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# a car is moving 15 m\s north needs to stop in 100 m. what acceleration is required? A. +2m/s2 B. -15 m/s2 C.-1m/s2 D. 0 m/s2?

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- NCSLv 71 month ago
There are a couple of methods.

by kinematics:

v² = u² + 2ad

(0m/s)² = (15m/s)² + 2 * a * 100m

a = -225m²/s² / 200m ≈ -1 m/s² ◄ C

by work/energy:

KE becomes friction work:

KE + W = 0

½mv² + m*a*d = 0

mass m cancels

a = -v² / 2d = -(15m/s)² / 2*100m ≈ -1 m/s² ◄ C

- oubaasLv 71 month ago
(0-Vi^2) = 2*a*d

a = -(Vi^2)/2d = -225/200 = -1.125 m/sec^2 (-1.1 with just two sign. digits)

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