Newton’s Second Law
Newton’s 2nd Law
When a coin is flipped in an airplane moving at constant
velocity, where will the coin land?

In the persons lap
 Newton’s 1st Law
 A body in motion will stay in motion.
Newton’s 2nd Law
If the coin is flipped as the plane accelerates
during take off, where will it land?
 Behind the person.
 Newton’s 2nd Law
 The coin is in constant motion while the plane
accelerates
Newton’s 2nd Law
If the coin is flipped as the plane decelerates
while landing, where will it land?
 When the plane decelerates as the coin is
flipped, the coin will stay in constant motion as
the plane slows down.
Newton’s 2nd Law
When acceleration is equal to zero, the net force is also ZERO.
Whenever something isn’t accelerating, all of the forces
balance out to zero.
FORCE. (One Newton is about the
same weight as a ¼ lb. hamburger.
The Newton is a unit of
Newton’s 2nd Law
If 18 tons of thrust are required for an airplane to move at a
constant velocity (
zero acceleration ), how much friction
force is there?
 18 tons  constant velocity = 0 acceleration
Newton’s 2nd Law
If more thrust were used, what would happen to the
plane’s speed?
 Increase in speed (acceleration)
 Find a higher constant velocity
Newton’s 2nd Law
If the thrust were lessened, what would
happen to the plane’s speed?
 Decelerate
 The air resistance > the thrust
Newton’s 2nd Law
Acceleration is directly proportional to net force

a ~ Fnet
As the net force increases, how does acceleration change?
Acceleration increases
Newton’s 2nd Law
Acceleration is defined as the
rate of change of velocity.
Newton’s 2nd Law
Acceleration is produced by a net FORCE being
applied to an amount of matter.
Write the equation for Net Force:
Fnet = ma
Newton’s 2nd Law
 This is Newton’s 2nd Law of Motion.
Newton’s 2nd Law
It was Galileo who dropped objects
and showed that they fall together with
the same acceleration (if air
resistance doesn’t count.) However,
he could never answer why! Isaac
Newton did!
Newton’s 2nd Law
 The relationship between mass and acceleration is an inverse one.
A ~ 1/m
 Inversely proportional
 As mass increases, acceleration decreases
 The relationship between force and acceleration is a direct one
A ~ F net
Directly Proportional
As Force increases, acceleration increases
Newton’s 2nd Law
In the parachute example, are both parachutes
accelerating? YES
You know that there is a NET force acting on them both!
As the parachute begins to fall, air resistance increases
because they are accelerating. The greater the speed,
the greater the air drag (resistance).
Newton’s 2nd Law
When air drag (resistance) equals the weight, the
force on a falling person wearing a parachute is
ZERO Newtons. Consequently, the acceleration is
zero since there is no net FORCE. The velocity is
CONSTANT; this is called TERMINAL velocity.
Newton’s 2nd Law
Newton’s 2nd Law is known as the Law of Acceleration:
The effect of an applied force is to cause the body to
move in the direction of the force.
Net FORCE produces acceleration! Force tends to
accelerate things, mass tends to resist acceleration.
Newton’s 2nd Law
We can use Newton’s 2nd Law to change mass to weight. Since
all objects accelerate towards earth because of gravity, we
replace “a” for acceleration in the equation wit “g”.
Fwt = mg
(same formula as F = ma)
The unit for force is the Newton. What is the Newton equivalent to using fundamental units?
kg*m/s2
(hint: Simply plug in the units for mass and acceleration.)