http://ww.chabotcollege.edu/faculty/shildreth/astronomy/06gravity.html

HW#4: Exploring Gravity vs. Magnetism Survey 

You can't blame gravity for falling in love. - Albert Einstein
 

25 points - Due: Thursday, 9/29 in class at 10:30 AM

You are encouraged to post this in our Blackboard Discussion Forum, or bring it to class on paper.

Late work receives reduced credit.

First, answer these questions yourself, record your own answers, and read the actual answers available below.  Then, ask 2-3 friends or family members to complete the same questionnaire, and explain the correct answers to them. The questions alone (as a separate handout or online page) are accessible at: http://www.chabotcollege.edu/faculty/shildreth/astronomy/gravityqs.html. 

Please record in a typed, spell-checked, short essay of at least 250 words:

Background to 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.
"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.  The exponent "10-11 means that the number really would be written as 0.000 000 000 067 (which is very small! ) m1 is the mass of one of the objects. m2 is the mass of the other object. r is the distance (separation) between the centers of mass of each object.

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.

4. For the following questions, consider a standard sized small refrigerator magnet. Decide whether each statement is TRUE or FALSE:

True or False?	Statement	Answer!
	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.
	While touching the refrigerator, the magnetic force on the magnet (from the refrigerator) is stronger than gravitational force (from the Earth)	Answer! TRUE! That is why the magnet sticks to the refrigerator, rather than falling to the ground!
	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).	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.
	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).	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.
	Magnetism can be canceled out.	Answer! TRUE!
	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.
	Magnetism depends on the type of materials involved.	Answer! TRUE! Consider metal vs. wood! And not all metals are inherently magnetic, either!
	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.

5. Take a look at the following images:

    Bruce McCandless, NASA Astronaut, flying untethered from the Space Shuttle, in 1984. (http://apod.nasa.gov/apod/ap050322.html)

    Practicing for a Shuttle Mission http://images.jsc.nasa.gov/lores/S84-47177.jpg

    Shuttle Astronauts "floating" http://www.rankopedia.com/CandidatePix/71506.gif

    Space Shuttle Astronauts moving while in orbit (YouTube) (http://www.youtube.com/watch?v=VDu9z4SCTmc)

Space shuttle astronauts in orbit around Earth seem to be floating inside (or outside!) 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 around them

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.

4. Consider a standard refrigerator magnet.

True The force of magnetism decreases with distance.

You can see this simply by noticing that the magnet is attracted 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.

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

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).

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.

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."

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.

 

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.

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.

 

5. 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.

True c) they are falling and their space craft is falling around them

This is an answer most people have a hard time understanding!  In some sense, Shuttle astronauts are like parachutists in free-fall. The only major difference between the shuttle astronauts and parachutists is that the shuttle is moving fast enough "sideways" around the Earth (at 17,000 miles per hour or so) so that, as they fall, the Earth actually curves away underneath them.

They fall continuously in a "controlled orbit."  Take a look at this YouTube video response from the BBC about orbits. (http://www.youtube.com/watch?v=YxRCyokN3g8&feature=related)

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.