Transcript Free Body Diagrams - Mr. Romero

```MAY THE FORCE OF PHYSICS
BE WITH YOU
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
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
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:
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