Newton’s First Law
INERTIA
An object at rest remains at rest,
and an object in motion
continues with constant velocity
unless it experiences a net
external force
Stationary objects don’t
want to move
Moving objects
don’t want to stop
When you are in a car that is accelerating,
you feel as if you are being pushed back.
Your body (actually the inertia of your
body) is resisting the increase in speed.
When the car comes to a sudden stop,
your body wants to continue
in a forward motion.
When stopping,
the coffee tends
to spill forward
When accelerating,
the coffee tends to
spill on you.
Sudden Stops seat belts help your
body resist the
tendency to keep
moving. (rapid
deceleration)
Sudden Starts head rests stop the
head from being
pushed back.
(rapid positive
acceleration)
What will happen without seatbelt?
What happens when you stop applying a
Misconception!! force to an object?
- If no force acts on a moving object,
it will eventually come to rest.
- Motion requires the presence on a force.
Truth!!
- Only a force can cause a change in the
motion of a moving object.
- An object moves at a constant
velocity or not at all if there is no
force acting on it.
External Force
A single force that acts
on an object as a result of
the interactions between
objects and its
environment
Net External Force
The vector sum of all
forces acting on a body.
Can be found using the
methods for finding
resultant vectors
Objects motion is the same
as if the net external force
was the only force acting
on the object
Do forces always cause an object to move?
NO!!
We need to talk about NET FORCES!
F2 = 10 N
F1 = 30 N
Fnet = 20 N
Equilibrium occurs when the net force
equals zero.
Object can be in equilibrium even when
many forces act on it
When equal force but
opposite direction, no
change in motion
If one team applies greater
force, acceleration will
occur in the direction of
that force
Equilibrium
Objects that are either at rest or are
moving with constant velocity are said
to be in equilibrium
When Σ FX= 0 and Σ FY = 0 then the
vector sum of ALL forces acting on the
body is equal to zero and the body is in
equilibrium