Calculate the gravitational force of the moon

AI Thread Summary
The discussion focuses on calculating the gravitational force exerted by the moon on two 1 kg blobs of water located on opposite sides of the Earth. The user initially miscalculated the forces by considering the attraction of the Earth to the moon instead of the blobs to the moon. After correcting the distances by accounting for the Earth's radius, the revised gravitational forces were found to be approximately 3.4E-5 N for the blob closest to the moon and 3.2E-5 N for the farthest blob. Clarifications were made regarding the units of gravitational force, confirming that they are indeed measured in Newtons. The conversation highlights the importance of accurately applying the gravitational formula and understanding the relevant distances in such calculations.
annjolino
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Homework Statement


COnsider two identical 1kg blobs of water on opposite sides of the Earth, one on the side facing the moon and the other on the side farthest away from the moon.

a)calculate the gravitational force of the moon on the blob on the side of the Earth closest to the moon. (considering that the distance i have for Earth to moon is centre to centre.)

b) calculate the force of the moon on the blob on the farthest side away from the moon.

Distance from Earth to moon = 3.84E8
Radius of Earth = 6.37E6
G= 6.67259E-11
Mass of Earth = 5.98E24
mass of Moon = 7.36E22

Homework Equations



F=G(m1m2)/d^2

The Attempt at a Solution



seeing how the blobs of water on the surface i assumed i would have to subtract the radius of the Earth from the distance to give me a more accurate distance from blob to moon.
3.84E8-6.37E6 = 3.78E8.

now factored that into equation

F=G(m1m2)/d^2
F=6.6725(5.98E24x 7.36E22)/3.78E8^2
F=2.06E20

thats a huge number does that seem correct..

for b) all i changed was the distance which i added the radius of the Earth instead which gave 3.9E8
F=1.92E20

i hope this is right...
 
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You are likely off by the factor of the mass of the Earth.

They only asked about attraction to the moon. The other mass that you are considering are the 1 kg blobs.

As to your distances You would do well to take the Earth moon distance and add a radius of Earth to one and subtract for the other.

Good luck.
 
Oh ok i see now i was actually calculating the attraction of the Earth to the moon instead of the blob to the moon
cheers thanks for that.
 
ok now i get Fnear= 3.4e-5
and Ffar= 3.2e-5

that sounds much better i think.
 
Quick question are the units for the gravitational force N ? seeing how in the equation there are 2 kg / m^2
 
annjolino said:
Quick question are the units for the gravitational force N ? seeing how in the equation there are 2 kg / m^2

Check out the units of the gravitational constant.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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