# Gravitational field question

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Thread starter 9 months ago
#1
Hi im a bit confused over 5 (e) of this paper below

http://www.ocr.org.uk/images/409055-...nian-world.pdf

mark scheme: https://www.ocr.org.uk/Images/410707...world-june.pdf

Why am I not allowed to use the equation T^2=(4pi^2r^3)/GM for this question? thanks.
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9 months ago
#2
(Original post by fiftythree)
Hi im a bit confused over 5 (e) of this paper below

http://www.ocr.org.uk/images/409055-...nian-world.pdf

mark scheme: https://www.ocr.org.uk/Images/410707...world-june.pdf

Why am I not allowed to use the equation T^2=(4pi^2r^3)/GM for this question? thanks.
This equation assumes that the orbiting body is moving around a stationary body of much larger mass, like the planets around the sun. But in your question both bodies are moving around their centre of mass.
1
Thread starter 9 months ago
#3
(Original post by David Getling)
This equation assumes that the orbiting body is moving around a stationary body of much larger mass, like the planets around the sun. But in your question both bodies are moving around their centre of mass.
Thx
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9 months ago
#4
(Original post by fiftythree)
Hi im a bit confused over 5 (e) of this paper below

http://www.ocr.org.uk/images/409055-...nian-world.pdf

mark scheme: https://www.ocr.org.uk/Images/410707...world-june.pdf

Why am I not allowed to use the equation T^2=(4pi^2r^3)/GM for this question? thanks.
You need to calculate the mass of S1. Kepler's equation can be used only for S1 and not S2
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