Kimberley_P
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A 2.400×10^22kg moon orbits a 7.200×10^24kg planet with an orbital radius of 2.500×10^8m. Calculate the gravitational potential at the point half way between the centres of the planet and its moon. You should take the Universal gravitational constant to be 𝐺=6.674×10^−11Nm^2kg^−2. [Note: more significant figures are used in this question than appear in earlier printings of the book.]
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BobbJo
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The potential at the midpoint = Potential due to moon + Potential due to planet
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Kimberley_P
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(Original post by BobbJo)
The potential at the midpoint = Potential due to moon + Potential due to planet
I got like 1.929x10^6 but its wrong
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BobbJo
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(Original post by Kimberley_P)
I got like 1.929x10^6 but its wrong
Yes it's wrong. Post the working
Potential varies inversely as distance so the formula is
 \phi =- \dfrac{GM}{r}
make sure to use correct formula
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Kimberley_P
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i did ((-2.4x10^22*6.674x10^-11)/2.5x10^8)+((--7.2x10^24*6.674x10^-11)/(2.5x10^8/2))
==-3.848x10^6 but still its wrong. so could you help me please???
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BobbJo
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(Original post by Kimberley_P)
i did ((-2.4x10^22*6.674x10^-11)/2.5x10^8)+((--7.2x10^24*6.674x10^-11)/(2.5x10^8/2))
==-3.848x10^6 but still its wrong. so could you help me please???
first one should be 1.25 x 10^8 in the denominator
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Kimberley_P
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(Original post by BobbJo)
first one should be 1.25 x 10^8 in the denominator
Thank You
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Kimberley_P
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HI, I have another question related to this which I can't solve

Calculate the gravitational potential at a point 6.800×10^8m from the centre of the planet in question F5.16 on the same side of the planet as its moon. You should take the Universal gravitational constant to be 𝐺=6.674×10^−11Nm^2kg^−2. [Note: more significant figures are used in this question than appear in earlier printings of the book.]

F5.16 A 2.400×10^22 kg moon orbits a 7.200×10^24 planet with an orbital radius of 2.500×10^8m. Calculate the gravitational potential at the point half way between the centres of the planet and its moon. You should take the Universal gravitational constant to be 𝐺=6.674×10^−11Nm^2kg^−2. [Note: more significant figures are used in this question than appear in earlier printings of the book.]
=-3.857x10^6 J/kg
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BobbJo
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(Original post by Kimberley_P)
HI, I have another question related to this which I can't solve

Calculate the gravitational potential at a point 6.800×10^8m from the centre of the planet in question F5.16 on the same side of the planet as its moon. You should take the Universal gravitational constant to be 𝐺=6.674×10^−11Nm^2kg^−2. [Note: more significant figures are used in this question than appear in earlier printings of the book.]

F5.16 A 2.400×10^22 kg moon orbits a 7.200×10^24 planet with an orbital radius of 2.500×10^8m. Calculate the gravitational potential at the point half way between the centres of the planet and its moon. You should take the Universal gravitational constant to be 𝐺=6.674×10^−11Nm^2kg^−2. [Note: more significant figures are used in this question than appear in earlier printings of the book.]
=-3.857x10^6 J/kg
draw a diagram to show the relative positions, then the distances to use are clearer

the gravitational potential at a point 6.800×10^8m from the centre of the planet in question F5.16 on the same side of the planet as its moon = gravitational potential due to planet at a point 6.8 x 10^8m from its centre + gravitational potential due to moon at a point (6.8 - 2.5) x10^8 m from its centre

the gravitational potential at a point 6.800×10^8m from the centre of the planet in question F5.16 on the same side of the planet as its moon = gravitational potential due to planet at a point 6.8 x 10^8m from its centre + gravitational potential due to moon at a point 4.3 x10^8 m from its centre
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Kimberley_P
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(Original post by BobbJo)
draw a diagram to show the relative positions, then the distances to use are clearer

the gravitational potential at a point 6.800×10^8m from the centre of the planet in question F5.16 on the same side of the planet as its moon = gravitational potential due to planet at a point 6.8 x 10^8m from its centre + gravitational potential due to moon at a point (6.8 - 2.5) x10^8 m from its centre

the gravitational potential at a point 6.800×10^8m from the centre of the planet in question F5.16 on the same side of the planet as its moon = gravitational potential due to planet at a point 6.8 x 10^8m from its centre + gravitational potential due to moon at a point 4.3 x10^8 m from its centre
thank you
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