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FORCES
episode II
newton strikes back
Physics
Mr. Maloney
A new era of physics

Aristotle (384-322 BC) thought that objects were
naturally at rest, would always stop themselves
and a force was needed for any motion.
 Descartes (1596-1650) thought that some unseen
vortex was constantly pushing on things to make
them stay in motion.
 Galileo (1564-1642) realized that the Greeks
weren't accounting for forces such as friction.
 Newton summarized Galileo's thoughts and
others through his three laws of motion.
© 2002 Mike Maloney
objectives
You will be able to
 describe how mass, force and acceleration
are related to eachother.
 describe the consequences of
Newton’s 2nd Law.
 graph data to find a relationship between
variables.
 Jump
to Post Lab
© 2002 Mike Maloney
Newton’s 2nd Law
 Newton’s
2nd law describes how a mass
behaves when forces act on it.
 We can guess these forces will produce a
change in motion but how are they related?
 ILD 2.2 (1st part)
© 2002 Mike Maloney
Newton’s 2nd Law
 How
does acceleration relate to force?
 Lets do a little experiment together as a
class to find out.
Force Sensor
Accelerometer
© 2002 Mike Maloney
Newton’s 2nd Law

How does this acceleration relate
to the force?
 The acceleration will be in the same direction
as the force.
 The magnitude of the acceleration increases
as the force increases
 The magnitude of the acceleration decreases
as the mass of the object increases
 Recreate our lab using this sim.
© 2002 Mike Maloney
N2L with multiple objects
 What
if there is more than one object
applying a force on some mass? What
is the effect now? (back to ILD)
 It is not just one force that determines
how an object will change it motion, it is
the sum total of all forces acting on the
object … what we call NET FORCE.
 Demos with 2 fans and 2 weights.
© 2002 Mike Maloney
Newton’s 2nd Law
 In
mathematical terms
 a = F/m
 acceleration = net force / mass
or more commonly written
 Force (net) = mass ∙ acceleration
 F = m∙a
 And since force and acceleration are
vectors, they will be in the same direction.
© 2002 Mike Maloney
MEASURING FORCES
 The
unit of force is the Newton {N}
 It is derived from its effect.
 F = m*a  [kg]*[m/s2]
 1 N = 1 kg•m/s2
 1 N is defined as the amount of force
required to accelerate 1 kg at a
rate of 1 m/s2.
© 2002 Mike Maloney
2nd Law Examples
 Accelerating
a car vs. truck
 Slowing down a car vs. truck
 Jogging vs. sprinting
 Pushing chair alone vs. someone in it
 Freefall
© 2002 Mike Maloney
objectives
Can you
 describe how mass, force and acceleration
are related to eachother.
 describe the consequences of
Newton’s 2nd Law.
 graph data to find a relationship between
variables.
 Jump
to Post Lab
© 2002 Mike Maloney
APPENDIX
© 2002 Mike Maloney
Inertia
 The
tendency of an object to resist a
change in motion is called its inertia.
 Objects with greater masses generally
have greater inertias.
 For our purposes, an object’s inertia is
basically measured by it mass.
 BACK
© 2002 Mike Maloney
Net Force

NET FORCE refers to the vector sum total
of all forces acting on an object. It is often
expressed as F
 For example, if there were two leftward forces
of 10 lb each, the NET FORCE would be 20
lb leftward.
 If there were one 10 lb rightward force and
one 8 lb leftward force, the NET FORCE
would be 2 lb rightward.
 What about if the forces were in X and Y?
 BACK
© 2002 Mike Maloney
Applied Force
 APPLIED
FORCE refers to a generic
force that is acting on an object.
 An APPLIED FORCE is basically any
force that is not a WEIGHT, NORMAL
FORCE, FRICTIONAL FORCE,
SPRING FORCE, or other named force.
 BACK
© 2002 Mike Maloney