Free Body Diagrams - Mr. Romero

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Transcript Free Body Diagrams - Mr. Romero

MAY THE FORCE OF PHYSICS
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Bell Ringer!
What is the weight of a
46.2 gram bird?
m = 46.2 g = .0462 kg
F = ma, F = (.0462 kg)(9.8
N/kg)
= 0.453 N
Net Force – Example 1
Finding
acceleration
9.0 N
5.0 kg
17.0 N
F = ma
Making to the right +
<+17.0 N – 9.0 N> = (5.0kg)a
8.0 N = (5.0kg)a
a = (8.0 N)/(5.0kg) = 1.6
m/s/s
Net Force – Example 2
unknown force
Some other
force is acting
on the block
F = ??
35.0 kg
Finding an
450. N
a = 9.0 m/s/s
F = ma
Making to the right +
<+450. N + F> = (35.0kg)(+9.0
m/s/s)
450. N + F = 315 N
F = 315 N - 450. N = -135 N (to the
left)
Check on Learning
Get your whiteboards and
calculators ready!
Find the acceleration:
3.0 N
5.0 kg
7.0 N
F = ma
Making to the right +
<7.0 N – 3.0 N> = (5.0kg)a
4.0 N = (5.0kg)a
a = .80 m/s/s
Find the acceleration:
3.0 N
23.0 kg
6.0 N
5.0 N
F = ma
<5.0 N – 3.0 N – 6.0 N> =
(23.0kg)a
-4.0 N = (23.0kg)a
a = -.1739 = -.17 m/s/s
Find the other force:
F = ??
452 kg
67.3 N
a = .12 m/s/s
F = ma
<67.3 N + F> = (452
kg)(.12 m/s/s)
<67.3 N + F> = 54.24 N
F = 54.24 N - 67.3 N
F = -13.06 = -13 N
A scalar is simply a number, a magnitude alone.
A force is usually shown as a vector, which includes both
magnitude and a direction.
Force (or free-body) diagrams show the relative magnitude
and direction of all forces acting upon an object. The object
must be isolated and “free” of its surroundings.
Bell Ringer
Calculate your weight in
Newtons.
HINT: You are going to
need to convert your
weight from pounds (lbs)
first…
This is a free-body diagram of the Statue
of Liberty. She is represented by a simple
box. The forces acting on her are labeled
with a magnitude and the arrow shows
direction. Notice the surrounding objects
are stripped away and the forces acting on
the object are shown.
2207250 N
2207250 N
W here represents the force of the weight
of an object.
W =2207250 N
N is the normal force, which represents
the force the ground is pushing back up
on the object.
N = 2207250 N
(Positive y-direction)
+y
Think of the diagram on an XY
plane.
W = 496210 N
If “up” is assumed to be the positive
direction, then N is positive and W is
negative.
N = 496210 N
+x
(Positive x-direction)
is
(Positive y-direction)
+y
W+N
(  is the Greek symbol for “sum” )
2207250 N
-2207250 N
2207250 N
2207250 N
The sum of the forces in the y is zero.
+x
(Positive x-direction)
The forces acting on the object cancel each other out.
•We know F = m * a, where “a” is acceleration.
•If a = 0, then F = m * 0 = 0.
•When  F = 0, the object is not accelerating.
•We can then say that the forces acting on the
object cancel each other out and it is in a state of
static equilibrium.
Bell Ringer
What do each of the terms stand for?
•
Fnorm (Normal force) = ground reaction force.
always perpendicular to the ground. (N)
•
Fgrav
•
Ffriction
•
Fair
•
Ftens or T (Tension)- pulling force exerted by
(Weight)
(Friction)
(Air resistance) – force of the air friction
a chain, cable, rope, or string on an
object
Create a free body diagram (FBD) for each of the following
situations. Draw a FBD of the gorilla:
N
W
Sitting Gorilla
Free Body Diagram of the Sitting
Gorilla (The box represents the
gorilla, W = weight of the gorilla,
N = Normal force)
Create a free body diagram (FBD) for each of the following
situations. Draw a FBD of the gorilla:
W
N
This is also an acceptable
diagram.
Sitting Gorilla
Draw a FBD of the wooden swing:
T1
T2
W
Parrot on wooden
swing hung by ropes
Free Body Diagram of the wooden
swing (The box represents the wooden
swing, W = weight of the swing and the
parrot, T represents the ropes that are
in tension supporting the weight)
Draw a FBD of the ring at point C:
A
B
C
TCA
TCB
D
TCD
Traffic Light
supported by cables
Free Body Diagram of the ring at
point C (T represents the force of the
cables that are in tension acting on
the ring)
Draw a FBD of the traffic light:
A
B
TCD
C
D
W
Traffic Light
supported by cables
Free Body Diagram of the traffic light
(TCD represents the force of the
cables acting on the light and W is
the weight acting on the light)
Class work.
Draw free body diagrams for the following scenarios.
1. A book is at rest on a tabletop.
A free-body diagram for this
situation looks like this:
2. A girl is suspended motionless
from the ceiling by two ropes. A
free-body diagram for this situation
looks like this:
4.
An egg is free-falling from a
nest in a tree. A free-body
diagram for this situation
looks like this:
5. A rightward force is applied to a
book in order to move it across a
desk with a rightward acceleration.
Consider frictional forces. A freebody diagram for this situation looks
like this:
6. A rightward force is applied to a book
in order to move it across a desk at
constant velocity. Consider frictional
forces. Neglect air resistance. A freebody diagram for this situation looks
like this:
7. A college student rests a backpack
upon his shoulder. The pack is
suspended motionless by one strap
from one shoulder. A free-body diagram
for this situation looks like this:
8. A skydiver is descending with a constant
velocity. Consider air resistance. A freebody diagram for this situation looks like
this:
9. A force is applied to the right to
drag a sled across loosely packed
snow with a rightward acceleration.
A free-body diagram for this
situation looks like this:
10. A football is moving upwards
towards its peak after having been
booted by the punter. A free-body
diagram for this situation looks like this:
11. A car is coasting to the
right and slowing down. A freebody diagram for this situation
looks like this: