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Push and Pull
Newton’s Laws
Newton’s First Law
An object at rest remains at rest, and an object
in motion continues in motion with constant
velocity (that is, constant speed in a straight line)
unless it experiences a net external force.
Also known as the “Law of Inertia”
Inertia
Tendency of an object to maintain its STATE OF MOTION
Proportional to MASS
Do these guys have a lot of inertia?
MORE MASS
means
MORE INERTIA
LOTS OF INERTIA
hard to…
GET MOVING or
STOP
Force
• A push or pull on an object
• May change the object’s STATE OF MOTION
• CONTACT FORCE – requires touching
– Normal, Tension, Friction
• FIELD FORCE – no contact required
– “Action over a distance”
– Gravity (Weight)
A block of wood is sitting motionless on a table.
What forces are acting on it?
Normal
Weight
FN
Fg
Normal Force is a
REACTION
force that any
object exerts
when pushed on
Weight is the force of
gravity
pulling an object
toward the
CENTER OF THE
EARTH
Net Force
• If there is no NET FORCE on an object it is at
equilibrium and either:
MOTIONLESS OR MOVING WITH CONSTANT VELOCITY
• So a “net” or “unbalanced” force will
– CHANGE AN OBJECT’S SPEED
• Changing speed means ACCELERATION
Force  Acceleration
• How much acceleration?
• Depends on:
– AMOUNT OF FORCE
• MORE FORCE = MORE ACCELERATION
– MASS OF OBJECT
• MORE MASS = LESS ACCELERATION
Newton’s Second Law
“The acceleration of an object is directly proportional to
the net external force acting on the object and inversely
proportional to the mass of the object.”
Fnet
a
m
Unit of force is the NEWTON (N)
Example
• A 2 kilogram box is pushed with a net,
unbalanced force of 10 newtons.
• What is the acceleration experienced by the
box?
a = Fnet / m
a = (10 N)/(2 kg)
a = 5 m/s2
Newton’s Third Law
“For every action, there is an equal and
opposite reaction”
FN
FT
Fg
Fg
Third Law Examples
• A firefighter directs a stream of water
from a hose to the east. In what direction
is the force on the hose?
There will be a force on the hose to the WEST
• A man getting out of a rowboat jumps
north onto the dock. What happens to
the boat?
The boat will move to the SOUTH