# Deceleration Formula

Deceleration Formula

Deceleration is the opposite of acceleration. It is the rate at which an object slows down. Deceleration is the final velocity minus the initial velocity, with a negative sign in the result because the velocity is dropping. The formula for acceleration can be used, recognizing that the final result must have a negative sign.

deceleration = (final velocity - initial velocity) / time

d = (vf - vi)/t

d = deceleration

vf = final velocity

vi = initial velocity

t = time

Deceleration Formula Questions:

1) You are driving at 100 m/s and a cat runs out in front of your car. You stop rapidly, which takes 5 seconds, and miss the cat. What is your deceleration?

Answer: The initial velocity, vi = 100 m/s. The final velocity, vf = 0 m/s. The time, t = 5 sec.

d = (vf - vi) /t

d = (0 m/s - 100 m/s)/ 5 sec

d = -100m/s / 5 sec

d = - 20 m/s2

2) You and a friend are driving on the highway at 150 km/hr when you see a police car ahead. You decelerate to 120 km/hr, which takes you 2 seconds. What is your deceleration in km/sec?

The initial velocity, vi = 150 km/hr x 1/3600 hr/sec = 0.042 km/sec. The final velocity, vf = 120 km/hr x 1/3600 hr/sec = 0.033 km/sec. The time, t = 2 sec.

d = (vf - vi) /t

d = (0.033 km/sec - 0.042 km/sec) / 2 sec

d = - 0.009 km/sec / 2 sec

d = - 0.0045 km/sec2