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First
Law of Motion
An object that is at rest will
remain at rest unless a nonzero net (or total) force is exerted on
it. This one is fairly easy to believe and sounds intuitive enough.
However, the next part of the first law might sound less plausible.
Simply stated, an object moving at a constant velocity will continue
to move at a constant velocity (moving at a constant speed and in a
straight line) unless a nonzero net (or total) force acts upon the
object. Recall that constant velocity means that the object is
moving at a constant speed and in a constant direction.
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At this point, you might be
thinking to yourself about something you saw in the world that
contradicts the statement I just made. For instance, think of a car
rolling in a straight line while in neutral. If what I stated above
were true, then the car should be able to roll in a straight line at
a constant speed forever. This is obviously not true in real life
because everyone knows the car eventually comes to a stop. This
certainly seems to prove that Newton's first law of motion is false.
Or does it? I assert that the above observation is consistent with
Newton's first law which states that an object moving at a constant
velocity will continue moving in that fashion unless a nonzero net
force acts upon that object. You might already know the answer.
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It is true that the car would
continue to move in a straight line at a constant speed if there was
no net force acting on the car. However, is there really no net
force acting on the car? In fact, there is a nonzero net force
acting on the car, causing it to slow down. And, you probably
already know what that force is. The force responsible for slowing
down the car is friction. Therefore, the above observation about a
car slowing down while in neutral is not inconsistent with Newton's
first law of motion. It just seems to contradict it at first. If
this is confusing, pause for a moment and think about it for awhile.
This is a pretty surprising fact to
most people when they first hear it. If we were able to remove all
the friction between the ground and a ball, once you start the ball
rolling, it would roll on forever in a straight line.
This is a perfect tie-in to
Newton's second law. We just discovered that objects like to move in
straight lines and at constant speeds unless a force acts upon them.
In fact, when a force acts on an object, the force causes the object
to change its velocity. In other words, forces cause objects to
accelerate.
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Second
Law of Motion
Simply stated, a force causes an
object to accelerate. Whenever you see an object accelerating, there
must be an external force acting on the object because, as stated in
Newton's first law, objects move at a constant velocity unless acted
upon by an outside force.
Mathematically, Newton's second law
of motion can be expressed by the following formula: a = F/m where a
= acceleration, F = force, and m = mass.
What this formula tells us is that
force causes an object to accelerate. However, it also tells us that
the acceleration an object feels, in response to an applied force,
does not solely depend on the amount of force applied. It also
depends on the mass or inertia of that object. It also tells us that
the more mass an object has, the less it accelerates in response to
an applied force. This makes intuitive sense. For instance, if I
apply the same force to a cotton ball and an elephant, the cotton
ball would experience a greater acceleration than the elephant
because the elephant has much more mass or inertia. Therefore, an
object with a greater mass has a better tendency to resist a change
in its motion when an external force is applied to that object. In
other words, we say that the elephant has more inertia than the
cotton ball.
Part of the beauty of math is that
all this can be elicited from looking at the formula above in a much
more compact form without reading an entire paragraph of
explanation.
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Third
Law of Motion
Newton's third law states that
whenever a force is exerted, an equal and opposite force arises in
reaction to this force. In other words, every force has an equal and
opposite reaction force.
For example, when you push on a
wall, the wall will also push back on you with an equal and opposite
force. By the way, the "Newtons" in the figure above is
the unit in which force is measured. In what follows, I will write
"N" in place of "Newtons". For example, 5
Newtons of force will be written as 5 N.
Some of you might be wondering why
you don't move backwards even though the wall is pushing you
backwards. How very astute. The reason why you don't move backwards
when you push against a wall is because static friction is pushing
you back with an equal amount of force to the right so that you
don't move anywhere. In the above example, static friction would be
pushing the person to the right with 5 N of force, so that the
person would experience zero total (or net) force, hence the person
does not move.
This brings up an important point,
i.e., that forces add up. We will come back to this point later when
we discuss force in more detail.
So, if Newton's third law is true,
and the wall pushes back on us just as hard as we push back on it,
there must be some way of seeing that in the real world. Well, there
certainly is. If you have ever gone ice skating or in-line skating
(notice I'm not using the word Rollerblading) or roller skating (if
you are really old), you can probably recall the example to follow.
Recall that the only reason why you
didn't move when you pushed against the wall was because there was
friction pushing you back to the right. Well, if you go skating, ice
skating for example, you are reducing the friction between you and
the floor because ice is very slippery. As a result, there isn't
enough friction to compensate for the wall pushing you back. If you
push against the wall while ice skating, you will move backwards as
a result of the reaction force to you pushing against the wall.
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