37. The net charge carried by John whose mass is m is roughly

q

=

(0.0001)

mN

A

Ze

M

=

(0.0001)

(90 kg)(6.02

× 10

23

molecules/mol)(18 electron proton pairs/molecule)

1.6

× 10

−19

C



0.018 kg/mol

=

8.7

× 10

5

C ,

and the net charge carried by Mary is half of that. So the electrostatic force between them is estimated
to be

F

≈ k

q(q/2)

d

2

=



8.99

× 10

9

N

· m

2

C

2



(8.7

× 10

5

C)

2

2(30 m)

2

≈ 4 × 10

18

N .