# Does gravity exert greater force on a mass as it progress further from a gravitational body to a defined limit?

Whilst perusing a wikipedia article about arboreal movement I came across a line which implied that

"As an animal moves up an inclined branch, they must fight the force of gravity to raise their body, making movement more difficult."

I interpreted this (possibly incorrectly) that the author was saying gravity becomes more of a problem the higher you climb. This seemed at odds with my elementary school learning.

Does gravity exert a greater force on an object the further it gets from a larger body or is that pressure simply the result of atmospherics? Surely, all things being equal, climbing 1 metre above the ground is the same gravitational force as climbing 100 metres above ground?

Note: I had also assumed that at n distance gravity exerts a continually weakening effect which allows travelling bodies to slingshot into the gravitational orbit of a body and then back out again as gravity is weaker the further away from a body.

• Can you provide the link to that article? Feb 21 '17 at 12:18
• The line is As an animal moves up an inclined branch, they must fight the force of gravity to raise their body, making movement more difficult. That means only that it is more difficult to ascend an incline than to move horizontally. It does not imply it gets increasingly difficult to move with increased height. Feb 21 '17 at 12:21
• I'm voting to close this question as off-topic because it is based on a misinterpretation of the homework problem. Feb 21 '17 at 14:38
• @JonCuster I am not aware of a policy stating that misinterpretations are off-topic. Even though it regards homework, this question asks about a direct physical principle. Feb 22 '17 at 10:12
• @Steeven - The question was parsed wrong, leading to an apparent paradox with physics. The resolution is not physics, but English language usage. I could go either way, except that the OP even pointed out that it was possibly an interpretation problem. Feb 22 '17 at 13:59

As you hint in your answer, the force of gravity actually reduces as you get further away from the source, in this case the centre of the Earth. You can use the simple equation; $$F=\frac {GM_1m_2}{r^2}$$

but in this case, the mass of the Earth $M_1$, is so dominant, you can barely measure the reduction in the effort of the animal to climb.

I am grateful for StephenG's clarification of this point:

It's not the size of Earth's mass that matters, it's the radius of the Earth. The difference between 6371 km and (say) 10 m more is tiny, hence the gravitational acceleration is, to a good approximation, constant at reasonable heights close to the surface.

Atmospheric pressure can be discounted as trivial.

Note: I had also assumed that at n distance gravity exerts a continually weakening effect which allows travelling bodies to slingshot into the gravitational orbit of a body and then back out again as gravity is weaker.

I would be careful if you are thinking of this reason to explain probes using the gravity assist of planets, for example the Cassini spacecraft various excursions between planets to pick up speed.

If you search this site, you will find Gravity Assist answers that will explain how this slingshot effect works, and it is not as straightforward as you may think.

• It's not the size of Earth's mass that matters, it's the radius of the Earth. The difference between 6371 km and (say) 10 m more is tiny, hence the gravitational acceleration is, to a good approximation, constant at reasonable heights close to the surface. Feb 21 '17 at 13:18
• How close to the earth's core do we need to be for the earth to exert maximum gravitational pull? Feb 21 '17 at 13:23
• Well, actually it will reduce as we descend, as there will be mass above us pulling us back upwards. So the surface gravity is the maximum we feel. At the core, you would be weight less.
– user146020
Feb 21 '17 at 13:28

As others have stated, the gravitational force decreases the further from the center of mass you are: $$F = G\frac{mM}{r^2}$$

I would interpret the quoted comment as saying

"As an animal moves up an inclined branch, they must fight the force of gravity to raise their body, making movement more difficult [than a non-inclined branch]."

It should be noted that, while the force does in fact get weaker at higher altitudes, the effect is largely negligible for any height that an animal would be climbing.

Yes you are correct. The strength of gravity does not vary significantly within the region inhabited by animals, and certainly does not increase with height.

But I think you are making a fuss about nothing. The linked article is teaching about the ecology of animals. It is not teaching about physics. What the author probably means is that an animal that climbs up and down - eg between ground and trees - uses more energy than one which stays on the ground. Such movement is more difficult because it uses more energy.

• I've deleted the comments, some of which were inappropriate, and removed some unnecessarily inflammatory statements from the answer. I'd advise everyone involved to refrain from such inappropriate behavior in the future. Feb 22 '17 at 23:11