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 uses the meter as the unit of length. It uses the
kilogram as the unit of mass. And uses the second as the unit of
time. 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. Newton's
second law applies to freely moving objects or objects with no friction.
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 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.
All objects have mass. A
brick is an example of an object with lots of mass. A hammer
relies on it's 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
does not change. What does change on the
moon is the weight of an object. The weight is a measure of the pull
between the object and the moon. It is about one sixth the pull the
object experienced on the earth. The mass of an elephant is
considerable but a mosquito also has mass.
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.
If we wish to know just how velocity changes
with constant acceleration it is proportional to the time.
The force of gravity on the earth causes free falling objects to accelerate
at the rate of 32 feet per second per second. If an object
is in free fall it is accelerating at 32 meters per second per
second. It's velocity is given by V = A x T.
After 1 second the object is moving at 32 meters per second or V =
32 x 1. After 2 seconds the object is moving at 64 meters per second
or V = 32 x 2. After 3 seconds the object is moving at 96 meters
per second or V = 32 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 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. That is it moves faster and faster. 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. The long rifle barrel
provides 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 provides less time and this results in a lower velocity.
Following there are several 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
is a video on Newton's 3 laws.