The weight of an object is the force due to the gravitational attraction of
the earth. Sometimes we weigh an object because we are concerned with this
force (is this truck so heavy that it will collapse the bridge?), but more
commonly what we really want to know is how much stuff we have (how much
coal is in the truck?). This wouldn't work if the same object weighed
very different amounts in different places. In fact,
the weight of an object does vary by a few percent as you move around the surface
of the earth (good enough for measuring out potatoes),
and by even more elsewhere
in the universe.
For those of us who stay on Earth, the gravitational force on an object (its
weight) seems to be the same everywhere, with the result that most people do
not clearly distinguish between mass and weight. However, mass is an invariant
property of an object, while the weight of an object varies slightly from
place to place even on the surface of the earth, and more so in the universe
in general (when you visit the Moon, you will find it much easier to climb
stairs because you weigh much less. But your belt will still be too tight,
and your mass has not changed). In an attempt to avoid this confusion,
the force scale only measures Newtons. So you can use it to determine the
gravitational force on an object, but not its mass. Later in this course
we will find a role for mass that has nothing to do with gravity;
until then we will characterize what other people would
call a 100 gram mass as being a weight of (very nearly) 1
Newton. As long as we are discussing forces, we will always use Newtons,
and
never use grams.
Direction of a force, and the Law of Interaction
Force is a quantity that has both a size and a direction. The gravitational
force is downwards but we can push and pull in other directions, too.
When we talk about a force, we have
to specify "who" is pulling on "what". For example, in a
friendly tug of war, Mr. B pulls on his pet alligator A, exerting a force called
"the force of B on the alligator." But there is another force present, "the force of the
alligator on B". These two forces are opposite in direction, and exactly equal.
The general statement is one of Newton's laws, The Law of Interaction:When object A exerts a force on object B, object B exerts an equal and
opposite force on A. This is a statement about interacting systems, and
is true for all kinds of forces without exception. Although in
this example A and B are not moving, the law does not depend
on this; it would still hold if B were throwing an Avocado or
just fanning the Air.
In the first picture, Al is testing the strength of a rope, by pulling on it.
In the second picture, Ben is helping.
With two people pulling on the rope, is it more likely to break?
-No, it is exactly the same. In the first picture, the force Al exerts on the tree
(with the aid of the rope) is equal and opposite to the force the tree
exerts on Al. In the second picture, Ben has replaced the tree but as far as the rope
is concerned, nothing has changed.
The Law of Interaction is pretty familiar to us. The way we can tell that
we are pushing on a door is that we can sense that the door is pushing on us.
The reason we are introducing this concept first is precisely that we are
so used to this facile translation
that it can get in the way, as when we accidentally subsitute "the force of the
door on us" when we need to be discussing "the force of us on the door."
Once that is out of the way, the Law of Interaction isn't very useful: it
relates two forces, but doesn't tell us anything about how big either is,
nor what effect it has.
The weight of an object is the force due to the gravitational attraction of
the earth. Sometimes we weigh an object because we are concerned with this
force (is this truck so heavy that it will collapse the bridge?), but more
commonly what we really want to know is how much stuff we have (how much
coal is in the truck?). This wouldn't work if the same object weighed
very different amounts in different places. In fact,
the weight of an object does vary by a few percent as you move around the surface
of the earth (good enough for measuring out potatoes),
and by even more elsewhere
in the universe.
When we talk about a force, we have
to specify "who" is pulling on "what". For example, in a
friendly tug of war, Mr. B pulls on his pet alligator A, exerting a force called
"the force of B on the alligator." But there is another force present, "the force of the
alligator on B". These two forces are opposite in direction, and exactly equal.
The general statement is one of Newton's laws, The Law of Interaction:
When object A exerts a force on object B, object B exerts an equal and
opposite force on A. This is a statement about interacting systems, and
is true for all kinds of forces without exception. Although in
this example A and B are not moving, the law does not depend
on this; it would still hold if B were throwing an Avocado or
just fanning the Air.
