In physics how do we know when to use x=x0 + v0t +.5at^2 versus v^2=v0 +2a(chang
ID: 1773811 • Letter: I
Question
In physics how do we know when to use x=x0 + v0t +.5at^2 versus v^2=v0 +2a(change in x)? Could you please use the question below to explain, I know that a uses the first formula and b uses the second formula but I'm not sure why. Thank you:)
Il You are driving to the grocery store at 20 m/s. You are 110 nm from an intersection when the traffic light turns red. Assume that your reaction time is 0.70 s and that your car brakes with constant acceleration. a. How far are you from the intersection when you begin to apply the brakes? b. What acceleration w bring you to rest right at the intersection? c. How long does it take you to stop?Explanation / Answer
a)
tr = reaction time = 0.70 s
V = constant speed before brakes applied = 20 m/s
distance travelled before applying the brakes = V tr =20 x 0.7 = 14 m
distance from the intersection = 11 0 - 14 = 96 m
b)
X = stopping distance = 96 m
Vo = initial velocity = 20 m/s
Vf = final velocity = 0 m/s
a = acceleration
using the equation
Vf2 = Vo2 + 2 a X
02 = 202 + 2 a (96)
a = - 2.1 m/s2
c)
t = time to stop
Vf = Vo + at
0 = 20 + (-2.1) t
t = 9.52 sec