Forces and Newton’s Laws
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
Example of a force:
friction
We know it’s a force because it
affects the motion of objects
*Forces affect the motion of objects
Newton’s First Law: An object will remain
at rest or in motion at constantvelocity unless acted upon by a netforce
1000 N
1000 N
Fnet = 0 N
1000 N
700 N
Fnet = 300 N to
the right
300 N
(an unbalanced force)
Both balls have inertia: the tendency of
an object to resist a changein its motion
Inertia depends on mass; more mass,
more inertia
•Newton’s First Law is also called theLaw 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:
-A net force will cause
a mass to accelerate
-If force increases for the
same mass, acceleration
increases
-If mass increases for the
same force, acceleration
decreases
F = m a
F
m
a
Unit of force: the “newton”, N
N
kg
m/s2
Can be summarized in an equation:
Also called the Law of Acceleration
Example: A force of 25.0 N acts on an
object of 2.0 kg. Calculate theacceleration.
a =
F
m
=
25.0 N
2.0 kg
=
12.5 m/s2
Newton’s Third Law: Every action force is
accompanied by a reaction force equalin 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
-Rockets
-Walking, jumping
Action force: you on Earth
Reaction force: Earth on you
Action force: rocket on gas
Reaction force: gas on rocket
Which object has more inertia?
battleship
Which object has more momentum?
speedboat
An object must be in motion
to have momentum
Another force:
Gravity
It affects the motion ofobjects, so it’s a force
Acceleration of gravity:
- 9.8 m/s2
(in a vacuum)
All objects exhibit gravity; most are
too small to notice
Need planet-size objects for gravity to
become apparent
Cause objects to
have weight
Mass vs. Weight
Mass: the quantity of matter in an
object; measured in kg
Weight: the force caused by the gravitational pullon an object; measured in newtons (notice thatforce is also measured in newtons)
•Mass never changes; weight can change,depending on location
crystal quartz -- 1 kg piece
Falling Objects in an Atmosphere
Objects dropped initially accelerateat 9.8 m/s2 downward, due togravity
Fg
As they fall, they begin to
encounter air resistance,which increases with speed
Fg
Fa
Fg
Fa
Eventually the air resistance forcewill match the gravity force
What is the net force?
Fa
Fg
Fnet = 0
so acceleration = 0
so velocity change = 0
Object falls with a
constant velocity
( Fa = Fg )
This velocity is called Terminal velocity
Terminal velocity:
The greatest speed
at which an object can fall in
an atmosphere
