/s

^{2}Notice that the acceleration is

negative. The velocity is in one direction and thechangein the velocity is in theotherdirection as the Jeep slows down!

(c) 2001, Doug Davis; all rights reserved.