# Physics presentation

#### ACLerok

I found your website when looking for some help eith Physics.
November I have to make a presentation involving an experiment in
which two objects of different mass will hit the ground
simultaneously. The motion of these two objects is constant
acceleration correct? How would I easily explain why this is true to
someone? Also, since the objects hit the ground simultaneously, do
they have the same acceleration?? How would I find it? Thanks and i
hope you get the chance to help me out.

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#### zoobyshoe

Here's a start:

My little book of formulas says, "In free-fall motion, an object experiences negligible air resistance and a constant acceleration due to gravity.

All objects in free fall at the same location above the earth have the same acceleration due to gravity.

The acceleration due to gravity is a vector that points toward the center of the earth and is denoted by the symbol g. Near the surface of the earth, g has a magnitude that is approximately constant and is given by

g=9.80 m/s2 or 32.2 ft/s2.

#### jcsd

Gold Member
The canonical figure to two sig. figures is 9.81 ms-2,zoobyshoes (though of course it varies across the Earth's surface, but 9.81 will be correct for most places on the Earth).

Use the formula:

s = ut + &frac12;at2

this means that if the inital velocity, u and the displacemnet s, is the same then if they take the same time they must of had the same acceleration. The motion of these two objects won't quite be constant accelarion due to drag, but if you try to eliminate as much drag as you can by using heavy objects with a small surface area, then you needn't worry too much about it.

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#### Integral

Staff Emeritus
Gold Member
In General 2 bodies with mass have a force of gravity given by the equation

F=GMm/r2

Where G is the gravitational constant
G=6.67e-11 Nm2/kg2

M= Mass of body 1 (let this be the mass of the earth)
m= mass of body 2 (let this be the mass of something on the surface of the earth.

r is the distance between the centers of mass. let us call this the radius of the earth.

Newton gave us F=ma, this is the relationship between the mass of a body, the force acting on it and the resulting acceleration. A body of mass m on the surface of the earth has a gravitational force exerted on it by the first equation, if we use that as the force in Newtons law we get

GMm/r2= ma

now m is the same on each side so we can cancel it out resulting in

a =GM/r2

Notice that all of the factors on the right are constant, to find a simply compute the value on the left side. This is the quantity we call g.
I have given you G, you should be able to find values for the other quantities (Mass and radius of the earth) be sure that they are expressed in the same units as the value for G (that is kg and meters).

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#### zoobyshoe

Originally posted by ACLerok
How would I easily explain why this is true to someone?
I suppose you are talking about an audience that isn't necessarily interested in physics, because you ask how it can be "easily" explained. The important first step is to make sure you understand it backwards and forwards yourself.
One strategy I have for this is to go to the library and compare how the same information is presented in as many different sources as I can. In this instance I'd pull all the physics textbooks they have down and read what each says on the acceleration of gravity. Encyclopedias, too.

I like the way Integral refered to Newton II as a "relationship". This is an important thing to be conscious of when you're trying to grasp, or trying to explain, what any formula is about: it expresses how the different values relate to each other. The better you understand that relationship the easier it is to explain it to a non-scientific person, in language they can understand.

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