Download Forces part1

Interactions
• A force is an interaction between 2 objects
An Introduction to Forces
Forces-part 1
– Touching
– At a distance
• See the Fundamental Particle Chart
(http://www.cpepphysics.org/images/2014-fund-chart.jpg)
PHYS& 114: Eyres
• Identifying Interactions
– Choose a system: circle the object
– Name the force and both interacting objects
Drawing force diagrams
Constructing force diagrams
1.
2.
3.
4.
Sketch the situation.
Circle the system.
Identify external interactions.
Place a dot at the side of the sketch
representing the system object.
5. Draw force arrows to represent the external
interactions.
6. Label the forces with a subscript containing two
elements.
• Example: a rock sinking into sand
© 2014 Pearson
Education, Inc.
© 2014 Pearson
Education, Inc.
Four Types
• weight:
Symbol W
– Pulling
– Between 2 objects with mass
Force Notation
• I am pushing down on a book that is lying on the
table.
• Tension: Symbol T
– Pulling
– Must have a rope or string or spring attached
• Normal: Symbol N or sometimes n
– Pushing
– Due to contact with a surface
• friction: Symbol f
– Opposite direction to motion or the direction of potential motion
– Rubbing (kinetic friction) or “Stickiness” associated with “trying to move
it” (static friction)
Force Type
By
On
Symbol
Direction
Weight
Earth
Book
W E on B
Down
Normal
Me
Book
NM on B
Down
Normal
Table
Book
NT on B
Up
Symbol W
1
Adding forces graphically
Adding forces graphically
(Cont'd)
• Draw the vectors head to tail.
• Draw the vector that goes from the tail of
the first vector to the head of the second
vector.
– This is the sum vector, also called the
resultant vector.
– In this case this vector is the net force (it is not
a new force, but rather the combined effect of
all the forces being exerted on the object).
© 2014 Pearson
Education, Inc.
Testing the relationship between
the sum of forces and the motion of
the system object
© 2014 Pearson
Education, Inc.
Reasoning without
mathematical equations
• Motion and force diagrams and the rule relating
motion and force can be used to reason
qualitatively about physical processes:
– To determine the relative magnitudes of forces if you
have information about motion
– To estimate velocity changes if you have information
about forces
© 2014 Pearson
Education, Inc.
Testing the relationship
between the sum of forces and
the motion of the system object
© 2014 Pearson
Education, Inc.
Inertial reference frame
• An inertial reference frame is one in which an
observer:
– Sees that the velocity of the system object does not
change if no other objects exert forces on it or
– Sees no change in the velocity if the sum of all forces
exerted on the system object is zero
• Make sure the unknown representation is
consistent with the known representation.
• In noninertial reference frames, the velocity of
the system object can change even though the
sum of forces exerted on it is zero.
© 2014 Pearson
Education, Inc.
© 2014 Pearson
Education, Inc.
2
Newton's first law of motion
• For an observer in an inertial reference frame,
the object continues moving at constant velocity
(including remaining at rest):
– When no other objects exert forces on an system
object or
– When the forces exerted on the object add to zero
• Inertia is the phenomenon in which an object
continues to move at constant velocity when the
sum of the forces exerted on it by other objects
is zero.
Newton’s Laws
• Newton’s 1st
Law
If there is no net Force, there is a constant
velocity.
What might the x vs t graph look like?
What might the v vs t graph look like?
What might the a vs t graph look like?
© 2014 Pearson
Education, Inc.
Newton’s Laws
• Newton’s 1st
Law
If there is no net Force, there is a constant
velocity.
What might the x vs t graph look like?
What might the v vs t graph look like?
What might the a vs t graph look like?
Newton's second law of motion
• Observation experiments help us construct
the following relationship between the sum
of forces on a system object and the
system object's motion:
• The symbol α means "is proportional to."
For example, if the sum of the forces
doubles, then the acceleration doubles.
© 2014 Pearson
Education, Inc.
Newton's second law of motion
• Observation experiments help us construct
the following relationship for the
proportionality between the acceleration of
a system object and the system object's
mass:
Cause-effect relationships
• The equation we deduced for Newton's second
law is:
• The right side of the equation (the sum of the
forces being exerted on the system) is the cause
of the effect (the system's acceleration) on the
left side.
© 2014 Pearson
Education, Inc.
© 2014 Pearson
Education, Inc.
3
Newton’s Laws
• Newton’s 2nd
Newton’s Laws
• Newton’s 2nd
Law
ΣF = ma
Law
F = ma
What might the x vs t graph look like?
What might the x vs t graph look like?
What might the v vs t graph look like?
What might the v vs t graph look like?
What might the a vs t graph look like?
What might the a vs t graph look like?
1st Law or 2nd Law?
Graph Sketches?
1st Law or 2nd Law?
Graph Sketches?
X=0
X=0
x vs. t
Constant Speed
therefore
∆v=0 and a=0
v vs. t
therefore
1st Law
1st Law or 2nd Law?
Graph Sketches?
a vs. t
1st Law or 2nd Law?
Graph Sketches?
X=0
X=0
x vs. t
Speeding up
therefore
∆v not 0 and a not 0
v vs. t
therefore
2nd Law
a vs. t
4
1st Law or 2nd Law?
Graph Sketches?
1st Law or 2nd Law?
Graph Sketches?
X=0
X=0
x vs. t
Slowing down
therefore
∆v not 0 and a not 0
v vs. t
therefore
2nd Law
Gravitational force
a vs. t
Weight
• The weight of the object on a planet is the
force that the planet exerts on the object.
• In everyday language, the normal force
that a scale exerts on you (which balances
the downward force you exert on it) is your
weight.
• We will not use the term "weight of an
object" because it implies that weight is a
property of the object rather than an
© 2014 Pearson
interaction
between two objects.
Education,
Inc.
© 2014 Pearson
Education, Inc.
Newton’s Laws
Newton's third law of motion
• Newton’s 3rd Law
If
Object 1 pushes on Object 2
then
Object 2 pushes on Object 1
• When two objects interact, object 1 exerts a
force on object 2. Object 2 in turn exerts an
equal-magnitude, oppositely directed force on
object 1.
On 2 By 1
N21
On 1 By 2
N12
• This can help to identify forces
• These forces are exerted on different objects
and cannot be added to find the sum of the
forces exerted on one object.
© 2014 Pearson
Education, Inc.
5