50. ••• A proton is fired into a uniform electric field, opposite to the direction of the field. The proton’s speed upon entering the field is 3.15 × 105 m/s, and it comes to rest 5.25 cm after entering the field.

(a) What is the electric field strength?

(b) What is the proton’s velocity when it is only 3.50 cm into the field? [Hint: There is more than one answer. Why?]