Force equals Mass times Acceleration
This section explains Newton's second law of motion
or force is equal to mass times acceleration. It
is represented by F = M x A.
It is easier to understand some of these concepts
if we use the European system of units. This system of units
replaces the unit of length (one foot) by the meter. It uses the
kilogram as the unit of mass. The unit of time remains the
second. This system is called the MKS or meter kilogram second system.
While the kilogram is not exactly the same type of unit as our
pounds it may be regarded as being equal to 2.2 pounds.
THREE THINGS AT ONCE:
Before we start it is important to point out that it is hard to
understand new ideas. Sometimes they need to be thought
about for a while. In this section we have to learn 3 new concepts
and learn to understand how all three interact with each other.
This is not easy. It may take some time. In
the end however it will be well worth the effort. These three new
concepts are force, mass, and acceleration.
FORCE:
Force is the thing that a elastic band exerts when
is it is stretched. Other forces include the force exerted on a
car by the engine and wheels. The force a person on a bicycle
exerts when he pushes on the pedals. The force the wind exerts
on a side of a building. The most well known of all forces is gravity.
In some cases when a force is applied the object moves.
In other cases ( like the wind example) the object does not move but the
object is still subject to a force. Here is an example
of a force. It is a YouTube video of a truck tug of war. Not
only is the force present in the strap between the two trucks but the driving
wheels exert a force between the ground and the truck. Force
in the MKS system is measured in Newton's.
WEIGHT:
Strictly speaking the weight of an object does not enter into an explanation
of F = M x A. This is because the weight of an object is produced
by a combination of the mass of the object as well as the mass of the earth.
If we were to go to the moon our weight would be one sixth what it is on earth.
If we were in space we would be weightless. However in
more practical terms the weight of an object may be considered to be proportional
to it's mass. This holds true as long as the object is on the earth.
For the moment we will replace the term "weight" with the term "force".
MASS:
All objects have mass. A brick
is an example of an object with lots of mass. A few objects
(a hammer for example) rely on their mass to function. While
other tools may be made out of aluminum to reduce their weight a hammer
will never be. It needs it's mass to function. So let
me repeat as this is important. Mass is an inherent part of
any object. It does not change. Even if taken to the moon
the mass of an object (or even us) does not change. The
mass of an elephant is considerable but a mosquito also has mass.
ACCELERATION:
Acceleration is a way of measuring a constant increase
in velocity. If an object is accelerating then it
is moving faster and faster. If a object has zero acceleration then it
may be stopped or it may be moving at a constant speed.
A dragster coming off the line is an example of an accelerating vehicle
while a car moving at 60 miles per hour on the highway has zero acceleration.
Likewise a parked car has zero acceleration. The best
example of acceleration is to drop something to the ground. As
we all know the higher it is dropped from the faster it goes and thus the
harder it hits. Drop an egg from 1 inch and no problem but try 2 or
3 feet and there are different results. This is true for
both objects and people.
If we wish to know just how velocity changes with
constant acceleration it is proportional to the time. That
is if an object is accelerating at 2 meters per second per second
then the velocity is given by V = A x T. After 1 second
the object is moving at 2 meters per second or V = 2 x 1.
After 2 seconds the object is moving at 4 meters per second or V
= 2 x 2. After 3 seconds the object is moving at 6 meters per
second or V = 2 x 3 etc.
THREE NEW CONCEPTS:
Now that these three terms have been defined we need to look at how they
interact with each other. The rule of interaction is termed
Newton's second law or force is equal to mass times acceleration.
For an example of F = M x A consider a rifle bullet.
Before being fired the bullet is a piece of lead and is not moving.
When the rifle is fired the explosion causes a quantity of gas to builds
up in back of the bullet. This gas exerts a force on the bullet.
This force causes the bullet to move down the barrel of the gun.
Now the important point. The bullet does not simply move, It
is accelerated all the way down the gun barrel. When the bullet exits
the gun barrel it's acceleration stops and it travels at a constant velocity.
It now continues with this velocity to the target. Typical exit velocities
of a rifle bullet are about
2000 feet per second or 600 meters per second. This is why a rifle
bullet is more powerful than a pistol bullet. There is more
time for the expanding gas to push on the bullet and this gives more time
for the gas to accelerate the bullet. The short barrel of a pistol
limits it's power.
EXPERIMENTS:
Following there are severel experiments to illustrate this point.
Experiment 1 The
Shopping Cart
Experiment 2 Illustration
of F = M x A
Experiment 3 The
Electric Drill
Experiment 4 Pulling a Piece
of Paper out From Under a Glass of Water.
Experiment 5 A Brick on a
String
Experiment 6 Dropping a Coin into an Empty
Glass
Here
is a video on Newton's 3 laws.
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