Question

An electric vehicle starts from rest and accelerates at a rate of 2.0 m/s2 in a straight line until it reaches a speed of 20 m/s. The vehicle then slows at a constant rate of 1.0 m/s2 until it stops. (a) How much time elapses from start to stop? (b) How far does the vehicle travel from start to stop?

Answer 1

(a) total time elapsed =10+20 =30s

Answer 2

total distance travelled =100 +200 =300 m

Answer 3

v = u + at
20 = 0 + 2t1
t1 = 10 sec

2nd phase:-
v = u - at
0 = 20 - 1 x t2
t2 = 20 sec

total time = t1+t2 = 10 + 20 = 30 sec
(b) 1st phase:

s = ut + 1/2at^2
s1 = 0 + 1/2 x 2 x (10)^2
s1 = 100 m
2nd phase:

By s = ut - 1/2at^2
s2 = 20 x (20) - 1/2 x 1 x (20)^2
s2 = 200 m

total distance(s) = s1+s2 = 100 + 200 = 300 m

Answer 4

woohoo. you are legend!

Answer 5

sorry matricked but no working...no medal...i got the first one already though... the second one confused me

Answer 6

don't hate the player...hate the game...

Answer 7

see for calculating time use velocity/acc for both retardation and acc as the vehicle starts from and stops to 0.
for distance use distance= average velocit * time and in each case its (20+0)/2
thus its easy for others to do orally

Answer 8

average velocity * time is used whenever acc. or retardation is constant ...
it helps in getting the answers quickly it time is known