Exploring Gravity

The law of gravity is given by the equation:

FG is the force of attraction between the two objects. G is the universal gravitational constant.  It is basically a conversion factor to adjust the number and units so they come out to the correct value.  This is a universal constant so it is true everywhere, on Earth, on Pluto, on Alpha Centauri, and in the most distant galaxies that we can see. (In this formula, "N" refers to Newtons, a unit of FORCE; "m" refers to meters, a unit of length, and "kg" refers to kilograms, a unit of mass.) m1 is the mass of one of the objects. m2 is the mass of the other object. r is the radius of separation between the center of masses of each object.

The force of earth's gravity upon you is your weight.  If you were on the moon, you would have the same mass, but a different weight, because the force of the moon's gravity upon you would be less.

For the following survey, record who you asked, what educational background they have, what they answered, and most importantly, WHAT YOU DID to teach the correct answers, which follow the survey.

So... what does this formula imply about the following? Decide which object will create a larger gravitational force on you!

1)  The force of gravity on you from a pound of lead compared with the force from a pound of feathers?

a) The force from lead will be larger than the force from feathers.

b) The force from feathers will be larger than the force from lelad.

c) The forces will be the same.

Answer? C) The forces will be the SAME! GRAVITY DOESN'T DEPEND ON MATERIALS.

2) The force of gravity on you from a pound of liquid water, compared with the force from a pound of solid ice, compared with the force from a pound of gaseous water vapor?

a) The force from liquid water will be largest.

b) The force from solid ice will be largest.

c) The force from gaseous water vapor will be largest.

d) The forces will be the same.

Answer? d) The forces will be the SAME! Gravity doesn't depend upon the STATE of matter.

 

3) The force of gravity on you from a pound of sugar, in the form of a solid sugar cube, a small, dense, compressed sugar pill, or a large ball of spun cotton candy.

a) A solid sugar cube will have the largest force.

b) A compressed sugar pill will have the largest force.

c) A ball of spun cotton candy will have the largest force.

d) The forces will be the same.

Answer? d) The forces will be the SAME! Gravity doesn't depend on SHAPE.

1. Consider a standard sized small refrigerator magnet. Indicate T or F for each of these:

True or False?	Statement	Answer!
	a) The force of magnetism decreases with distance.	Click Here for Answer! TRUE - both forces decrease with the SQUARE of the distance; if you 10 times farther away, both forces are 100 times weaker.
	b) While touching the refrigerator, the magnetic force on the magnet (from the refrigerator) is stronger than gravitational force on the magnet (from the Earth)	Answer! TRUE! That is why the magnet sticks to the refrigerator, rather than falling to the ground!
	c) When held a fraction of an inch (a few millimeters) away from the refrigerator, so that the magnet is not touching, the magnetic force on the magnet (from the refrigerator) is stronger than gravitational force on the magnet (from the Earth).	Answer! TRUE! You can feel the attractive force between the magnet and the refrigerator; release it, and the magnet will jump and stick to the refrigerator door.
	d) When held a few inches (about 10 centimeters) away from the refrigerator, so that the magnet is not touching, the magnetic force on the magnet (from the refrigerator) is stronger than gravitational force on the magnet (from the Earth).	Answer! FALSE! The magnet will fall to Earth when released rather than jump to the door. WHY? Because magnetism can be both "positive" & "negative" [North & South]; consequently it can be effectively cancelled out.
	e) Magnetism can be canceled out.	Answer! TRUE!
	f) Gravity can be canceled out.	Answer! FALSE! Gravity only comes in one "flavor" - attractive. Even "anti- matter" is still subject to an attractive form of gravity.
	g)Magnetism depends on the type of materials involved.	Answer! TRUE! Consider metal vs. wood! And not all metals are inherently magnetic, either!
	h) Gravity depends on the type of materials involved.	Answer! FALSE! All matter creates a gravitational attraction. Even energy, including light, is subject to a gravitational field, and also creates an attraction towards other matter.
	i) Gravity stops after a certain distance.	Answer! FALSE! Gravitational force extends an infinite distance into space; it gets weaker, but doesn't disappear. Gravity can't be stopped, or shielded, or reduced by intervening matter.

2. Space shuttle astronauts in orbit around Earth seem to be floating inside their spacecraft because (choose the ONE best answer):

a) there is no force of gravity on them

b) the force of gravity on them is much less than it is on Earth

c) they are falling, and so is their spacecraft

d) the Earth's magnetic field supports them and their spacecraft

e) they are massless in space

f) they are closer to the Moon, and the Moon's gravity is actually helping to keep them suspended.

1. Consider a standard refrigerator magnet.

a) True The force of magnetism decreases with distance.

You can see this simply by noticing that the magnet is attracted strongly to the refrigerator when close, but farther away, it no longer feels a strong enough force to make it leap across space and attach itself to the refrigerator.  Or you can hold two magnets near one another, and feel the repulsive or attractive forces between them (depending upon the way you hold them).  Farther away, those forces are weaker.

b) True While touching the refrigerator, the magnetic force on the magnet (from the refrigerator) is stronger than gravitational force on the magnet (from the Earth).

The forces acting on the magnet include gravity and magnetism, but since the magnet does not fall, magnetism must be stronger here.

c) True When held a fraction of an inch (a few millimeters) away from the refrigerator, so that the magnet is not touching, the magnetic force on the magnet (from the refrigerator) is stronger than gravitational force (from the Earth).

Once you let the magnet go, it is still pulled onto the refrigerator's metal surface rather than falling to the ground. Magnetism is a strong force if the distance between objects is smaller than the distance between a magnet's two poles (North and South).

d) False When held a few inches (about 10 centimeters) away from the refrigerator, so that the magnet is not touching, the magnetic force on the magnet (from the refrigerator) is stronger than gravitational force (from the Earth).

Since the magnet now falls to Earth at this distance, gravities force must now be stronger than the magnetic attraction between the refrigerator and object. Note that the mass of the magnet does not change, and its distance from the center of Earth doesn't, so the gravitational force on the magnet is constant throughout your experiment. So it must be that the magnetic force is getting much weaker with distance.

e) True Magnetism can be canceled out.

As the above experiment shows, far away from a magnet, the magnetic force somehow has dropped substantially. Magnets have two opposite poles, and close to either one, the electrons in a metal will feel a significant force. But from some distance away the poles effectively shield each other, and "cancel out."

f) True Magnetism depends on the type of materials involved.

Compare wood and metal! You can also add here that magnetism depends on the motion of charges; in the centers of planets like Jupiter or Saturn, the hot liquid hydrogen in rapid rotation as the planets spin creates enormous spinning currents which generate the large magnetic fields those planets have. But remember that magnetism won't hold onto moons or rings! It is gravity that acts at greater distances, holding on to all matter.

g) False Gravity can be canceled out.

As far as we understand today, gravity is a universal attractive force, with no possible way to shield it, reverse it, or cancel it. In this, gravity is extremely different than magnetism.

h) False Gravity depends on the type of materials involved.

Examine the universal law of gravity to notice that there is absolutely no dependence on the type of matter involved (solid, liquid, gas), nor on its shape nor composition. The key is simply how much matter is involved; a kilogram of lead, a kilogram of water, and a kilogram of air would all generate and respond to the same gravitational force.

i) False Gravity stops after a certain distance.

The law of gravity doesn't have a limit - nowhere in the formula does it say that after 300 miles, or 3000 miles, or 30 trillion miles, the force will disappear.

2. Space shuttle astronauts in orbit around Earth seem to be floating because:

a) there is no force of gravity on them

Not so: if gravity cannot be shielded nor canceled, they still feel a force of gravity

from Earth. They would still feel a force if they were at the moon, even though

that force would be much less.

b) the force of gravity on them is much less than it is on Earth

Not so: The shuttle operates just 250-350 nautical miles above the Earth's surface,

and so is only about 5-8% farther from the center of the Earth than we are on the

planet's surface. The force of gravity is a bit less - perhaps 95% as much.  But it is not

zero, and certainly much stronger there than at the distance of the moon. 

True c) they and their space craft are falling

The only major difference between the shuttle astronauts and parachutists is that

the shuttle is moving fast enough "sideways" around the Earth so that, as they fall,

the Earth actually curves away underneath them. They fall continuously in a

controlled orbit.

d) the Earth's magnetic field supports them and their spacecraft

Not so: from the refrigerator magnet, you know that the force of magnetism, while

powerful, does not extend for tremendous distances with the power to levitate large

objects like the Shuttle.

e) they are massless in space

Not so: there is no way to eliminate the mass of the shuttle nor astronauts.

f) they are closer to the Moon, and the Moon's gravity is actually helping to keep

them suspended.

Not so: At 4,300 miles or so from the Earth's center, the Shuttle is still almost 240,000

miles from the Moon! The force of gravity from Earth is still MUCH larger.