Download Newton`s First Law

Forces and Newton’s Laws
force: a push or a pull
* Forces affect the motion of objects
History
Aristotle: Natural state of
bodies is at rest
Galileo: Natural state of bodies is
at rest or in motion at
constant velocity
Newton: Extended Galileo’s
ideas; summarized in
three Laws of Motion
Newton’s First Law: An object will remain at rest or in
motion at constant velocity unless acted
upon by a net force (an unbalanced force)
1000 N
1000 N
Fnet = 0 N
700 N
Fnet = 300 N to
the right
1000 N
300 N
Both balls have inertia: the tendency of an object to
resist a change in its motion
Inertia depends on mass; more mass, more inertia
•Newton’s First Law is also called the Law of Inertia
Examples:
-When you are riding down the road in a car and you
come to a sudden stop, you lurch forward in the car
(your inertia keeps you moving forward)
-When you start after being stopped , you lurch
backward in the car, but not really
-Really, you stay at rest (because of your inertia), and
the car moves forward underneath you, until an
unbalanced force (the seat) exerts a force on you
Newton’s Second Law: Law of Acceleration
- A net force will cause a mass to accelerate
- The acceleration is
directly proportional
to the force
- The acceleration is inversely
proportional to the mass
- The acceleration is in the same
direction as the force
Also called the Law of Acceleration
Can be summarized in an equation:
F = ma
Unit of force: the “newton”, N
Example: A force of 25.0 N acts on an
object of 2.0 kg. Calculate the acceleration.
a =
=
F
m
25.0 N
2.0 kg
=
12.5 m/s2
Mass vs. Weight
Mass: the quantity of matter in an
object; measured in kg
Weight: the force caused by the gravitational pull
on an object; measured in newtons
•Mass never changes; weight can
change, depending on location
crystal quartz -- 1 kg piece
Newton’s Third Law: Every action force is
accompanied by a reaction force equal in size and
opposite in direction
-Also called the Law of Action-Reaction
Examples:
-the recoil of a cannon after firing
Action force: cannon on ball
Reaction force: ball on cannon
-Walking, jumping
Action force: you on Earth
Reaction force: Earth on you
-Rockets
Action force: rocket on gas
Reaction force: gas on rocket
*Forces affect the motion of objects
Example of a force: friction
We know it’s a force because it
affects the motion of objects
Falling Objects in an Atmosphere
Objects dropped initially accelerate
at 9.8 m/s2 downward, due to
gravity
As they fall, they begin to
encounter air resistance,
which increases with speed
Eventually the air resistance force
will match the gravity force
What is the net force?
Fg
Fa
Fg
Fa
Fg
Fnet = 0
( Fa = Fg )
Fa
so acceleration = 0
so velocity change = 0
Object falls with a
constant velocity
Fg
This velocity is called Terminal velocity
Terminal velocity: The greatest speed
at which an object can fall in
an atmosphere