Transcript Dynamics Free body Diagrams and Systen Schema

Dynamics: Intro & Application of
Newton’s Laws
Drawing Free-Body-Diagrams
AP Physics Development Committee
May 2010 – New Wording for free-body
or force diagrams. (p. 149 C&E)
Students will be directed to "draw and label
the forces (not components) that act on the
[object]," where [object] is replaced by a
reference specific to the question, such as
"the car when it reaches the top of the hill."
Any components that are included in the
diagram will be scored in the same way as
incorrect or extraneous forces.
Drawing Free-Body-Diagrams
In addition, in any subsequent part asking for
a solution that would typically make use of
the diagram, the following will be included.
"If you need to draw anything other than
what you have shown in part [x] to assist in
your solution, use the space below. Do NOT
add anything to the figure in part [x]." This
will give students the opportunity to
construct a working diagram showing any
components that are appropriate to the
solution of the problem. This second diagram
will not be graded.
Drawing Free-Body-Diagrams
Binder pp. 19-21
Force = Interaction
Force: interaction between an agent
and an object causing a push or pull
Two kinds of forces
1. Contact
2. “Non-Contact” (long-range
field forces due to
gravitational, magnetic, and/or
electric fields)
Force Types
Contact
Supportive (normal or ⊥)
Tension (rope/chain)
Friction or Drag (always
oppose motion)
Other push or pull
Non-Contact
Gravitational
N
T
f
P
G
System Schema
block
Identify the
interactions
table
earth
System Schema
block
label the
interaction
types
N
G
table
G
earth
System Schema
block
Dot around the
system of
interest
N
G
table
G
earth
System Schema
block
You are only
interested in
the forces
that cross the
dotted line!
N
G
table
G
earth
Agent/Object Notation
Type of force
FT
Object the force acts on.
“Feeler”
A/O
Agent that produces the force.
“Dealer”
If the agent can't be identified, the
force doesn't exist!
Constant Velocity
FN T/B
block
N
G
table
G
earth
FG E/B
Constant Velocity
FN T/B
When the object is
moving, include a velocity
vector off to the side
v
FG E/B
Note: the velocity vector does NOT
touch the dot.
Changing velocity
FN T/B
v
block
N
f
Ff T/B
G
table
G
earth
FG E/B
Changing velocity
FN T/B
Ff T/B
v
a
FG E/B
Note: the acceleration vector does
NOT touch the dot.
Non Perpendicular Forces
Object slides without friction
FN R/B
v
block
N
G
ramp
G
earth
FG E/B
Non Perpendicular Forces
FN
Ff
Ramp ll/B
Another form of A/O
notation
Components should not
appear on the FBD!!
Ramp /B
= FN
= f
FG
E/B=
Wt
Unambiguous Force Labeling
FT
Rope1/B
= T1
FT
FG
E/B=
Rope2/B
Wt
= T2
Forces – Relative lengths
FT
T
FN T/B = FN
Ff
Tll/B
R/B=
θ
= f
v
FG
E/B
= mg
Ambiguity in HW is OK
Ff
OR
v
FG
E/B=
mg
A/B=
v
D
FG
E/B=
mg
NEWTON'S LAWS
FIRST LAW
Object at rest or moving
with constant velocity.
Isaac Newton
(1642-1727)
ΣF = 0 (Equilibrium)
Vectors should be written
in component form:
ΣFx = 0
ΣFy = 0
2005B2
2005 B2. A simple pendulum
consists of a bob of mass
1.8 kg attached to a string
of length 2.3 m. The
pendulum is held at an angle
of 30° from the vertical
by a light horizontal string
attached to a wall, as
shown.
a. Draw a free-body diagram
labeling the forces on the
bob in the position shown.
FT
s1/B =
T1
FT
FG
E/B=
s2/B=T2
mg
b. Calculate the tension in the horizontal string.
T1
60°
T2
mg
ΣFH = T2 – T1 cos 60º = 0
ΣFV = T1 sin 60º - mg = 0
T1 sin 60
mg
=
T2
T1 cos60
mg
1.8 kg (9.8 m/s )
T2 =
=
tan 60
tan60
2
T2 = 10.18 N
c. The horizontal string is
now cut close to the bob, and
the pendulum swings down.
Calculate the speed of the
bob at its lowest position.
q
h
L
h = L - Lcosθ
U0 = K f
1 2
mgh0 = mv f
2
v f = 2 gh = 2(9.8 m/s )(2.3 m(1-cos30 )
2
= 2.5 m/s
The End
(for now)