deceleration

Question

A car is travelling at 20 m/s south and slows to 15m/s.

What is the deceleration?

There is no time stated in the question, but  the textbook gives the answers; -1m/s South,  or +1 m/s North.

I can't work out how to get there.

Answer 1

To answer this question, we need one more piece of information: Either the time it took to decelerate, or the distance over which the deceleration took place. If you know the time t, the acceleration is

a = (v₂ - v₁)/t

 

Since v₂ < v₁, this will give a < 0, and hence it is a deceleration.

 

If you know the distance x over which the deceleration took place, you can get the acceleration from the kinematic equation

 

v₂² - v₁² = 2ax

 

This too will give a < 0, hence a deceleration.  Either way, essential information is missing from the problem statement.

Answer 2

Hi Hagrid,

 

I'm surprised that the problem doesn't say how long it took the car to get from 20m/s to 15 m/s. 

 

Just assume it's equal to 1 second

 

The formula for deceleration is (final velocity - initial velocity) / time taken

deceleration = (15 -20)/(1)

 

 

deceleration = -5