astronomy question--True or False--You exert a gravitational pull on
the earth that is smaller than the gravitational pull the earth exerts
on you.
I say true, since gravity is proportional to mass, and mass of a
person
is less than mass of earth, but others say false, since Newton's 3rd
law says forces must be equal. Is my answer correct, and if not, why
not? Professor marked my answer as wrong, but I disagree, since I
think even though Newton's law of gravity makes it clear that the net
gravity force between two objects is related to the product of the 2
masses divided by radius squared, BUT I still think I exert less
gravitational pull on the earth than the earth does on me. Help!

Hello and thank you for your question.

Here are the two sides of the question, but only one side is correct:
"Question 9: The gravitational force of the earth pulling on you is
greater than the gravitational force of you pulling on the earth.
TRUE 
·         earth's gravity is obviously much greater because it
attracts many other objects with its gravitational force.
·         The earth's mass is much greater than mine and so its force
on me is greater
·         People's gravitational force doesn't pull on the earth, but
the gravitational force of the earth pulls on people
[or]
FALSE
·         No because the forces are an action-reaction pair and
therefore exert the same amount of force.
·         this is because both the forces are equal, if the earth
exerted more force we would be sucked into it...and if we exerted more
then we would FLY!!
·         According to Newton, every force has an EQUAL and opposite
force. So the force you put on the earth has to be the same as the
force it puts on you."
Physics 100
http://webug.physics.uiuc.edu/cc/parkland/phy111/spring01/discussion/Week6/week6.htm

Unfortunately the weight of authority (no pun intended!) is on your
teacher's side.

"[N]ot only does the Earth pull on you, you also pull just as hard on
the Earth.
"The gravitational force one object exerts on another due to gravity
is effected by three things.
  The mass of the first object. 
  The mass of the second object. 
  The distance between the objects."
How Gravity Sucks
http://www.paladin-magazine.com/sciTECH_gravity.htm

"A natural question is: if the earth exerts a force on us and forces
cause acceleration, why aren't we accelerating? The reason is that
when you are standing on the ground there are two forces acting on
you. The first is the force of gravity, and the second is an opposite
resistive force acting up on you from the ground. When you add these
two forces, the net force is zero and you don't move."
Kidspace
http://www.space.gc.ca/kidspace/1-edu_res/resources/all/microgravity/gravity.asp

"Simply stated, [Newton's Third Law] means that any time object A
exerts a force on object B, then B also exerts an equal and opposite
force on A. The Earth exerts a gravitational pull on you; you exert an
equal and opposite gravitational pull on the Earth. You press down on
the ground with a force equal to your weight; the ground presses up on
you with equal force.

"Here is a question which helps to illuminate the third law: 

"A horse is pulling a cart. By Newton's third law, the cart pulls back
on the horse with the same force which the horse is pulling the cart
forward. The forces are equal and opposite, so how can the cart move?

"The key is to consider only the forces acting on the cart. The
backward force is on the horse, so we ignore it. The only force which
acts on the cart is the forward pull, so the cart moves forward.

"What about the horse? The only force we've mentioned acting on the
horse is the backward pull of the cart. The only way the horse can
move forward is if there is another, larger force propelling him
forward. This force is exerted by the ground, which pushes forward on
the horse's hooves as it pushes back on the ground. Note that the
source of this force is friction; if the ground were frictionless it
would provide no forward- directed force. If you are one of the few
readers of Victorian novels you may recall passages in which
horse-drawn carriages are stranded by ice on the streets of London
because the horses could not get enough traction on the ice. By the
way, if it were not for friction then car tires would spin helplessly,
our feet would slide out from under us, and so on."
PHY 241 General Physics I [Google cache]
http://216.239.37.100/search?q=cache:3g2ZdwQwjGgC:faculty.uvi.edu/dstorm/classes/00phy241/mods/lect_11.html+You+exert+a+gravitational+pull+on+the+earth&hl=en&ie=UTF-8

Thanks again--isn't Physics Fun?

Seach terms used:
exert gravitational force

Richards answer is certainly beyond reproach  as
   it is basd on meticulous search meeting Google standards :-)

  But sometimes Less is More, and dragging in the Third
  Law seems to me to be both unnecessary and confusing.

 Answer is contained in the Newton's law of gravity,
 indeed is contained in the Drhulas question itself:

 Pull is proprtional to
                             product of the 2 masses, namely 

  mass of Earth  * mass of  Drhula   / r * r        (pull on Earth on Drhula)
  mass of  Drhula *  mass of Earth   / r * r        (pull of Drhula on Earth)

 Both pulls (forces) have same magnitude.

 To say :
            "earth's gravity is obviously much greater ..."
 may sometime confuse student by using ambiguous term. 

 Physical quantities are:  mass, force, acceleration, ...
 What is meant by 'gravity as a quantity' is unclear.

 The mass of Earth is bigger,
 the forces of attraction between the Earth and small object 
 have same magnitude and opposite direction.

If Earth's gravity is equal to Average Joe's, then why will Average
Joe die if he jumps off a cliff?

Both the Earth and Joe are accelerating towards each other. Both he
Earth and Joe exert the same force on each other. However, if you
remember, F=ma, so acceleration = Force / mass, so Joe's acceleration
is far greater than the Earth's, due to his lighter mass.
Also, remember that Joe's acceleration is the same as my acceleration
(9.8 m/s^2 aprox.), even if my mass is different. That's because, if
Joe is heavier, the force between him and the Earth is greater, so the
increase in mass balances the inceas in force, so the acceleration is
the same.

Dear Drhula,

I think I can help you here.

The bad news is that the answer is "false" and that your professor is
right.  The gravitational force the earth exerts on you and the
gravitational force you exert on the earth are indeed a Newton's Third
Law pair of forces, and so are equal in magnitude and opposite in
direction.

The confusion arises when you say "gravity is proportional to mass",
because (as Hedgie says) you are here not identifying exactly what
physical quantity you are describing by "gravity".  You cannot talk of
the gravitational _force_ of a single body - in this case, you -
because no force exists unless you have another body for it to be
exerted on.  What you can, however, talk about is the gravitational
_field strength_ created by a single body.  It may help to understand
that force and field strength are quite separate quantities if we
notice that they have different dimensions and therefore different
units: the SI unit of force is, of course, the newton (N), but the
unit of gravitational field strength is the newton per kilogram (N/kg)
(which can also be expressed as metre per second squared (m/s2)).

When you say "I say true, since gravity is proportional to mass, and
mass of a
person is less than mass of earth" you are thinking of the
gravitational field strength that your mass creates around you, not a
force.  This is indeed very much less than that of the earth - as you
can see by noting the fact that objects around the room do not fall
towards you in the way that they do towards the earth!  But the force
exerted on a body in a gravitational field is simply the product of
the body's mass and the field strength.  So if you need to find the
gravitational force that you exert on the earth, you need to take into
account not only the very much smaller field strength that you create
but also the very much larger mass that the earth has in experiencing
your gravitational field.  Put simply, the force the earth exerts on
you is its large field strength multiplied by your small mass, whereas
the force you exert on the earth is your small field strength
multiplied by its large mass: either way, the magnitude is the same
(though the directions are opposite).

I hope this helps.

Carnegie