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Lecture PowerPoint
Physics for Scientists and
Engineers, 3rd edition
Fishbane
Gasiorowicz
Thornton
© 2005 Pearson Prentice Hall
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Chapter 4
Newton’s Laws
Main points of Chapter 4
• Forces
• Newton's first law – inertia
• Newton’s second law – F = ma
• Newton’s third law – action-reaction forces
• Noninertial frames
• Identifying forces; free-body diagrams
• Finding the motion
4-1 Forces and Newton’s First Law
• Force is a vector
• Forces are additive – vector sum of all forces
acting on an object is the net force:
(4-2)
4-1 Forces and Newton’s First Law
Different Ways of Stating the Law of Inertia
•When there is no net force acting on an object,
that object maintains its motion with a constant
velocity.
4-1 Forces and Newton’s First Law
Different Ways of Stating the Law of Inertia
•When an object moves with constant velocity,
the net force acting on it must be zero.
4-1 Forces and Newton’s First Law
Different Ways of Stating the Law of Inertia
•When there is no net force on an object, it will
be at rest in a reference frame that is one of a
set of reference frames moving at constant
velocity to one another.
4-1 Forces and Newton’s First Law
Constant-velocity equations of motion:
(4-3)
(4-4)
(4-5)
Common forces:
Gravity
Tension (in rope, string, wire, etc)
Normal force (exerted by one surface on
another)
4-1 Forces and Newton’s First Law
Relative Motion – who
is at rest?
No correct answer –
each point of view is
as good as the other
4-2 Newton’s Second Law of Motion
An object acted upon by a net force accelerates.
(4-6,7)
4-2 Newton’s Second Law of Motion
Second law is more than just a definition of
force: can use it to measure forces and masses,
and to predict motion
4-2 Newton’s Second Law of Motion
Units of force: newtons
(4-10)
Observers in different reference frames
agree on forces but disagree on velocities –
cannot tell who is moving and who at rest by
measuring forces
4-3 Newton’s Third Law of Motion
When a force due to object B acts on
object A, then an equal and opposite
force due to object A acts on object B.
(4-15)
4-4 Noninertial Frames
Are they both accelerating
downward, acceleration g?
Or, as man sees he is at rest
with respect to woman, and
believes she should be
subject to gravitational force
but doesn’t feel one himself,
is there something
counteracting gravity?
4-4 Noninertial Frames
Answer can be found by identifying forces;
if there are none that can account for
motion, then frame must be accelerating.
4-5 Using Newton’s Laws:
Identifying Forces and Free-Body Diagrams
A good free-body diagram is essential in solving
force problems.
4-5 Using Newton’s Laws:
Identifying Forces and Free-Body Diagrams
External and Internal Forces
Look at two-block system –
internal forces are those exerted
by one block on the other (or
within a single block)
External forces come from
outside the two blocks; net
force in Newton’s second
law is net external force
4-5 Using Newton’s Laws:
Identifying Forces and Free-Body Diagrams
Forces on a sled:
A sled sliding down a hill has three forces acting on
it – gravity, the normal force, and friction
4-5 Using Newton’s Laws:
Identifying Forces and Free-Body Diagrams
We see that two of the forces are
at right angles to each other:
and choose those as
our coordinate axes:
4-5 Using Newton’s Laws:
Identifying Forces and Free-Body Diagrams
More on drawing free-body diagrams:
• Identify and isolate object; make sketch with clear labels
• Identify and draw all forces, with reasonably accurate
directions and magnitudes, emanating from a single point on
object
4-5 Using Newton’s Laws:
Identifying Forces and Free-Body Diagrams
More on drawing free-body diagrams (cont):
•Draw coordinate axes so that as many forces as possible are
along one or the other
• Separate forces into components if necessary
• Arrow showing acceleration, if you draw one, should be
clearly distinguishable from forces; same with net force
4-6 Using Newton’s Laws: Finding the Motion
Have found net force, and can then find
acceleration – now need to use kinematics laws
to find motion
For constant force, acceleration is constant –
have already developed equations for this
Variable forces need to be treated individually
4-6 Using Newton’s Laws: Finding the Motion
Example: spring force
This last equation is the one that will need to
be solved for x(t) – later.
Summary of Chapter 4
• Newton’s first law: if net force on object is
zero, object moves with constant velocity
• Newton’s second law:
(4-6,7)
• Units of force: newtons
• Newton’s third law: interacting objects
exert equal and opposite forces on each
other:
(4-15)
Summary of Chapter 4, cont.
• Observers in different reference frames
moving at constant velocity with respect to
each other measure different velocities but the
same forces – not true for accelerating frames
• If forces are known, can find motion of object,
or vice versa
• Internal forces do not affect object’s motion