How did they figure the 9.8 m/s2 acceleration, and how does the earth know to pull harder on a heavy object...

Question

and lightly on a light object, so they fall together???

Answer 1

Rate of acceleration can be easily measured. Just drop an object with negligible air resistance from a known height, time its descent, and find the value of g assuming that it is a constant force.

Gravity is proportional to mass. The more massive something is, the greater the gravitational force affecting it. As a result, an object twice as massive gets pulled with twice the force. Since it also has twice the inertia, it accelerates at the same rate as any other object would, regardless of the mass.

Answer 2

The Earth doesn't pull any harder regardless of the mass of an object. The pull due to gravity is 9.8 m/s^2 at sea level which is a constant that accelerates a feather at the same rate as an elephant.

Gravitational pull varies inversely proportionally to the distance between two objects (the earth and the elephant) but for most gravity calculations the value is assumed to be constant, since distance changes near the earths surface have almost no effect of the pull.

The force created is the result of the mass of an object multiplied by the acceleration, however since inertia is also proportional to mass the acceleration of these object matches, neglecting wind resistance.

Answer 3

Semi-easy way to calculate this your self.

Get a long pendulum, the longer the better (a good physics lab would probably have a 2 meter long pendulum).

Place a weight somewhere along the pendulum. [*Edit: Pull the pendulum back so that when you let it go], gravity will cause it to start swinging. Count how many times the pendulum swings in a given amount of time (a minute, 30 seconds, whatever). Repeat this a couple times and average the time. From this, you can calculate the time it takes to complete half of one swing (pendulum pointing straight down).

Tape a piece of copier paper and a piece of carbon paper to the pendulum with the carbon against the paper. At the same time you release the pendulum, release a marble or ball bearing (a good physics lab would use one electromagnet to hold the ball bearing and one electromagnet to hold the pendulum in place. Both electromagnets would be part of the same electrical circuit, which could be disabled with a switch. This would release the ball bearing and pendulum as close to simultaneously as you can get.)

Measure how far down along the pendulum the ball bearing collided with the pendulum (you'll have to move the paper and try again if the collision misses the paper completely).

Now you know how far the ball fell in a given amount of time. If you assume acceleration is constant, you'd have to solve two simultaneous equations to get the ball's final velocity and its acceleration.

Moving the weight will change the period of the pendulum. Time the new period and repeat the experiment. Repeating the experiment allows you verify whether or not constant acceleration was a valid assumption (it turns out is was, so you're spared having to solve four simultaneous equations).

Answer 4

The gravitational pull between 2 objects is based on their individual mass and the distance between them.

9.8m/s2 is the acceleration with which an object will fall towards Earth if no other force (air friction) acts on the object. Galileo was the one who first hypothesized all objects would fall toward Earth (or on any other planet) at the same rate. The crew of Apollo-15 proved his hypothesis on the Moon, by dropping a hammer and a feather. Both objects fell at the same rate toward the Moon's surface (the Moon has gravity but no air, so the only force acting on the objects was gravity).

Answer 5

If you drop an object, you can measure its acceleration. I've had my students do this in their physics lab many times - there are actually many other ways you can measure g.

There is a force of gravity between earth and every object on earth (actually there's a force of gravity between every two objects in the universe!). That force of gravity depends on the mass of earth and the mass of the object, and the distance between their centers. Since Earth is so big, the distance between the center of earth and the center of that other object is essentially just the radius of the earth (assuming that object is "near" earth's surface and not orbiting high above the atmosphere). So with a heavier object, the force between it and earth is greater, and with a lighter object, the force between it and earth is less.

The force of gravity between earth and the heavier object is greater, but the heavier object also has more inertia (resistance to movement), and those two things cancel each other out, so all objects fall with an acceleration of g = 9.8 m/s2.

You can see this if you use Newton's Second Law: acceleration = force divided by mass. The force of gravity on any object is the weight of that object, which is mass times g. So acceleration = mass times g divided by mass = g.

Answer 6

To experimentally derive the acceleration due to gravity, one way would be to drop an object in an enclosure empty of air. Pretty good data can however be obtained by dropping very dense objects that do not have too much of an aerodynamic drag.
The earth does not pull harder on a heavy object. The more massive object has more matter in it, and each gram is pulled equally; in essence an object twice as large as another is pulled like two of the lighter objects falling together.

Answer 7

because the earth is all knowing

seriously.

We did this years ago in physics class. One way to measure acceleration due to gravity is to measure the change in velocity of a block sliding down an inclined ramp as a function of time. Alternately you can simply drop an object and measure the velocity as it falls but it's a bit harder. The incline maintains a manageable velocity. Typically ticker tapes are attached to the block and pulled through a device that marks the tape at a particular frequency. Gives you distance vs time. (ie velocity)

The equation for velocity of an object as it free falls is

v = velocity initial + a x t
where a is your acceration above. t is time. mass is not part of the equation.

if you want to know about how and why gravity works and exists, here's a start...
http://en.wikipedia.org/wiki/Gravity
http://www.bartleby.com/173/
http://sharma.newtheory.org/NatureOfGravitation.htm

Answer 8

Objects would have less weight if the gravity of the earth were lower, so they would be easier to pick up. Objects would, however, retain their mass. While it would be easier to push a car down the street, this is due to less friction force (which depends on weight). A massive object like a car would still be difficult to accelerate due to it's large mass because the force required is proportional to mass and not weight - F=m*a. Once the car was up to speed, it would be easier to keep it there. (Again due to lesser friction force)

Answer 9

The above answers give the method to determine the acceleration due to gravity at the earth's surface. Massive objects do not "pull" on one another! Two objects interact with one another because each warps space-time according to the laws of General Relativity. This warping of space-time is what causes them to accelerate toward one another. Figure out if you will how a massive object acts on space-time to cause it to warp. Space-time must be "something" in order for gravitons to act on it. And how does a massive object, especially a sub-atomic particle, generate the influencing gravitons to act on space-time? Where in an electron and from what do they arise?