gravity stuff

[pizza1994]

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A 0.2-kg apple rests on the surface of the Earth. Approximate the gravitational force exerted by the apple on the Earth. (Note: The universal gravitational constant, G, is 6.7 × 10-11 N-m2/kg2, the mass of the Earth is 6.0 × 1024 kg, and the radius of the Earth is 6.4 × 106 m.)

Answer: 2 N
Source: TPR

can someone explain why we dont use the g=GM1m2/r^2 equation forms?

 

[sillyjoe]

You absolutely can and still can get the same answer.

However, it is much easier to apply Newton's 3rd Law and realize that the force exerted by the earth on the apple (gravity) will be equal in magnitude to the force exerted by the apple on the earth.

 

[pizza1994]

You absolutely can and still can get the same answer.

However, it is much easier to apply Newton's 3rd Law and realize that the force exerted by the earth on the apple (gravity) will be equal in magnitude to the force exerted by the apple on the earth.

how do you do it using the equation? what mass do you use>?

 

[sillyjoe]

The mass of the earth and the mass of the apple.

 

[pizza1994]

The mass of the earth and the mass of the apple.

so conceptually now why do we use both masses? Since the apple is on the earth then cant we just use g=Gm/r^2 because w=mg ??