|1st Law of Motion
Motion doesn't change
without an outside force..
Chapter 14 - Force & Motion
Newton's Laws of Motion
|2nd Law of Motion
F = ma
This explains how much a
force will change the motion
of an ojbect.
|3rd Law of Motion
For every action...
the force will effect the
motion of both of the objects
2nd Law F = ma
You may be surprised to learn that Newton wasn't the genius
behind the law of inertia. But Newton himself wrote that he
was able to see so far only because he stood on "the
shoulders of Giants." And see far he did. Although the law of
inertia identified forces as the actions required to stop or
start motion, it didn't quantify those forces. Newton's second
law supplied the missing link by relating force to acceleration.
This is what it said:
|When a force acts on an object, the object accelerates in the
direction of the force. If the mass of an object is held constant,
increasing force will increase acceleration. If the force on an
object remains constant, increasing mass will decrease
acceleration. In other words, force and acceleration are directly
proportional, while mass and acceleration are inversely
Technically, Newton equated force to the differential
change in momentum per unit time. Momentum is a
characteristic of a moving body determined by the product
of the body's mass and velocity. To determine the
differential change in momentum per unit time, Newton
developed a new type of math -- differential calculus. His
original equation looked something like this:
where the delta symbols signify change. Because
acceleration is defined as the instantaneous change in
velocity in an instant of time (Δv/Δt), the equation is
often rewritten as:
The equation form of Newton's second law allows us to
specify a unit of measurement for force. Because the
standard unit of mass is the kilogram (kg) and the
standard unit of acceleration is meters per second
squared (m/s2), the unit for force must be a product of the
two -- (kg)(m/s2). This is a little awkward, so scientists
decided to use a Newton as the official unit of force. One
Newton, or N, is equivalent to 1 kilogram-meter per
second squared. There are 4.448 N in 1 pound.
So what can you do with Newton's second law? As it turns
out, F = ma lets you quantify motion of every variety. Let's
say, for example, you want to calculate the acceleration of
the dog sled shown below.
|If you want to calculate the acceleration, first you need to
modify the force equation to get a = F/m. When you plug in
the numbers for force (100 N) and mass (50 kg), you find that
the acceleration is 2 m/s2.
Now let's say that the mass of the sled stays at 50 kg
and that another dog is added to the team. If we assume
the second dog pulls with the same force as the first (100
N), the total force would be 200 N and the acceleration
would be 4 m/s2.
|Notice that doubling the force by adding another dog
doubles the acceleration. Oppositely, doubling the mass to
100 kg would halve the acceleration to 2 m/s2.
Finally, let's imagine that a second dog team is attached to
the sled so that it can pull in the opposite direction.
|If two dogs are on each side, then the total force pulling
to the left (200 N) balances the total force pulling to the
right (200 N). That means the net force on the sled is zero,
so the sled doesn’t move.
This is important because Newton's second law is concerned
with net forces. We could rewrite the law to say: When a net
force acts on an object, the object accelerates in the direction
of the net force. Now imagine that one of the dogs on the left
breaks free and runs away. Suddenly, the force pulling to the
right is larger than the force pulling to the left, so the sled
accelerates to the right.