I need your help please.?

Question

An electron, starting from rest and moving

with a constant acceleration, travels 4 cm in

7 ms.

What is the magnitude of this acceleration

in km/s2? Answer in units of km/s2.

Answer 1

d = 0.04 m

t = 0.007 s

d = ut + 0.5at^2

0.04 = 0 + 0.5a(0.007^2)

a = 0.04/[0.5(0.007^2)]

= 1632.65 m/s^2

= 1.6 km/s^2 to two significant figures

OR

average speed = (beginning speed + final speed)/2

average speed = 0.04/0.007 = 40/7 m/s

40/7 = (0 + final speed)/2

final speed = 80/7 m/s

acceleration = (final speed - beginning speed )/time

= (80/7 )/0.007 = 80000/49

= 1.6 km/s^2