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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