Newton's Second Law

In the presence of a NET FORCE, an object experiences an ACCELERATION

directly proportional to the NET FORCE

inversely proportional to the MASS of the object.

Remember, though,

-- F is the NET force

-- m is the mass which that net force acts on.

We often turn this around and write it as

F is the NET force acting on an object


m is the mass of the object which the force F acts upon.

What are the UNITS of force in

F = m a

A force of ONE unit

will give an object of 1.0 kg mass

an acceleration of 1.0 m/s/s ;

this force is known as

ONE NEWTON (1.0 N) .

1 N = ( 1 kg ) ( 1 m/s/s )


F = m a

Force will be measured in newtons

A force of 1 N will give

a mass of 1 kg

an accelertion of 1 m/s/s.

1 N = ( 1 kg ) ( 1 m/s/s )

12 N = ( 3 kg ) ( 4 m/s/s )

A force of 12 N could give

a mass of 3 kg

an accelertion of 4 m/s/s

12 N = ( 2 kg ) ( 6 m/s/s )

A force of 12 N could give

a mass of 2 kg

an accelertion of 6 m/s/s .


We have already seen that all objects fall with the same acceleration, 9.8 m/s/s (which we approximate as nearly 10 m/s/s).

We call this free fall.

When such objects fall, the only force acting on them is their weight, the force of gravity.

The only force on a body in freefall is the force of gravity. We call this its weight.

Since it accelerates at 9.8 m/s/s,

that weight must be

w = (mass ) x (9.8 m/s/s),

w = m g


The weight of an object is the force of gravity on that object.

Weight, since it is a force, will be measured in units of newtons (N).

Mass will be measured in kilograms (kg).

If an object is in equilibrium -- at rest -- then, the net force on the object must be zero.

That is, the sum of all the forces on an object is zero when the object is in equilibrium.


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Free Fall

(C) 2003, Doug Davis; all rights reserved