Peoples Physics Book Version-2

http://www.ck12.org Chapter 6. Newtons Laws Version 2

For any object a heighthabove the surface of the earth, the force of gravity may be expressed as:

F~G=Gmearthmob j

(rearth+h)^2

[4]

Now we make the approximation that

rearth+h≈rearth

then, we can rewrite [4] as

F~G=Gmearth

r^2 earth

︸ ︷︷ ︸

gearth~

×mob j=mob j×~g [4] Gravity on Earth

We can do this because the quantity in braces only has constants; we can combine them and call their productg.
Remember,this is an approximation that holdsonlywhen the r in formula [3] is more or less constant.

We call the quantitymgan object’sweight. Weight is different from mass — which is identical everywhere — since
it depends on the gravitational force an object experiences. In fact, weight is the magnitude of that force. To find the
weight of an object on another planet, star, or moon, use the appropriate values in formula [4].

Normal Force

Often, objects experience gravitational attraction but cannot move closer together because they are in contact. For
instance, when you stand on the surface of the earth you are obviously not accelerating toward its center. According
to Newton’s Laws, there must be a force opposing gravity, so that the net force on both objects is zero. We call such
a force theNormal Force. It is actually electromagnetic in nature (like other contact forces), and arises due to the
repulsion of atoms in the two objects. Here is an illustration of the Normal force on a block sitting on earth: