Stuff you asked about:
Derek
You should tell everyone in the classroom to scoot in
as far as possible so that people don't have to crawl
through 20 people in a row to get a seat. Telling
everyone to scoot in as far as they can towards the
middle will allow for more orderly seating.
I know that this is probably not why you have this rule, but I
would like to thank you for allowing us to attend other
lectures and have our iclicker points counted. I overslept
my 10AM lecture on Thursday but was able to still get
credit by attending the 12 PM lecture. You guys rock.
Stuff you asked about:
Have trouble just listing what forces are acting on the object in question and then
decomposing them into the X and Y components (or when to do this.)
Can you go over the box on a spring in the elevation?
Free body diagrams has always been confusing for me, especially when it involves
inclined planes or elevators but once I get the diagram the problem is easy to solve.
Can you help to establish basic ground rules and things to keep in mind when we draw
these?
I just wanted to say, I really like the demos in lecture. They make physics seem real.
For the elevator problems, are we talking about instantaneous when the elevator starts
to accelerate? Or as it is accelerating constantly? Does it matter?
Love the line It is what it needs to do what it does.
how in the holy heck did newton know that things accelerate at 9.81 m/s^2 if
there is no air present. did he live in a vacuum. what the heck.
Can you go over the universal gravitation stuff? Pleeeeeeaseeee?
The prelecture would be easier to understand if the examples weren't
theoretical and you just inputed random numbers into the problem to
give us a numerical sense of the problem.
What does an equal sign with 3 lines mean?
Physics 211
Lecture 5
Today’s Concepts:
a) Free Body Diagrams
b) Force due to strings
c) Force due to springs
d) Force due to gravity
Classical Mechanics
“I would like more free body diagram examples please.”
Free Body Diagrams
1)Choose system (object or group)
2)Draw forces acting on object
-Gravity
-Things touching (contact)
T
m
mg
Free Body Diagrams
“Third Law Newton needs explaining. Why is gravity and
normal force not 3rd law?”
Free Body Diagrams
Free Body Diagrams
Tension Force
Direction: Parallel to rope, always pulling.
Magnitude: As much or little as needed for Fnet=ma
T
m
mg
Checkpoint
BOX IN THE ELEVATOR
A box of mass m is hung with a string
from the ceiling of an elevator that is
accelerating upward. Which of the
following best describes the tension T
in the string:
A) T < mg
B) T = mg
C) T > mg
“Tension acts upward on the box, while mg (weight) acts
downward on the box. Since the box accelerates upward,
T must be greater than mg."
"it is what it has to be to do what it does"..
a
Watch out for subtle mistakes…
Many people got the right answer with
explanations like this one:
Because the elevator is accelerating
upwards the box is being pulled down
relative to the elevator, meaning that
there must be extra tension to keep the
box hanging.
Don’t view problem from accelerating
reference frame or you may get confused
T
m
mg
Just Remember: Net force causes acceleration
a
Aside: How a Spring Scale works
F
F
There is a spring in there
F
F
Dx
The dial just shows how much it has been stretched, Dx.
Tension Clicker Question
A block weighing 4 lbs is hung from a rope attached to a scale. The
scale is then attached to a wall and reads 4 lbs. What will the scale
read when it is instead attached to another block weighing 4 lbs?
?
m
A) 0 lbs.
m
B) 4 lbs.
m
C) 8 lbs.
Normal Force
Normal force is the force perpendicular to the surface that
prevents the objects from passing through each other.
1) Direction: Perpendicular to surface and out
2) Magnitude: As much or little required keep objects separate.
"It is what it has to be to do what it does." This statement
confused the hell out of me.
Checkpoint
A block sits at rest on a horizontal frictionless surface. Which of the
following sketches most closely resembles the correct free body
diagram for all forces acting on the block? Each arrow represents a
force.
A
B
C
D
The horizontal frictionless surface is pushing up on the box,
to keep it from falling prey to evil gravity, which is pulling
down on the box.
Clicker Question
A block accelerates down a frictionless inclined plane. Which of the
following sketches most closely resembles the correct free body
diagram for all forces acting on the block? Each arrow represents a
force.
A
B
C
- Gravity is down.
- Normal is perpendicular to surface.
D
In the pre-lecture question, I was asked to choose which free body
diagram matched a box sliding down a frictionless slope. As such,
shouldn't there be an arrow pointing downwards parallel to the slope?
Spring Force
Direction: Parallel to spring: Push or pull
Magnitude:F=-kx
My Physics 100 TA said that one of the greatest traps in the spring force physics
is that many students use -k even when they already pointed the Spring force in
the direction of restoration…does this mean that when you point the Fspring
vector in the direction of restoration, the negative sign is already accounted for
and you can thus just use a positive k in your calculations?
Springs Example
“can we go over springs more in depth in lecture?”
A 5 kg mass is suspended from a
spring with spring constant 25 N/m.
How far is the spring stretched from
its equilibrium position?
F
 ma
kDx  Mg  0
Mg
5*9.8
Dx 

k
25
A) 0.5 m
B) 2 m
C) 5 m
Checkpoint
A box of mass m is hung by a spring
from the ceiling of an elevator.
When the elevator is at rest the
length of the spring is L = 1 m. If the
elevator accelerates upward the
length of the spring will be:
A) L = 1 m
B) L < 1 m
C) L > 1 m
The box is accelerating upward,
so the spring force must be
greater than the gravity force.
Thus, L is larger than normal
length.
a
L
m
Accelerating reference frames can confuse…
there will be more force pulling down on the
box causing it to stretch the spring
As in the previous problem, the acceleration
will induce a downwards force on the box.
This force will stretch the spring to a length
greater than what it was originally.
a
L
m
The same principle with the string, because
the there will be a 'reaction force'
downwards, the spring will stretch to be
longer than its original length.
Remember: Net force causes acceleration
Clicker Question
You are traveling on an elevator
up the Sears tower. As you near
the top floor and are slowing
down, your acceleration
A) is upward
B) is downward
C) is zero
V
a
mg
N
Clicker Question
You are traveling on an elevator
up the Sears tower, and you are
standing on a bathroom scale
that measures N.
As you near the top floor and
are slowing down, the Normal
force is.
a
mg
A) More than your usual weight
B) Less than your usual weight
C) Your usual weight
N
Clicker Question
A cart with mass m2 is connected to a mass m1 using a
string that passes over a frictionless pulley, as shown
below. The cart is held motionless.
m2
The tension in the string is
A) m1g
B) m2g
C) 0
m1
Clicker Question
A cart with mass m2 is connected to a mass m1 using a string
that passes over a frictionless pulley, as shown below.
Initially, the cart is held motionless, but is then released and
starts to accelerate.
a
m2
After the cart is released, the tension in the string is
A) = m1g
B) > m1g
C) < m1g
a
m1
Universal Gravitation
Near surface of earth r = Rearth
Mearth m
F G 2
Rearth
 6.67  1011 m3 / kg s2 
5.97  1024  kg 
m / s2  m

9.8
m
2
6.4  10 m
6
r

Mm
FGravity  G 2 rˆ
r
Careful - r is measured from the center of Mars!
r


Fnet  ma
Mm
v2
G 2 m
r
r
v  GM / r
USE ALGEBRA!
r
d
t
v
2r
t
GM / r
3
r
t  2
GM
Careful, check units. Asks for time in
hours, not seconds!
r
v  GM / r