[OCR] 4
Topic: Atomic Structure
(a) Explain why gravitational potential is always negative whereas electric potential can have positive and negative values, given that both potentials are zero at infinity.\n$\qquad$\n$\qquad$\n$\qquad$\n$\qquad$\n$\qquad$\n$\qquad$\n$\qquad$\n$\qquad$\n$\qquad$\n$\qquad$ [11.0m]
Part 1: (b) Fig. 4.1 shows how the gravitational potential $\phi$ from the surface of a planet varies with distance $r$ from the centre of the planet.\n
\nFig. 4.1
[1m]
Part 2: (i) A moon orbits the planet. The mass of the moon is $1.48 \times 10^{23} \mathrm{~kg}$ and its orbital radius is $1.07 \times 10^{6} \mathrm{~km}$.\n\nUsing Fig 4.1, calculate [1m]
Part 3: 1. the orbital speed of the moon,\norbital speed $=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots$$ [2m]
Part 4: 2. the total energy of the moon. [2m]
Part 5: (ii) A rock is projected vertically upwards with a speed of $45 \mathrm{~km} \mathrm{~s}^{-1}$ from the surface of the planet. The resistive forces on the rock by the planet's atmosphere are negligible.\n\nUsing Fig 4.1, [1m]
Part 6: 1. show, with clear working, that the rock will not escape the planet, [2m]
Part 7: 2. determine the maximum distance of the rock from the surface of the planet.\n$$\text { distance }=\ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots \ldots$$ [2m]
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