What keeps the planets in orbit around the sun?

Aproton is projected toward a fixed nucleus of charge +ze with velocity vo. initially the two particles are very far apart. when the proton is a distance r from the nucleus its velocity has decreased to 1/2vo. how far from the nucleus will the proton be when its velocity has dropped to 1/4vo?

What ia the energy of an object because of its potential and kinetic energies

Which form of pure carbon is so hard that it can be used in cutting tools?