# Deceleration of a particle

1. Feb 19, 2014

### TheShiny

1. The problem statement, all variables and given/known data
"A car is approaching traffic lights. The car is travelling with speed 10ms-1. The driver applies the brakes to the car and the car comes to rest with constant deceleration in 16s. Modelling the car as a particle, find the deceleration of the car."

2. Relevant equations
v = u + at

3. The attempt at a solution
[STRIKE]s =[/STRIKE], u = 10ms-1, v = 0ms-1, a = ?, t = 16s

v = u + at
0 = 10 + 16a
a = -10 ÷ 16
a = -0.625ms-2

The answer given is 0.625ms-2. Why is the answer positive and not negative?

2. Feb 19, 2014

### TheShiny

If the particle (the car) is slowing down it would have negative acceleration.

So, to go from acceleration to deceleration multiply by -1.

Therefore the deceleration would be -1 * -0.625 = 0.625

Is that why the answer is positive or have I misunderstood?

3. Feb 19, 2014

### lendav_rott

The car's acceleration is -0.625 units, its deceleration is 0.625 units :) Game of words
You have understood correctly.

Last edited: Feb 19, 2014
4. Feb 19, 2014