Transcript force - Cloudfront.net

Forces in one
Dimension
Chapter
4
Classical Mechanics

Describes the relationship between the ________ of
objects in our everyday world and the ______ acting
on them.

Conditions when Classical Mechanics does not apply:


Very tiny objects (< atomic sizes)
Objects moving near the speed of light
Forces

Commonly imagined as
a _____ or _____ on
some object.


_______ quantity (F)



Forces results from
interactions between
objects.
Magnitude
Direction
There are two classes
of forces: __________
and __________.
Forces causes objects to _________ the way
they _________.



Objects can change
their __________.
Objects can ______
or ________.
These changes to
motion helps us to
_____ the object
being acted upon by a
force or a series of
forces.

This object is also
known as the system.
Contact and Field Forces
• Contact forces result from ______________between
___________.
• Field forces act between _____________________.
Contact Forces

A contact force exists when an
object from the external world
________ a system and thereby
exerts a force on it.


The _________ force in the picture
to the right is a contact force.
Whenever a force is exerted on
an object, the _______ of the
object can change, even rigid and
inflexible objects.

These changes can be permanent.
Fundamental Forces
 These
______________
force
 ______________
force*
 _____________
force *
 __________

Decreasing Strength
fundamental
forces are all
_____ forces.
 Types:
Free-Body Diagrams


These are ___________that help us in _________
involving ________.
They help us to ________ and _______ the forces
acting on an object.
Free Body Diagram




Use a free body diagram in order
to ________________which we
are applying Newton’s laws.
Must _________ all the forces
acting on the object of interest.
Choose an appropriate
coordinate system.
If the free body diagram is
incorrect, the solution will likely
be incorrect.
Sample Problem
Draw a free-body diagram for the following scenario: A
flowerpot falls freely from a window sill. Ignore any forces
due to air resistance.
Force and Acceleration



A ______ applied to an object cause the object to experience a
_________________, specifically the object ___________.
Acceleration is a vector quantity, thus a force can induce a change
in _________ or a _______________.
The applied force and the resulting acceleration are also
dependent upon the _________ of the object.

More on this later
Units of Force

SI unit of force is a
Newton (N)
kg m
1N  1 2
s

US Customary unit of
force is a pound (lb)

1 N = 0.225 lb
Combining Forces

Since forces are __________ they can be added and
subtracted like any other vectors.


We will deal with the simple horizontal case now and deal with
more complex vector additions in the next chapter.
The ____________is the vector sum of all the vectors
acting on an object.
Sample Problem
In the figure below, two forces are acting upon the block.
Calculate the net force acting on the block. The positive
direction is towards the right.
4N
-2 N
Newton’s First Law
Explain Newton’s first law of motion in
your own words.
Newton’s First Law
If the net force ΣF exerted on an object is
zero, the object continues in its original state
of motion. That is, if ΣF=0, an object at rest
remains at rest and an object moving with
some velocity continues with that same
velocity.

An object moves with a _________ that is _______
in magnitude and direction, unless acted on by a
_________ net force.

The net force is defined as the _________ sum of all the
________________exerted on the object.
ΣF = Sum of All External Forces
C

The net force (ΣF)
is the vector sum
of all the external
forces acting on
an object.

External force
B
A
ΣF = A + B + C

Any force that results
from the _________
between the ______
and its
____________.
Newton’s first law of
motion in action.
F
v
Thus an object will not change
any aspect of its motion unless
acted on by external forces.
v
Inertia

Is the __________ of
an object to _______
in its ______ motion.


In the _________ of a
force.
Thought experiment




Hit a golf ball.
Hit a bowling ball with
the same force.
The golf ball will travel
farther.
Both resist changes in
their motion.
Mass

A ________of the ________
of an object to changes in its
motion due to a force.





The ______ the mass, the ____
it ________ under the action of
a given force.
This definition allows us to relate
mass to motion.
In chemistry mass is the amount
of matter an object possess.
SI units are kg
Scalar quantity
Equilibrium


An object either _________or moving with a ________
velocity is said to be in equilibrium.
The ___________acting on the object is ______ (since
the acceleration is zero.)
F  0
Newton’s Second Law
The _____________ of an object is _______
proportional to the net force acting on it and
_________ proportional to its mass.

Newton’s 1st law deals with what happens when
no net force acts on an object. Newton’s 2nd law
deals with what happens when a force does act
on an object.
Newton’s Second Law
 Acceleration
is ________ proportional
to the net force.
a
F
Newton’s Second Law
 Acceleration
is ________ proportional
to the net mass.
a
1
m
Newton’s Second Law

Newton’s 2nd law stated in equation form is as
follows:
ΣF = The sum of all the forces acting
on an object.
Some Final Notes About Forces
 __________

cause changes in motion.
Motion can occur in the ________ of
forces.
 All
the forces acting on an object are
added as ________ to find the net force
acting on the object.

m is not a force itself
Using Newton’s 2nd Law


Newton’s 2nd law can be used to solve for __________,
directly and ________, __________, and ____
indirectly.
Steps for solving Newton’s 2nd law problems:
1.
2.
3.
4.
5.
6.
Read the problem
Draw a free-body diagram
ID all the forces acting on the object
Add the force vectors together
Apply the total force to solve for either mass or acceleration
Apply the kinematics equations to solve for velocity,
displacement , or time.
Sample Problem
Anudja is holding a pillow, with a mass of 0.30 kg, when Sarah
decides she wants it and tries to pull it away from Anudja. If
Sarah pulls horizontally on the pillow with a force of 10.0 N
and Anudja pulls with a horizontal force of 11.0 N, what is the
horizontal acceleration of the pillow.
Gravitational Force
________________between any two
objects.
 This is the weakest of the field force, especially
when we are talking about the ___________
particles.
 This force can be fairly strong when discussing
____________ objects.

Fg
Fg
Gravitational Force
 Expressed
by Newton’s Law of Universal
Gravitation:

Every particle in the Universe ________ every
other particle with a ________ that is ________
proportional to the _______of the _________
between them.
m1 m 2
Fg  G 2
r
Weight

The magnitude of the
gravitational force acting
on an object of mass m
near the Earth’s surface
is called the _______ w
of the object.

w = m • g is a special case
of Newton’s Second Law


g is the acceleration due to
gravity
g can also be found from
the Law of Universal
Gravitation.
More about weight

Weight is not an ______________of an object.


Weight depends upon _________.



Mass is an inherent property.
Weight varies with the _________ from the Earth’s
surface.
g _________ with ________ distance from the
Earth’s surface.
The equation for weight derived above can be
used to calculate the acceleration on an object
falling at the surface of any massive object if the
radius of the more massive object is known.
Drag Force

When an object moves through any kind of _____ (air,
water, etc.) the fluid exerts a __________on the
moving object in the direction _______ to its motion.
The drag force is dependent
on the ______ of the object,
the ________ of the object,
and the _______ of the fluid
that the object is moving
through.

Terminal Velocity

This occurs when an object is
accelerating due to ________.


During free fall the object
experiences two forces: ______ and
____________.
_______ a falling object the force
of gravity is greater than the air
drag.


As the falling object picks up speed,
the upward drag force then ______
the downward gravitational force.
The constant velocity that is reached
at this point is known as the
________________.
Interacting Forces

Forces always come in
________.



These force pairs are
known as _________
pairs.
These pairs will be _____
in _______ but _______
in __________.
__________________
summarizes the
relationship between two
forces.
Newton’s Third Law
_______________is premised on the fact that
forces in nature do not act alone.
 Forces always act in ___________ pairs.
 These opposing forces are always ________ in
magnitude.

Newton’s Third Law
If object 1 and object 2 interact, the force
exerted by object 1 on object 2 is equal in
magnitude but opposite in direction to the
force exerted by object 2 on object 1.


Therefore, single isolated forces _____________.
F12  F21
Newton’s Third Law: Identifying
the Interacting Pairs

F12 may be called the
______ force and F21
the ________ force.


Actually, either force
can be the action or
the reaction force.
The action and
reaction forces act
on __________
objects.
Forces Acting on an Object
Newton’s Law uses
the forces acting on
an object.
 n and Fg are acting
on the object.
'
 n ' and Fg are acting
on other objects.

Sample Problem
Identify the interacting pairs in the figure below.
Some Action-Reaction Pairs

n and n '

n is the _________,
the force the table
exerts on the TV.
 n is always
____________ to
the surface.
 n 'is the reaction –
the TV on the table.

n  n '
Normal Forces (n)
These are forces that are always directed
_________ and run __________ to a surface
of an object and act upon the object.
 n ________ the ________ of the object it is
directed towards.
 The __________ may vary depending upon
the object exerting the force.
 n = Fg so long as the object exerting the
normal force can _________ the weight of
the object resting upon it.

Applications of Newton’s Laws

Assumptions

Objects behave as
particles.



Can ignore
rotational motion
(for now)
Masses of strings or
ropes are negligible.
Interested only in
the forces acting on
the object.

Can neglect
reaction forces
More Assumptions – Ropes
 Ignore
any ______________of the rope.
 Ignore the ______ of the rope.
 The magnitude of the force exerted along
the rope is called the ___________.
 The tension is the _______ at all points in
the rope.
Solving Newton’s Second Law
Problems


Read the problem at least once.
Draw a picture of the system.





Label each force.



Use labels that bring to mind the physical quantity involved.
Use the kinematic equations when needed.
Apply Newton’s Second Law


Identify the object of primary interest
Choose an appropriate coordinate system
Draw a free-body diagram
Indicate forces with arrows
The x- and y-components should be taken from the vector
equation and written separately.
Solve for the unknown(s)
Sample Problem
A 50.0 kg bucket is being lifted by a rope. The rope will
not break if the tension is 525 N or less. The bucket
started from rest, and after being lifted 3.0 m, it is moving
at 3.0 m/s. If the acceleration is constant, is the rope in
danger of breaking?
The Laws of
Motion
Chapter 4
The End