Newton’s 2nd Law
Seed Question: What happens to the acceleration of an object in deep space if you double
the force on it, while keeping the mass constant?
Exploration:
Part 1: Let’s answer the seed question.
If you double the force,
you will double the
acceleration because:
F
m
a
aαF
(m = constant)
2F
“is proportional to”
m
2a
From the a vs F lab graph
Part 2: What happens to the acceleration of an object in deep space if you double its mass
while keeping the force constant?
F
m
If you double the mass, you
will cut the acceleration in half
because:
a
a α 1/m
(F = constant)
F F
From the a vs 1/m lab graph
2m
a/2
Big Idea:
a α F/m
“is prop to”
Which means a = constant (F/m)
Things will be simple if the constant is 1.
Looking at the units, we want:
m/s2 = 1 (N/kg)
In other words, using metric units we define: 1 N = 1 kg•m/s2
then
a = F/m
But physicists don’t like fractions so multiply both sides by m:
Newton’s 2nd Law:
N
Fnet = ma
m/s2
kg
In words: The net (total) force on an object equals the object’s mass times its acceleration.
Note: Fnet is a vector: it has magnitude and direction.
For Further Contemplation: ?