Help please on my physics homework?

Question

A car traveling 45km/h slows down at a constant 0.50m/s2 just by letting up on the gas. (a) the distance the car coasts before it stops, (b) the time it takes to stop, and (c) the distance it travels during the first and fifth seconds.

Answer 1

Your units are inconsistent.  Kilometers and hours in one, meters and seconds in the other.  We'll have to make them consistent.  For no good reason, let's use meters and seconds.

45 km/hr * 1000 m/1 km * 1 hr/3600 s = 12.5 m/s

a = (vf - v0)/t

a = -0.50 m/s^2 (can you explain why it's negative?)

vf = 0

v0 = 12.5 m/s

So,

t = (vf - v0)/a = (0 - 12.5 m/s)/(-0.50 m/s^2) = 25 s

(Two significant digits.  You should be able to explain why.)

s = v0t + (1/2)a t^2 = (12.5 m/s)(25 s) - (1/2)(0.50 m/s^2)(25 s)^2 = 156.25 m = 160 m (two significant digits.  Same comment.)

I'm going to let you get part (c).  Use the same equation we used just now.  For the first second, you have everything you need.  For the fifth second, you'll have to figure out how fast it's traveling at the beginning.  Use the equation we used for the time and you'll be fine.