Newton’s first law, often called the Law of Inertia states the following:
An object in motion tends to stay in motion,
and an object at rest tends to stay at rest unless acted upon by an outside
force.
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or -
An object with no net force acting on it will move at a constant velocity (which may be zero).
This means that when an object is moving, it should keep on moving at the
same speed and in the same direction unless some outside force is acting on
it. It also means that an object at rest ( not moving ) will stay at rest
until some force causes it to start moving. If you’ve ever seen a video of
astronauts in space, this law makes more sense. The astronauts can float without
moving because there is no net force acting on them. If they push off the
wall of the space ship, they will float away at a constant speed and direction
and won’t stop until they collide or push off something else.
In order to understand
how an object will move, you just need to know what forces are acting on it.
Let’s describe some of the most common forces. The first is one that has been
acting on you your entire life: gravity. Earth’s gravity
is constantly pulling you down towards the center of the earth. When you jump
into the air, you exert a force that pushes you up from the ground. Since
this force is greater than gravity, you go up. Once you’re in the air, gravity
becomes the only force acting on you and pulls you back down.
A second familiar
force is called contact force. When two objects are touching each other, they
are exerting a contact force. When
you push or throw something, you exert a contact force on it. Go back to the
jumping example. After you jump into the air and begin to fall back down due
to gravity, the Law of Inertia says that you should continue that downward
motion. If gravity had its way you’d keep falling towards the center of the
earth, but you stop when you hit the ground because the ground exerts a contact
force on you. When you’re standing on the ground, Earth’s gravitational force
is pulling you down, but the ground is exerting a contact force pushing you
up. These two forces have the same magnitude, or strength, but act in opposite
directions, so they cancel each other out. Even though both of these forces
continue to act on you, we say there is no net force
because they cancel each other out. The term ‘net force’ refers to the directional
sum of all the forces. When two forces act in the same direction, they add
together. When they act in the opposite direction, one is subtracted from
the other. If two forces that are equal in strength act in the opposite directions,
they cancel each other out completely.
A third force you encounter all the time but may not realize
it is friction. Friction is a force that opposes motion between two
objects in contact. If you push a box across a floor and then stop pushing,
it will slide for a while but soon come to a stop because friction opposes
the motion between the box and the floor. If you go to a skating rink and
push that box across the ice, it will slide further because there is less
friction between the box and the ice. It will still eventually come to a stop.
Hovercraft, in fact, were invented as a way of reducing this sliding friction!
Another form of friction is called wind resistance. If you stick your
hand out of the window of a fast moving car, you feel a force pushing your
hand back. This is caused by friction between your hand and the air it’s moving
through. If you stick your hand out of a moving hovercraft, you will also
feel the wind resistance, but something interesting happens. Hovercraft are
designed to have so little friction as they move that the small amount of
wind resistance that occurs when you stick your hand out to the side of the
hovercraft is enough to make the hovercraft turn in that direction!
We now have a better concept of what forces are, and we
know from Newton’s first law that if there is no net force on an object, its
velocity doesn’t change. Exactly how the velocity changes if there is a net
force acting on the object is described by Newton’s Second Law. It is most easily
stated with the following formula:
F = m · a
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or -
Force = Mass · Acceleration
This means that anytime a net force acts on an object, that object will accelerate.
It also states that more massive objects require more force to achieve the
same acceleration as less massive objects. This is why you have to push harder
to move a big desk than a small chair.
Given what we know about velocity and acceleration, this
second law really agrees with the first law. The equation for the second law
states that a force leads to an acceleration, so if no net force is acting
on an object, it will not be accelerated. No acceleration means that the velocity
doesn’t change, so no force equals constant velocity… just like the first
law states!
Knowing the relationship between force and acceleration
means we can clear up one of the most common misconceptions in all of science:
the relationship between mass and weight. It’s very common to believe that
kilograms are just the SI (System International) equivalent of pounds and
vice versa, but this isn’t true. The pound is a measure of weight in the Imperial
unit system, and its equivalent in SI is the Newton, named after Sir Isaac
Newton. Weight is actually a measure of the force that gravity exerts
on an object. The kilogram is a unit of mass in SI. Although practically never
used, the unit for mass in the Imperial system is called a slug. Refer to
the unit conversion sheet to see exactly how these units are related.
The pound is therefore a measure of force exerted by gravity
while the kilogram is a measure of the mass of an object. When you convert
between pounds and kilograms, you’re essentially using Newton’s second law.
The reason we can convert so easily is because of the way gravity acts. On
earth, gravity acts to accelerate all objects the same, regardless of the
mass. If you drop a bowling ball and a pebble from a bridge, they will hit
the bottom at the same time because gravity accelerates them the same. If
acceleration is constant for all masses and forces, then you can easily interchange
between the two. Keep in mind, however, that mass and weight are different
concepts. If you’ve seen a video of astronauts on the moon you would have
noticed that they seem to weigh very little as they almost float around on
the surface of the moon. This is because they really do weigh less on the
moon than on the earth. Their mass, on the other hand, stays exactly the same.
We can now finish up with Newton’s Third Law, which states:
For every action there is an opposite and equal reaction.
This means that for every force that is exerted on an object, the object exerts
back an equal force in the opposite direction. This can be demonstrated with
two pool balls. If the cue ball runs into the eight ball, it will exert a
force on the eight ball, causing it to accelerate away. At the exact same
time, the eight ball will exert the exact same amount of force on the cue
ball in the opposite direction, which causes the cue ball to decelerate. Essentially
this means that all forces exist in pairs. You push against a wall, it pushes
back. If you bang your head against a wall, you’re exerting a force on the
wall. Keep doing this and the headache you’ll get will demonstrate the wall’s
force exerting back on you! When a rifle is shot it forces the bullet forward,
but the bullet, in turn, pushes back on the rifle. You can see this in the
way the gun recoils after a shot, and you can feel it in your shoulder. It
may be hard to believe, but this also works for gravity. The earth is pulling
you with a force due to your mass, but you are also gravitationally pulling
back on the earth with the same force. The earth doesn’t really notice this
however, because its mass is so huge that the earth’s acceleration caused
by you is very small according to Newton’s second law.