During a solar eclipse calculate forces.etc

[lettertwelve]

During a solar eclipse...calculate forces...etc

Homework Statement

During a solar eclipse, the moon, Earth, and sun lie on the same line, with the moon between Earth and the sun.
(a) What force is exerted on the moon by the sun?
__________N
(b) What force is exerted on the moon by Earth?
__________N
(c) What force is exerted on Earth by the sun?
__________N

Homework Equations

F=(G*M1*M2)/R^2

The Attempt at a Solution

scroll down.

 

[mgb_phys]

R is the distance between the centres of the objects -- not the radius.
(it's R because it's the radius of the orbit)

 

[lettertwelve]

R is the distance between the centres of the objects -- not the radius.
(it's R because it's the radius of the orbit)

ok so for part A i did:

G=6.67*10^-11
Mass of moon: 7.36*10^22 kg
Mass of sun: 1.99*10^30 kg
dist between sun and moon: 1.5*10^11 m

(6.67*10^-11)(7.36*10^22)(1.99*10^30) ALL OVER (1.5*10^11) and i got 6.51e31

but it says it's incorrect...
should i be converting to Newtons?

 

[D H]

G=6.67*10^-11 -- you are missing the units! G is not a number like pi. It is a physical constant, which means its value depends on the units in which it is represented.

 

[lettertwelve]

G=6.67*10^-11 -- you are missing the units! G is not a number like pi. It is a physical constant, which means its value depends on the units in which it is represented.

alright then.. i don't know what the units would be in this case for G. i just know it's a universal constant. i never knew i would have to convert it somehow, and either way i still don't know how right now :(

 

[D H]

It is 6.673*10^-11 m³/kg/s². So of course you got the wrong answer when you mixed meters and kilometers.

 

[lettertwelve]

It is 6.673*10^-11 m³/kg/s². So of course you got the wrong answer when you mixed meters and kilometers.

thank you, but...that is very confusing, and my teacher failed to mention those units in class. now i REALLY am confused as to how to incorporate it into the equation!

 

[D H]

Sorry. For some reason I saw you using kilometers. You did use meters.

You're problem is here:

(6.67*10^-11)(7.36*10^22)(1.99*10^30) ALL OVER (1.5*10^11) and i got 6.51e31

What is the equation for the gravitational force?

 

[lettertwelve]

What is the equation for the gravitational force?

for which planet? i know that for Earth we should multiply the number by 10 to get it in Newtons from Kilograms

 

[D H]

I am talking about Newton's universal law of gravitation. You applied it incorrectly.

 

[lettertwelve]

I am talking about Newton's universal law of gravitation. You applied it incorrectly.

it's Fnet=ma
and F of gravity ~ (m1*m2)/d^2

how did i apply it incorrectly

 

[D H]

You divided by the distance, not the square of the distance.