Force equals Mass times Acceleration
Or F=MA
A page describing one of the foundations of Physics.
This is a series directed at understanding various principles
of physics.
This section explains Newton's second law of motion
which is force equals mass times acceleration or
F=MA. It is easier to understand some of
these concepts if we use the MKS metric system. In the MKS system
force is measured in newton's, mass is measured in kilograms and acceleration
is measured in meters per second per second.
2.2 pounds = 1 kilogram ( on the earth)
Part one. Understanding the three terms.
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 the object does not move but the object is still subject to
a force. Force in the MKS system is measured in newton's.
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. The first question
to come up when discussing mass is how is it related to weight.
The answer is while mass never changes the weight of an object can change.
A rock for instance placed under water will weigh less, If
an object is taken from the earth to the moon it's mass is unchanged but
it's weight is 1/6 what it was on earth.
For an example of this consider the true story that
when the astronauts landed on the moon they smuggled along a golf
club and two golf balls. When the balls were hit they received
the same velocity as they would have on earth. This was due to
the fact that their mass was unchanged and the mass of the golf club was
unchanged as well as the velocity of the golf club. However as the
balls flew away the moon's gravity pulling them down was only one sixth
of what is was on earth and they traveled a lot further.
Similarly If we had a electric car that could accelerate
from zero to sixty in twenty seconds and sent that car to the moon
then it would still accelerate from zero to sixty in twenty seconds.
The force of gravity does not enter into it. So let
me repeat as this is important mass is an inherent part of any object.
It does not change. The weight of an object is
a measure of the force of the earth pulling on it. Weight can
change. Even a helium filled balloon has a small amount of mass
in spite of the fact that it is not attracted to the earth.
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.
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. After 2 seconds the object is moving at 4
meters per second. After 3 seconds the object is moving at 6 meters
per second etc.
There is one more example that must be mentioned. This
is the effect of gravity. It happens that the acceleration due
to gravity is 9.8 meters per second per second. If 2 objects are
dropped off a cliff with a mass of 1 kilogram and a mass of 2 kilograms
the force of gravity on the first it is given by 1 x 9.8 or 9.8 newtons
the force of gravity on the second is given by 2 x 9.8 or 19.6 newtons.
Thus while the second object has twice the force pulling on it it
will also resist this force twice as much so the net effect is that both
objects fall with the same constantly increasing velocity.
A few extra points,
1) It is generally accepted that there
are 2.2 pounds in a kilogram. Strictly speaking this is not correct
but as long as the objects using it are on the earth it works fine.
2) Besides mass there is another
term to learn. This term is "inertia" the inertia of a
body is another way of expressing is resistance to change. If
the body is standing still it's inertia is a way of expressing
it's resistance to motion If a body is in motion it's inertia
is a way of expressing it's resistance to changing that motion.
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 glassfull of water.
Experiment 5 A Brick on a String
Experiment 6 Dropping a coin into an empty
glass
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