3. (a) With a understood to mean the magnitude of acceleration, Newton’s second and third laws lead to

m2a2 = m1a1 =⇒ m2 =

(6.3 × 10−7 kg

) (7.0m/s2

)

9.0m/s2= 4.9 × 10−7 kg .

(b) The magnitude of the (only) force on particle 1 is

F = m1a1 = k|q1||q2|

r2 =(8.99 × 109) |q|2

0.00322 .

Inserting the values for m1and a1 (see part (a)) we obtain |q| = 7.1 × 10−11 C.