the brakes on a car permit it to accel at the rate of -.80m/s/s how far to stop if the car is going 60 km/h?

Answer 1

Ok Issac,
You should know that when dealling with a question, a very good idea is to change all the units to SI units.
here, m/s² is a SI unit but km/h is not
to change km/h to the SI unit of velocity(i.e. m/s), you simply need to substitute the equivalents.
i.e. 1km = 1000m and 1 hr = 3600 sec
so 60km/h = 60 (1000m/3600sec) = 60 * 1000/3600 = 16.667m/s

the formula which relates "distance","acceleration" and " velocity" and does not contribute to " time" is:
V² - Vo² = 2ad
Where:
V = final velocity(since we want to stop the car, V = 0)
Vo = initial velocity = 60km/h = 16.666666666666667m/s
a = rate of acceleration = -0.8m/s²
d = unknown distance.
Now you need to substitute the values:
V² - Vo² = 2ad
=> 0 - (16.6666666666666667)² = 2(-0.8)d
=> d = (16.6666666666666667)²/1.6
=> d = 173.611m

I hope you got the procedure Issac.
Best of Luck dude!!!

Answer 2

Pull out your kinematic equations, and take a look at this one in particular.

vf² = vi² + 2ax

vf = 0
vi = 60 km/h = 16.667 m/s
a = -0.8 m/s²

0 = (16.667)² + 2 (-.8)x

x = 173.611 m

With significant figures, we would say x = 170 m = 0.17 km

Answer 3

apply newton second law :
0=(60*5\18)*(60*5\18) + 2*-.8*x therfore 173.61 metres

Answer 4

The time to stop is given by v = a*t. v = 60km/hr 16.7 m/s. Therefore t = 80/16.7 = 4.8 sec. The distance traveled in that time is s = 0.5*a*t^2 = 921.6 m