EK 1001 phyiscs number 165

[2010premed]

An object with mass m is held near the surface of he earth. If r is the distance between the centers of gravity of the earth and the object, and M is the mass of the earth, which of the following represents the gravitational force on the earth due to the object?
A. Gm/r^2
B. GM/r^2
C. Mg
D. mg

Why is the answer D?

---

Members don't see this ad.

 

[Phantastic]

Well, they asked for the gravitational force on the earth, not the acceleration, so that rules out A and B:

F=ma=GMm/r^(2), so both the answers that look like GM/r^(2) are solving for the acceleration of the earth.

F(1 on 2) = -F(2 on 1)...Newton's third law is why the answer is D.

 

[surag]

Well, they asked for the gravitational force on the earth, not the acceleration, so that rules out A and B:

F=ma=GMm/r^(2), so both the answers that look like GM/r^(2) are solving for the acceleration of the earth.

F(1 on 2) = -F(2 on 1)...Newton's third law is why the answer is D.

THIS IS WRONG. I really dislike how people confuse others with info they themselves don't understand.

First off gmm/r2 is a force equation. Second it is only used when r is significant such that it's value is larger than or nearly the same as the radius of earth since that is where it's center of gravity is.

However since the statement is near earths surface r does not matter and the weight of the object due to the force ofgravity takes this into account. Therefore mg is correct since as the above user correctly said newtons third law applies.

 

[Phantastic]

THIS IS WRONG. I really dislike how people confuse others with info they themselves don't understand.

First off gmm/r2 is a force equation. Second it is only used when r is significant such that it's value is larger than or nearly the same as the radius of earth since that is where it's center of gravity is.

However since the statement is near earths surface r does not matter and the weight of the object due to the force ofgravity takes this into account. Therefore mg is correct since as the above user correctly said newtons third law applies.

My, my. That's the pot calling the kettle black. You should check your logic, and not deter this poster from a good explanation.

You're right: GMm/r^2 is a force equation. However, you'll notice that choices A and B exclude one of the respective masses each...so, that makes them accelerations (look at the units).

As my answer above states: F=ma=GMm/r^2 --> therefore, a=GM/r^2, so neither A nor B could be correct, because we are looking for a force. (which has units of Newtons, just so we get that straight: kg*m/s^2)

Therefore, only choices C and D stand as forces.

 

[2010premed]

Thanks this helped a lot. I didn't see how it was "force" so I was solving for g. Thanks again!