A space probe of mass m m\,m is launched radially outwards from the surface of a planet of radius RRR. The graph shows how the gravitational potential energy EpE_{\text{p}}Ep of the probe varies with its distance r r\,r from the center of the planet.
What physical quantity is represented by the gradient of the tangent to this curve at distance r0r_0r0?
The magnitude of the gravitational force acting on the probe at r0r_0r0.
The gravitational potential of the planet's field at r0r_0r0.
The gravitational field strength of the planet at r0r_0r0.
The escape velocity of the probe from r0r_0r0.