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S18 PH 121 6 intro
Transcript of S18 PH 121 6 intro
Motion in 1-D
(cc) photo by Metro Centric on Flickr
(cc) photo by Franco Folini on Flickr
(cc) photo by jimmyharris on Flickr
(cc) photo by Metro Centric on Flickr
if F = 0
then a = o
and v = ?
gravity & weight
Types of Friction
Drag and Velocity
Sum of forces = Zero
Velocity also = zero
Velocity is constant
Only works on a particle model, just a dot,
we'll deal with rods and massive objects later
1. Find Acceleration
2. Use kinematics to find motion
Components, Components, Components
Lots of Algebra
Free body diagrams
Use acceleration in equations
Hints: Look for knowns (time, distance, velocities)
Repeat same steps
each time, always!!
Weight, Gravity, Mass
Newton's law of gravity
Yet we have said F = mg
Weight w = mg
What about elevators?
- An apparent weight!
Take a closer
SUM it all up!!
If an objects Net Force
is Zero then
A. Its not moving
B. It has constant velocity
C. Either / or Depends
In using Newton's Laws in
problems it is good to use:
B. Vector Components
D. Kinematic equations
E. All the above, in order!!
If I wanted to have an apparent weight double my normal weight
what should my acceleration be?
A. 2 g
(g can change though)
"Could you please go over static vs dynamic equilibrium?"
" Can we go over the figure 6.1 with the man weighing himself in the accelerating elevator?"
"Is it appropriate to say "apparent weight" when talking about objects accelerating? "
" how do we measure mass?"
"Why is the cup "Accelerating" with the astronaut? from the pre-quiz questions?"
A student stands on a scale in an elevator that is accelerating at 2.5 m/s2. If the student has a mass of 78 kg, to the nearest newton what is the scale reading?
Static friction is probably the most complicated.
It is the force of friction if the object is not moving. This is the same idea as static equilibrium.
It can range from zero, up to a maximum value.
It takes the form of:
Where n is the normal force
and mu is the coefficient of
Things to note with this form of friction
It is not equal to that maximum value unless
it is just starting to slip.
Thus when these problems are written they look
"what minimum force is require to start a book sliding"
"What is the maximum angle that a car can park if it is
not to slide..."
This range however is 1-to-1 linear
from zero up to that max value.
Most common, constant, smaller than max static friction
Always opposes motion. Has the value of:
One often sees graphs for this like the following
slope = 1
Closer to Static (breaks bonds vertically)
Here is a little table to compare these coefficients for you.
Rubber on concrete
Steel on Steel (dry)
Steel 0n Steel (lubricated)
Wood on wood
Wood on Snow
Ice on Ice
Causes of Friction
Bonds form between the surfaces, max value is when bonds are broken.
Some bonds, not nearly as much
Rather surfaces 'protrude' into the other surface, and then gets 'scrapped off'
This is why tread wears out.
Similar to static, but the bonds are broken vertically
Similar to kinetic in that not nearly as many bonds form
With this visualization comes lubricants.
Liquids often help make the atoms not touch each other, but filling the space between
the substances, causing them to move by without
In this text they use a simplified model that accounts for
Size (cross sectional, needs to be .001 - 10 m)
Speed must be relatively small
Just moving through atmosphere close to earth
Complicated force. Always opposes motion.
Depends on speed and viscosity. It takes different forms.
C is a constant that describes how Aerodynamic the object is (ranges form 0-1.5)
rho is the the density of the material (air)
A is cross sectional area
v is velocity
As the speed of an object increases, (due to force of gravity) the Drag force increases. Eventually these forces equal and cancel.
This removes the acceleration and that final speed reached is call the terminal velocity.
Fg = D
DRAG and Terminal Velocity
Which is the largest force of friction?
d. Max static
e. Can't be determined
Which is true when acting on an object?
c) Both acceleration and drag depend on mass
d) Neither acceleration nor drag depend on mass
a) Drag depends on mass, but acceleration does not.
b) Acceleration depends on mass, but drag does not