Physics 231
General University Physics
Spring 2007
Lecture 9
Today
Force as a Vector
Superposition of forces
Newton’s III Law
Types of Forces
Weight is Gravitational Force
`Contact’ Forces
Force is a Vector
Demo: tennis ball and basketball
Discussion: Projectile Motion Trajectory
a
vs
v
Acceleration is a Vector and Newton’ II
Law implies that
Force
is a
Vector
Superposition of Forces
Reminder: additive property of forces
Superposition of Forces
Forces are vectors
Forces add
Forces add as vectors
1
2
3
...
=
net
i
F
F
F
F
F
Σ
=
+
+
+
G
G
G
G
G
Newton’s Second Law
a=
Σ
F
/
m
When viewed from an inertial frame, the
acceleration of an object is directly proportional to
the
net
force acting on it and inversely
proportional to its mass
Force is the cause of change in motion, as
measured by the acceleration (the effect)

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