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/s2
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.
vi = 70 (mi/h) [1.61 km/mi][1000 m/km][h/3600 s]
vi = 31.3 m/s
x - xi = 186 ft [0.305 m/ft]
x - xi = 56.7 m
Now we are ready to use
v2 = vi2 + 2 a (x - xi)
0 = (31.3 m/s)2 + 2 a (56.7 m)
a = - [979.7 m2/s2] / (113.4 m)
a = - 8.64 m/s2
Notice that the acceleration is negative. The velocity is in one direction and the change in the velocity is in the other direction as the Jeep slows down!
(c) 2001, Doug Davis; all rights reserved.
