Warm-Up!
Do objects stop moving once they are
set in motion in space?
A. Never
B. Always
C. No, unless something gets in the way
Which set of arrows do not represent
equal and opposite force vectors?
A. F1 F2
B. F1 F2
C. F1 F2
Inertia and Newton’s Laws
Isaac Newton
• 18th Century Scientist
• Newton’s 3 Laws of Motion
• Newton’s Law of Gravitation
Galileo’s Thought Experiment
© The Physics Classroom, Tom Henderson 1996-2007
Newton’s Thought Experiment
This thought experiment lead to
Newton’s First Law.
© The Physics Classroom, Tom Henderson 1996-2007
Newton’s 1st Law of Motion
• Objects will not change
their motion without help
– force.
• INERTIA is this resistance
to change in motion
• MASS is how we measure
inertia
• The boulder example
Balanced Forces
• Forces can be
applied but inertia
is overcome only if
unbalanced forces
exist – Leggo my
Eggo.
• Balanced forces
result in constant
motion
Constant Motion
• Motion which is either stationary (0) or
not accelerating and in a straight line
Forces and Equilibrium
• If the net force (SF) on a body is zero, it is in
equilibrium.
• An object in equilibrium may be moving relative
to us (dynamic equilibrium).
• An object in equilibrium may appear to be at rest
(static equilibrium).
Newton’s First Law
• Newton’s 1st Law is also called the Law of Inertia.
• A body stays at rest or at constant
velocity unless acted upon by an
unbalanced force.
• This law is commonly applied to the horizontal
component of velocity, which is assumed not to
change during the flight of a projectile.
• SF = 0
The Walking Man
• http://phet.colorado.edu/sims/movingman/moving-man_en.jnlp
Sample Problem: Three forces act upon a 3.0 kg body
moving at constant velocity. F1 = (4i – 6j + k) N and F2 = (i –
2j - 8k) N. Find F3.
Newton’s Second Law
• Newton’s 2nd Law says that a body acted upon by
a net unbalanced force will accelerate.
• The acceleration is proportional to the net force
and inversely proportional to the mass. It is in
the direction of the net force.
• SF = ma
• Sample Problem: Two forces, F1 = (4i – 6j + k) N and
F2 = (i – 2j - 8k) N, act upon a body of mass 3.0 kg. No
other forces act upon the body at this time. What do you
know must be true?
Newton’s Third Law
• Newton’s 3rd Law is commonly stated “for every
action there exists an equal and opposite
reaction”.
• If A exerts a force F on B, then B exerts a force of
-F on A.