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THIRD LAW
LAW OF INTERACTION
For every action, there is an equal but opposite
reaction.
When a force a applied to an object, the object
exerts a force that has the same magnitude but
opposite in direction.
These are called action-reaction forces (Two forces
+ two objects).
Box on table
Table on box
THIRD LAW
LAW OF INTERACTION
ACTION-REACTION FORCES
Action-reaction forces present on a
moving car-Earth system
The road
pushing
the car
forward
The car’s
tire
pushing
on the
road
Normal
force of
the Earth
pushing
up on the
car
Contact force
of the car
pushing
down on the
Earth
The road’s
friction on
the car’s
tires
Earth’s
Gravity
pulling the
car down
The cars
friction on
the road
The car’s
gravity pulling
the Earth up
Steps to construct a free-body diagram
1. Sketch the Forces – Identify and sketch all of the
external forces acting on an object.
2. Isolate the Object of Interest – Replace the object
with a box. Apply each of the forces acting on the
object to that point.
3. Choose a Convenient Coordinate System – If the
object moves in a known direction, it is often
convenient to pick that direction for one of the
coordinate axes.
4. Resolve the Forces into Components – Determine
the components of each force in the free-body
diagram.
5. Apply Newton's Second Law to each Coordinate (up,
down, left, right) Directions – Analyze motion in
each coordinate direction using the component form
of Newton's Second Law.
FREE BODY DIAGRAM
Foamcrete is a substance designed to stop an airplane
that has run off the end of a runway, without
causing injury to passengers. It is solid enough to
support a car, but crumbles under the weight of a
large airplane. By crumbling, it slows the plane to a
safe stop. For example, suppose a 747 jetliner with a
mass of 1.75 X 105 kg and an initial speed of 26.8
m/s is slowed down to a stop in 122 m. What is the
magnitude of the retarding force exerted by the
Foamcrete on the plane?
We need to find the acceleration of the
plane first.
Given:
This leads to the kinematics equation
to find the acceleration. Plugging in
the given values and solving for a
gives:
Now we need to use Newton's Second
Law to find the force. The only
direction we are worried about is the
x direction. Therefore, all of the force
in that direction needs to be added up
and set equal to the mass and
acceleration.
The force will be negative because we
are slowing the plane down.
RELEASED TAKS 2004
RELEASED TAKS 2006