# Physics Aqa unit 4 exam question help?

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#1
I cant understand this question. Can someone please explain it, thanks.
The red circled option is the answer btw.

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6 years ago
#2
(Original post by Moddapukka)
I cant understand this question. Can someone please explain it, thanks.
The red circled option is the answer btw.

Posted from TSR Mobile
The gravitational force acting on an object is a vector quantity. The forces acting at the midpoint sum.
Spoiler:
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The forces acting at the midpoint from the two masses in the question are of opposite direction but equal in magnitude. Therefore the sum of the gravitational field (forces) at the midpoint will cancel.

Gravitational potential is a scalar quantity and is defined as the work done against gravity to bring a mass to a given point in the gravitational field. Gravitational potentials sum.

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The gravitational potential from each mass is given by:

U = -Gm/r

But m1 = m2 in this case (equal mass) then U = -2Gm/r

So the gravitational potential from both masses at the mid point is:

U1 + U2 = 2U since both mass and distance is the same.

Hence Utot = 2(-2Gm/r) = -4Gm/r
1
#3
I get that G. Force is vector and G. Potential is scaler, i also understand why the field strength is zero but i still dont get why V(G. Potential)= -4Gm/r. Could you please just explain where the 4 in the -4GM/r came from and why when m1=m2, V=-2Gm/r.
Sorry for the late reply and thanks for the previous explanation.

Posted from TSR Mobile
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6 years ago
#4
(Original post by Moddapukka)
I get that G. Force is vector and G. Potential is scaler, i also understand why the field strength is zero but i still dont get why V(G. Potential)= -4Gm/r. Could you please just explain where the 4 in the -4GM/r came from and why when m1=m2, V=-2Gm/r.
Sorry for the late reply and thanks for the previous explanation.

Posted from TSR Mobile
You're just adding the totals together. -GM/r + -GM/r = -2GM/r. M = (2M) because the masses of both points are equal, therefore V = -4GM/r.
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#5
Oh i understand it now. Its because r=r/2 for the first step so V=-Gm/r turns into V=-2Gm/r and then you just add it because its a vector. Thanks for all your inputs.

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