Forces

Urban and Civil

Nov 16, 2013 (4 years and 5 months ago)

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KINEMATICS

DYNAMICS

Describes motion.

The forces that cause
motion.

Elementary Definition:

Force

a push or a pull

Not totally accurate though.

True Physics Definition:

Force

any action that can cause a
change

of
motion in an object

Vector

quantity

Forces have magnitude and
direction

May be a
contact force

or a
field force

Contact forces
result from physical contact between two
objects

Field forces
act between objects that are not in physical
contact

CONTACT FORCES

FIELD FORCES

Elastic Force (
F
elas

)

Frictional Force (
F
fric
)

Applied Force (F
A
)

Tensional Force (
F
tens
)

Normal Force (F
N
)

Strong Nuclear Force

Electromagnetic Force

Gravitational Force (
F
g

)

Electric Force (
F
elec

)

Magnetic Force (
F
mag

)

Physical contact must be present during the exertion of
the force.

Elastic Force
(
F
elas

)

due to stretching or compression of
an elastic object (rubber band, springs, basketball
bouncing)

Tensional Force
(
F
tens
)
-

due to objects pulled by a rope or
cable
-
like object

Frictional Force (
F
Fr
)

any force that
opposes

motion due
to two types of matter coming in contact (air resistance,
cleats on a grass field)

Physical contact must be present during the exertion of
the force.

Applied Force
(F
A
)

generic term for any other type of
force (boxer strikes another fighter, wrecking ball hits
the building)

Normal Force (F
N
)

force that acts
perpendicularly

when
an object is on a surface

From strongest to weakest …

Strong nuclear force

holds particles in nucleus
together

Electromagnetic force

electric and magnetic fields.
Holds particles together so that they can bend,
stretch or shatter.

Gravitational

attractive force that exists between all
objects

Must identify 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.

The force is the tension
acting on the box

The tension is the same at all
points along the rope

are the forces
exerted by the earth and
the ground

g
and
n F
The object is ...
The object is ...
The forces are...
The object is ...
All are true
0%
0%
100%
0%
0%
1.
The object is at
rest.

2.
The object is at
equilibrium.

3.
The forces are
balanced.

4.
The object is on a
horizontal
surface.

5.
All are true

True
False
0%
0%
1.
True

2.
False

The object wil...
The object wil...
The object wil...
The object has...
0%
0%
0%
0%
1.
The object will
eventually come
to a stop.

2.
The object will
speed up.

3.
The object will
continue moving
at a constant rate.

4.
The object has a
negative
acceleration.

Going up and g...
Going up and l...
Not moving.
Going down and...
Going down and...
0%
0%
0%
0%
0%
1.
Going up and
gaining speed.

2.
Going up and
losing speed.

3.
Not moving.

4.
Going down and
gaining speed.

5.
Going down and
losing speed.

In freefall.
On a level sur...
Not moving.
Going down and...
0%
0%
0%
0%
1.
In freefall.

2.
On a level
surface.

3.
Not moving.

4.
Going down and
losing speed.

1642

1727

Formulated basic concepts
and laws of mechanics

Universal Gravitation

Calculus

Light and optics

Newton’s First Law (Law of Inertia)

An object
moves with a constant velocity unless acted
upon by a net force (nonzero).

Net Force

vector sum of all external forces acting
on an object

Also referred to as the Law of Inertia

Inertia

the tendency of an object to remain in
its present state (at rest or in motion at constant
velocity)

The acceleration of
an object is directly
proportional to the
net force acting on
it and inversely
proportional to its
mass.

Weight is considered a force

F
g

= m x 9.8 because a = 9.8 m/s
2

Object has weight whether it is falling, resting, or
being lifted. Gravity is considered a constant

This does not mean that our weight is constant,
however.

SI unit of force is a Newton (N)

US Customary unit of force is a pound (lb)

1 N = 0.225 lb

2
s
m
kg
1
N
1

Forces cause
changes

in motion

Motion can occur in the absence of forces (if they are already
in motion)

All the forces acting on an object are added as
vectors to find the net force acting on the object

m is
not

a force itself

Newton’s Second Law is a vector equation

Forces
always

occur in
pairs
.
(action / reaction pairs)

(ex: push on a door, we
feel door push back)

Newton’s Third Law

when
one object exerts a force
on a second object, the
second object exerts a
force on the first that is
equal in magnitude, but
opposite in direction

Action
-
reaction forces

equal in magnitude,
opposite in direction.

Also, action
-
reaction force pairs ALWAYS act on
DIFFERENT objects.

A single isolated force cannot exist.

Where in real
-
life do
we try to reduce
weight to reduce
required force?

Friction

the force that
opposes the motion
between two
surfaces that are in
contact.

-

surfaces, at
microscopic level,
actually have many
small
inconsistencies

There are two main kinds of friction:

static friction

the force that opposes the
start of motion

sliding or kinetic
friction

the force
between surfaces while in motion

M

Normal

weight

applied
force

friction

To keep object
moving at a
constant
velocity, apply
constant force
equal to the
force of
friction.

a = 0

F
net

= 0

To calculate force of friction (F
F
), use the
equation:

F
Fr

=
μ
F
N
or
μ
=
F
Fr

/F
N

μ

= coefficient of friction

μ

= ratio of the frictional force to the normal
force

Frictional force is directly proportional to the
μ
.

F
fr

is always parallel to the surface and opposite
the motion of the object.

F
N

is force normal, or force perpendicular to
surface.

μ

represents coefficient of friction, and changes
with surface

Air resistance is the net force
of the air molecules
striking a moving object

-

another source of friction

-

without air resistance
(like in a vacuum, all
objects would fall
unimpeded at the
acceleration of gravity.

When force of air resistance equals the force
of gravity, terminal velocity is reached.

-

velocity becomes constant

For example, terminal velocities:

ping pong ball

6 m/s

skydiver
-

60 m/s

skydiver w/chute

5 m/s

How does a parachute work?

How else could a diver change his/her terminal
velocity?