GEC 224
Newton’s
Laws of
Motion
Background
Sir Isaac Newton (1643-1727) an English
scientist and mathematician famous for his
discovery of the law of gravity also
discovered the three laws of motion.
He published them in his book Philosophiae
Naturalis
Principia
Mathematica
(mathematic
principles
of
natural
philosophy) in 1687. Today these laws are
known as Newton’s Laws of Motion and
describe the motion of all objects on the
scale we experience in our everyday lives.
“If I have ever made any valuable discoveries, it has
been owing more to patient attention, than to any
other talent.”
-Sir Isaac Newton
NEWTON'S LAWS OF MOTION
There are three of them.
They explain the motion of an object as
resulting from the forces acting on the object.
Newton’s Laws of Motion
SUMMARY
1. An object in motion tends to stay in motion and an
object at rest tends to stay at rest unless acted upon by
an unbalanced force.
2. Force equals mass times acceleration
(F = ma).
3. For every action there is an equal and opposite
reaction.
Newton’s First Law
An object at rest tends to stay at rest
and an object in motion tends to stay
in motion unless acted upon by an
unbalanced force.
What does this mean?
Basically, an object will “keep doing what it was doing” unless
acted on by an unbalanced force.
If the object was sitting still, it will remain stationary. If it
was moving at a constant velocity, it will keep moving.
It takes force to change the motion of an object.
Balanced Force
Equal forces in opposite
directions produce no motion
1st Law
• Unless acted upon by
an unbalanced force,
this golf ball would sit
on the tee forever.
Unbalanced Forces
Unequal opposing forces
produce an unbalanced force
causing motion
1st Law
• Once airborne,
unless acted on
by an
unbalanced force
(gravity and air –
fluid friction) it
would never
stop!
If objects in motion tend to stay in motion, why don’t
moving objects keep moving forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon them.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
Newton’s First Law
(law of inertia)
•MASS is the measure of the amount of
matter in an object.
•It is measured in Kilograms
Newton’s First Law
(law of inertia)
• INERTIA is a property of an object that describes
how much
______________________
it will resist change tothe motion of
the object
mass means more inertia
• more _____
____
Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an
object is changing speed.
Newton’s 2nd Law
Newton-2
• The acceleration produced by a net force acting on an
object is directly proportional to the magnitude of the
net force and in the same direction as the net force,
and the acceleration is inversely proportional to the
mass of the object.
• Acceleration = net force/mass
• a=Fnet/m
Mass and Acceleration
• Mass resists acceleration, this is the principle of inertia
• We call this relationship inversely proportional
• Acceleration ~ 1/mass
What does F = ma mean?
Force is directly proportional to mass and acceleration.
Imagine a ball of a certain mass moving at a certain
acceleration. This ball has a certain force.
Now imagine we make the ball twice as big (double the
mass) but keep the acceleration constant. F = ma says
that this new ball has twice the force of the old ball.
Now imagine the original ball moving at twice the
original acceleration. F = ma says that the ball will
again have twice the force of the ball at the original
acceleration.
The Unit Newton
• Newton’s 2nd law says a = Fnet / m
• So Fnet = ma by algebra
1N = 1kg  m / s
2
• 1 Newton of force is the amount of force necessary to
accelerate 1 kg at 1 m/s/s
• This is why 1 kg weighs 9.8 N on Earth, because the
acceleration due to gravity on earth (g) is 9.8 m/s/s
Pressure
• Pressure = Force/Area
• Pressure is directly proportional to force but is not the
same thing as force
• 10 N of force exerted by pushing on someone with the palm of
your hand
• 10 N of force exerted by pushing on someone with a pin
P=
F
A
P
=
F
A
Which has the smaller surface
area -- point of pin or palm of
hand?
Pin has smaller area and
larger pressure
Pressure Units
SI: N/m2 = Pascal = Pa
equal weights
Named for Blaise Pascal (1623 – 1662)
French mathemetician & physicist
More Pressure Information & Examples
Finding Acceleration
• Kinematics
v
a=
• Dynamics t
d = at
1
2
Fnet
a=
m
2
Two Ways to Find Net Force
• Fnet = Vector Sum
• FNET = S F
• Newton 2
• FNET = m a
Example: Pushing a Box on Ice.
• A skater is pushing a heavy box (mass m = 100 kg)
across a sheet of ice (horizontal & frictionless). He
applies a force of 50 N toward the right. If the box
starts at rest, what is its speed v after being pushed for
a time t = 5 s ?
v=?
F = 50 N
m
d=?
t=5s
a=?
Example: Pushing a Box on Ice...
• Start with Fnet = ma.
•
•
•
•
•
•
a = Fnet / m.
a = (50 N)/(100 kg) = 0.5 N/kg = (0.5 kg m/s2)/kg
a = 0.5 m/s2
Recall that vf = vi+ at (from definition of a)
So v = (0.5 m/s2) (5 s)
v = 2.5 m/s
v = 2.5 m/s
F = 50 N
m
d=?
t=5s
a = 0.5 m/s2
Example: Pushing a Box on Ice...
• Now, what distance will the block travel during the 5
seconds?
• d = ½ a t2
• d = (0.5)(0.5m/s2)(5 s)2
• d = 6.25 m
v = 2.5 m/s
F = 50 N
m
d=?
t=5s
a = 0.5 m/s2
Friction: force that resists motion
• force between the surfaces of two objects
• Examples: sliding friction, air resistance
• Friction acts in the direction opposite to motion
Friction Example
• A force of 5 N is used to drag a 1 kg object across the
lecture table at a constant velocity of 1 m/s. What is the
friction force opposing the motion?
• What is the acceleration of the object?
• Velocity constant – acceleration = 0
• What is the net force on the object?
• Acceleration = 0 → Fnet = 0
• What is the force of friction opposing the motion?
• 5N
5N
FN = 10 N
5N
Fg = 10 N
Friction Example
• A force of 5 N is used to drag a 1 kg object across the
lecture table at a constant velocity of 1 m/s. What is the
friction force opposing the motion?
• What is the force of friction opposing the motion?
• 5N
• Now a force of 13 N is applied to the object. What is its
acceleration?
• Fnet = 13 N – 5 N = 8 N
• a = Fnet/m = 8 N/1 kg = 8 m/s2
5N
FN = 10 N
13 N
Fg = 10 N
Newton’s Third Law
For every action there is an equal and opposite
reaction.
What does this mean?
For every force acting on an object, there is an equal
force acting in the opposite direction. Right now,
gravity is pulling you down in your seat, but
Newton’s Third Law says your seat is pushing up
against you with equal force. This is why you are
not moving. There is a balanced force acting on
you– gravity pulling down, your seat pushing up.
Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.
Vocabulary
Inertia:
the tendency of an object to resist changes
in its state of motion
Acceleration:
•a change in velocity
•a measurement of how quickly an object is
changing speed, direction or both
Velocity:
The rate of change of a position along
a straight line with respect to time
Force:
strength or energy
Vocabulary
Inertia:
the tendency of an object to resist changes
in its state of motion
Acceleration:
•a change in velocity
•a measurement of how quickly an object is
changing speed, direction or both
Velocity:
The rate of change of a position along
a straight line with respect to time
Force:
strength or energy