Question

A car slams on its brakes, coming to a complete stop in 4.0 s. The car was traveling south at 60.0 mph. Calculate the acceleration. mi/h/s

Answer 1

We need to apply the following expression

Vf = Vo + a.t

Where a is the acceleration
t is the time it takes to achieve Speed Vf
And Vo the initial speed 

In this problem Vo = 60 miles/hour
Vf =  0 (comes to a stop)
t = 4 seconds 

a = -60 [miles/hours] / 4 [s] = -15 miles/hour/second    (negative as it goes in                                                                                                  the opposite direction)

Answer 2

Answer:

The car is decelerating with the magnitude of 0.00416 mi/h².

Or

The car is decelerating with the magnitude of 15 mi/h/s.

Explanation:

Given that,

Time = 4.0 sec

Speed = 60.0 mph

We need to calculate the acceleration

Using equation of motion

$v = u+at$

Where, v = final speed

u = initial speed

a = acceleration

t = time

Put the value into the formula

$a= \dfrac{v-u}{t}$

$a=\dfrac{0-60.0}{4.0\times3600}$

$a =-0.00416\ mi/h^2$

We need to calculate the acceleration  in mi/h/s

$a = \dfrac{60.0\ mph}{4.0\ s}$

$a= -15\ mi/h/s$

Negative sign

Hence, The car is decelerating with the magnitude of 0.00416 mi/h².

Or

The car is decelerating with the magnitude of 15 mi/h/s.