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Chapter 4
Why things move as they do
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Force: Any external influence that causes a body
to accelerate
Friction: A force by one surface on another
surface, due to the roughness of the surface
Air Resistance: The resistive force that air
molecules exert on an object moving through the
air
Force of Gravity: Downward pull by Earth on
objects in Earth’s vicinity
4.1 Force: Why things
accelerate
Newton’s Theory: Forces cause acceleration,
and no force is needed to keep a thing moving
 Inertia: A body’s ability to stay at rest or to
maintain an unchanging speed and direction of
motion whenever no force is exerted on it
 Mass: The amount of Inertia in an object
(measured in kilograms)
 Acceleration = Force/mass
 Force is measured in Newtons (N)
 Newton is the amount of force that can give a 1
kg mass a 1 m/s acceleration
 Weight and Inertia are different things!
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4.2 Connecting Force and
Acceleration
Law of Motion
An object’s acceleration is determined by
the net force exerted on it by its
environment and by the object’s mass.
The direction of the acceleration is the
same as the direction of the net force.
Quantitatively, the acceleration is
proportional to the net force divided by
the mass
4.3 Newton’s Law of Motion
Weight: Net gravitational force exerted
on an object by all other objects
 Normal Force: The force, perpendicular
to a solid surface, that is exerted by any
solid surface on any object touching it
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4.4 Weight: Gravity’s Force on a
body
Law of Force Pairs
Every force is an interaction between two
objects. Thus, forces must come in pairs:
Whenever one body exerts a force on a
second body, the second exerts a force on
the first. Furthermore, the two forces are
equal in strength but opposite in direction.
4.5 The Law of Force Pairs
Resistive Forces: Any force that acts on
a moving body in a direction opposite to
the body’s motion
 Rolling Resistance: The resistive force
by a surface on a rolling object
 Rocket Propulsion: When material is
ejected from a vehicle, it exerts a reaction
force back on the vehicle because of the
law of force pairs. This force accelerates
the vehicle
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4.6 Newton meets the
Automobile