Download File

Ch.4
&
Net Forces
INERTIA
the tendency of an object
to resist any change in its motion
Inertia is a property of matter and does not
depend on the position or location of the object.
MASS
a quantitative measure of inertia
FORCE
“a push or pull”
NEWTON’S 1st Law of Motion
“Law of Inertia”
The velocity of an object remains constant
unless acted on by an unbalanced force.
The net force acting on an object is the
vector sum of all the forces acting on it.
Examples:
9 lb
8 lb
8 lb
4 lb
7 lb
12 lb
6 lb
8 lb
?
5 lb
4 lb
If an object is remaining at rest, it
is incorrect to assume that there
are no forces acting on the object.
We can only conclude that the
net force on the object is zero.
4 lb
3 lb
7 lb
EQUILIBRIUM
the condition of an object
such that it is not accelerating
Balanced Forces:
•no net force acting on the object
•no acceleration
•Balanced forces are in equilibrium.
SF = 0
SFx = 0 and SFy = 0
NEWTON’S 2nd Law of Motion
“Law of Acceleration”
Acceleration is directly proportional to
the net force and inversely proportional
to the object’s mass.
Fnet = m a
NEWTON’S 3rd Law of Motion
“Law of Interaction”
Short Version
“For every action there is
an equal and opposite reaction.”
More Scientific Version
When one object exerts a force on a
second object, the second exerts a force
on the first that is equal in magnitude,
but opposite in direction.
Newton’s Third Law deals with two forces
acting on two different objects.
Newton’s Third Law pairs of forces
always,
sometimes,
never
cancel each other out.
The only way for two forces to cancel
each other out is for them to be equal
and opposite and act on the same object!
How do Newton’s Laws of motion
apply to these situations?
•
•
•
•
•
•
•
an object rests in your hand
a ball is tossed upward
a car windshield hits a bug
a person sits on a table
a person jumps up from the floor
a baseball bat hits a baseball
a truck and car hit head-on
Net Forces
Forces are VECTORS
Net Force is the resultant vector of
all forces acting on an object.
The SI unit of force is Newton .
1N =1kg·m/s2
Rules for summing vectors are still the same.
Examples:
Net Force with more than 1 direction:
Fnet  Fx  F
2
Fx  F cos 
Fy  F sin 
2
y