A proton(m=1.67 * 10^-27 kg) is suspended at rest in a uniform electric field E. Take into account gravity and determine E. How do I find the field if I don't know what r is. A spacecraft makes a trip from the Earth to the Moon, 380000km away. At what point in the trip will the gravitational field be zero? The mass of the Moon is about 1/81 that of Earth. I understand this problem I think, but I want to make sure. I plug the numbers in for Newton's universal gravitation law. I have to fields, one exerted by the Earth, one by the moon. I set the radius in the moon's field as (380000000-r)meters. Right?