# Homework Help: Acceleration due to gravity.

1. Jul 23, 2009

### Petrucciowns

The universally accepted value of accleration due to gravity is 9.8 m/s ^2. My question is why would an experimental value differ from the accepted value. What factors would contribute to this difference?

2. Jul 23, 2009

### ralilu

Its not really a rigid universally accepted value. The acceleration due to gravity decreases ssllliiggghhhty as you go further from the center of the earth.

3. Jul 23, 2009

### zcd

Law of universal gravitation states that $$F=G\frac{m_{1}m_{2}}{r^{2}}$$. On earth, this would translate to differences in an object's mass and altitude.

4. Jul 23, 2009

### ideasrule

An object's mass has nothing to do with its acceleration (unless it's massive enough to affect the gravitational field significantly). For just about any experiment that doesn't use ultra-precise instruments, friction, air resistance, and errors in instrument readings completely overwhelm any inherent differences in acceleration.

5. Jul 23, 2009

### zcd

Sorry, I misread the OP. However acceleration would just be force of gravity divided by the object's mass so it condenses down to distance from center as ralilu said.

6. Jul 23, 2009

### Mentallic

The main factors are experimental errors as Idealsrule said, but the strength of gravity does differ very slightly at different points on the Earth because of the different heights to the centre of the Earth and it also depends on the composition of the matter between the surface and centre of the Earth (which also varies at different points)

7. Jul 24, 2009

### RoyalCat

Another factor that affects gravity is the object's distance from the equator, and this is for two reasons.
The earth isn't actually a perfect sphere.
So the distance to the center is maximal at the equator and minimal at the poles.
Another factor is the effect of earth's rotation. From the object's frame of reference, at any point on the earth's surface it is affected by two forces (For the sake of the following analysis I'll assume a non-massive object and uniformity of the earth's interior). The gravitational pull, $$F=m\frac{(GM_e)}{r^2}$$, where $$r$$ is the distance to the center of the earth, and the centrifugal force as a result of earth's rotation around its axis, $$F=m\omega^2 r$$, where $$r$$ is the minimal distance to the axis of rotation from the object's location.

Note that the centrifugal acceleration points opposite the direction of gravity, and so it weakens it.
If I recall correctly, some world records regarding the height of jumps were overturned because they were set at a high altitude near the equator.

Earth's gravity varies by about 0.4%, when you compare the poles and the equator.