To see if this is justified,
let us calculate the acceleration of the proton in the given electric
field:
p
p
e E
a
m
=
     
19
4
1
27
(1 6
10
C)
(2 0
10
N C
)
1 67
10
kg −
−
−
×
×
×
=
×
     
12
–2
1 9
10
m s =
×
which is enormous compared to the value of g (9 8 m s–2), the
acceleration due to gravity