Question

A box is given an initial leftward velocity of -20 m/s. Friction works against that motion, causing a constant acceleration of 4 m/s2 until the box stops. Hint: would acceleration be positive or negative here to slow the box down?
How far does the box travel before it comes to rest?
How much time does the box take to stop?

Answer 1

if v is - , a is +

v = Vi + a t
0 = -20 + 4 t
t = 5 seconds

x = Xi + Vi t + .5 a t^2
x = 0 - 20(5) + 2 (25)
x = -100 + 50
x = -50

Answer 2

I understand how you got the time but I dont really understand how x is -50
The box can travel in a negative?

Answer 3

sure, it has a negative velocity (leftward)
so it ends up at negative, not positive, x