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Forces
(Newton’s Laws of Motion)
Forces
 A force is a push or pull.

Forces are vectors.
 Force = mass(kg) x acceleration
 Units: kg*m/s2 = Newtons (N)


1 Newton is the force needed to accelerate 1
kg at 1 m/s2.
1 pound = 4.45 N
Newton’s Laws of Motion
 Three laws
 Relate force to motion.
 In a nutshell:
 Law 1: An object’s velocity doesn’t change
unless acted upon by a net force.
 Law 2: The acceleration of an object upon
which a force is acting depends on the amount
of net force being applied and on the mass of
the object.
 Law 3: For every force, there is an equal and
opposite counter-force. (Forces come in
pairs).
Net Force
 Net force means that forces are unbalanced.
Zero net force:
Gravity pulls down.
Rope pulls up.
Crate’s motion does
not change.
Net force: 
Rope snaps.
Gravity pulls down.
Crate accelerates
downward.
The First Law
 An object’s velocity doesn’t change unless
acted upon by a net force.

Inertia – the natural tendency of an object to
resist changes in its motion.


Objects at rest stay at rest.
Objects in motion stay in motion.
 Same speed.
 Same direction.

UNLESS a net force acts on them.
 Newton’s First Law = Law of Inertia
The First Law
 NO acceleration if NO net force.
 Net force = acceleration.
 CAREFUL!
 Don’t confuse movement w/ acceleration!
 If an object is moving at constant velocity, there is NO
NET FORCE acting on it!
There may be forces acting on it, but they are
balanced!
 Example: An airplane moving at a constant velocity of
200 mph north has many forces acting on it...
 But they all cancel out to zero, or the plane would be
accelerating!

Lift
The First Law
Drag
Weight
Thrust
Net force
Drag
Weight
Lift
Thrust
The First Law
 Inertia explains why:




You feel pushed back into your seat as your
car accelerates forward.
You feel thrown forward as your car slows
down.
You feel thrown to the side when your car
takes a tight curve.
Magicians can yank a tablecloth from beneath
a table setting without knocking anything over.
The First Law
 David Ginn’s tablecloth stunt demonstration
(complete with rabbit!) (YouTube link)
The Second Law
 The acceleration of an object depends on the
magnitude of the net force, and inversely on
the mass of the object.
a=
Fnet
m
 Big force = Big acceleration
 Big mass (more inertia) = Small acceleration
The Second Law
 Explains why everything falls w/ the same
acceleration (neglecting air resistance).


Heavier objects are pulled with more force but
have greater mass (inertia).
Lighter objects are pulled with less force but
have less mass (inertia).

In both cases, Fnet / m = 9.81 m/s2
The Third Law
 For every force, there is an equal and
opposite counter-force.

Or, you cannot push something without being
pushed back.
 Consider a swimmer:

The swimmer pushes back on the water using
his hands and feet.


The water accelerates backward.
The water pushes forward on the swimmer.

The swimmer accelerates forward.
The Third Law
 Consider walking:
 You push back against the Earth with your
feet.
 The Earth pushes forward on you, causing you
to accelerate forward.
 Does the Earth accelerate backwards?


YES! But only by a teeny tiny amount.
Why?



Because the magnitude of acceleration depends
inversely on the mass (2nd Law)
You  small mass  significant acceleration
Earth  huge mass  infinitessimal acceleration
A Problem for Consideration
 A farmer hooks his horse to the cart and says
“Pull, horse!”

The horse has studied a bit of physics and
says “Nope! I know about Newton’s 3rd Law.”
 The farmer says “What about it?”

The horse says “If I pull the cart, the cart pulls
right back on me with equal force, and neither
of us moves. Therefore, trying to pull the cart
is a hopeless task.”
 Obviously the horse isn’t totally correct, but
why?