Question
8 marksAnswer all parts. Use the stated idealised models; you do not need facts about BepiColombo's actual flight path. No calculation is required.
1(a)
A model spacecraft travels at constant speed in a circular orbit around Mercury. Ignore the Sun and other bodies.
A pupil says that constant speed means there is no acceleration and no resultant force. Explain why both parts of that statement are wrong, and identify the force that maintains the orbit.
[3 marks]
1(b)
During an earlier approach, the spacecraft is moving to the right relative to Mercury. A short thruster firing must reduce this rightwards speed. Ignore gravity during the short firing.
State the direction in which propellant must be expelled relative to the spacecraft. Use Newton's third law to explain the slowing, including why the thruster works in a vacuum.
[3 marks]
1(c)
Consider only the gravitational interaction between Mercury and the much less massive spacecraft. A pupil says, 'Mercury pulls harder on the spacecraft than the spacecraft pulls on Mercury because Mercury is larger.'
Correct the comparison of the two forces, and explain which body has the larger acceleration due to this interaction.
[2 marks]
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