Newton’s Laws of Motion
Mr. Rockensies
Regents Physics
Newton’s First Law of Motion
An object in motion remains in motion
unless acted upon by an unbalanced force.
 Better stated:

◦ If the Net Force is Zero, the velocity remains
constant.
◦ If FNET = 0 N
velocity is constant
Constant Velocity
2 possible velocity conditions
v=0
Static Equilibrium
v≠0
Dynamic Equilibrium
Both follow the same rules, velocity does NOT change.
WHY?
Objects naturally resist changes in motion due to their inertia.
Inertia = Mass
Newton’s 2nd Law
The first Law of Motion looked at
examples where there is no Net Force,
but what happens when there is one?
 FNET = ma or a = FNET/m

◦ FNET is the resultant force
◦ Units – 1 N = 1 kg•m/s2
◦ Direction: F and a have the same direction.

Weight = Fg = mg, 1 kg = 9.8 N
Solving Net Force Problems
1.
2.
3.
4.
Identify all Forces
Draw free-body diagram
Find FNET
FNET = ma
The Rocket
Fengine
Fg = m•g
FNET = Fengine – Fg = ma
Newton’s Third Law
-Action + Reaction
For every force exerted on one object by a second
object, the second object exerts an equal force,
opposite force on the first object.
Fman on wall
Fwall on man
These forces do not cancel out because they are NOT acting on the same object.
This is true for remote forces as well…
These forces do not cancel out because they are
NOT acting on the same object.
This is true for remote forces as well…
Earth
Fg
Fg
Moon
Related Problems