Transcript Newton`s Laws

Push and Pull
2-1-1 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
current state of motion
Do these guys have a lot of inertia?
• Inertia is a relative measurement…
– More inertia = MORE MASS
– More inertia = harder to CHANGE VELOCITY
Force
• A force is a push or pull that may change an
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 placed on a table and is motionless.
What forces are acting on it?
FN
Fg = WEIGHT
a force pulling any
object toward the
CENTER OF THE
EARTH
FN = NORMAL FORCE
a reaction force that any
object exerts
when pushed on
Fg
Net Force
• No NET FORCE on an object = EQUILIBRIUM… either
– MOTIONLESS
– MOVING WITH CONSTANT VELOCITY
• A “NET” or “UNBALANCED” force changes an object’s
VELOCITY
– This means that SPEED AND/OR DIRECTION change
– Also called ACCELERATION
Net Force  Acceleration
• How much acceleration a net force causes
an object to have depends on…
– amount of FORCE
• greater force = GREATER ACCELERATION
– amount of MASS
• greater 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 2.0 kilogram box is pushed with a net force of
10. newtons. What is the acceleration
experienced by the box?
a = Fnet / m
a = (10 N)/(2.0 kg)
a = 5.0 m/s2
Weight
• The force with which gravity pulls on an object.
g
Fg
m
Fg  mg
Example
• What is the weight of an object with a mass of
30 kilograms?
Fg = mg
Fg = (30 kg)(9.81 m/s2)
Fg = 294.3 kg·m/s2 or 294.3 N
Newton’s Third Law
“For every action, there is an equal and
opposite reaction”
FN
Fg
FT
Fg
Examples
• A firefighter directs a stream of water
from a hose to the east. In what direction
is the force on the hose?
To the WEST
Examples
• A man getting out of a rowboat jumps
north onto a dock. What happens to the
boat?
It moves SOUTH
Examples
• A 60 kilogram astronaut pushes against a
120 kilogram satellite with a force of 15
newtons. How much force does the
satellite exert on the astronaut?
15 newtons
End of 2.1.1 - PRACTICE