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Stuff you asked about:
I believe you said in the first lecture that there was no such thing as centripetal
force. In previous physics courses I have learned that centrifugal force does not
exist, but I thought that centripetal force did?
Can you explain inertial references more as in Newton's first law?
A girl in my dorm put a note on the bathroom door asking us not to slam the door
(which it does naturally on its own) after midnight… So I wrote on the note: "Talk
to Newton woman; for every action there is an equal and opposite reaction.
Sorry." She should take a physics class.
the examples in lecture were nothing like the discussion questions - disc. q's
were harder
Where in the world are all these closed off perfectly circular tracks that people in
these problems are always on?
Rest in piece bug, your sacrifice for my understanding of newton's 3rd law will
not be in vain.
Is there any way for the lab and discussion to be a little bit more fun and more
challenging?
Can we base our questions in some fictional place, I'm having a hard time
neglecting the Earth's Rotational Force on the example problems.
This could be for you…
Physics 211
Lecture 4
Today's Concepts: Newton’s Laws
a) Acceleration is caused by forces
b) Force changes momentum
c) Forces always come in pairs
d) Good reference frames
Newton’s 2nd Law
Acceleration is caused by force.
A bigger mass makes this harder
Checkpoint
The net force on a box is in the positive x direction. Which of
the following statements best describes the motion of the box :
A) Its velocity is parallel to the x axis
B) Its acceleration parallel to the x axis
C) Both its velocity and its acceleration are parallel to the x axis
D) Neither its velocity or its acceleration need be parallel to the x axis
B) The direction of the velocity depends on the
original velocity of the box, but the acceleration is
always the same direction as the force..
C) Velocity is always in the same direction as force,
and a force cannot exist if there is no acceleration.
Therefore, C is correct.
Concerns you raised:
“I am really bad at understanding the questions ask during the prelecture. Can
we go over some examples of momentum in class? I get the basic idea of it, but
I think it would help me if I could see it applied..”
Dv
F = ma
a =Dv/Dt
Dv same direction as F
D v = v 1 – v0
v0
v1
Clicker Question
A force F is applied to a small block, that pushes a larger
block. The two blocks accelerate to the right. Compare the
NET FORCE on the block with mass M, to the net force on the

block with mass 5M.

 F  ma
A) FM < F5M
B) FM = F5M
C) FM > F5M
Net Force
Same acceleration, so larger
mass has larger net force.
a
F
M
5M
Checkpoint
You are driving a car with constant speed around a horizontal circular track. The
net force acting on your car
A) Points radially inward toward the center of the circular track
B) Points radially outward, away from the center of the circular track
C) Points forward in the same direction your car is moving
D) Points backward, opposite to the direction your car is moving
E) Is zero.
“Why is Centrifugal force a fake force, yet when I am turning my car I feel
it instead of Centripetal force?”
“For the circular race track problem, what is causing the centripetal force to
go towards the circle and what is making the car feel the opposite force? If
we drew a free body diagram, would the car's force be facing towards the
circle or outside? Intuition tells me that the force would go to the outside,
but centripetal force is telling me it would go inside. Please explain!.”
When you are driving a car and turning to the left,
in which direction is the road pushing the car?
A) Left
B) Right
When you are driving a car and turning to the left,
in which direction is the car pushing you ?
A) Left
B) Right
Centripetal acceleration and force
1) Objects moving in a circle always have a component
of acceleration, called centripetal, which is toward
the center of the circle.*
2) Centripetal acceleration must be caused by a force:


Friction, gravity – whatever force keeps it moving in a circle.
This force is often called the “centripetal force”
3) There is no “new” kind of force here.
4) There is no such thing as centrifugal force.
* They can have also have tangential acceleration if their speed is not constant
Momentum & Force
“May we derive the connections between momentum and force and
acceleration? Math is the smoking gun to the run away criminal that is
physics...”
 F  ma
If m is constant
(Not really true
for spaceships)
dv
m
dt
d (mv )
dp


dt
dt
dp
So:  F 
dt
Which also means
Tablecloth & Dishes
dp 
  F  dt
if
p  mv
Lots of Momentum questions…


p  mv
Oooomf
More on Directions…

v

a
More on Directions…


p  mv


F  ma
dp
 F  dt
The direction of the Net Force is the direction of the change in momentum.
Newton’s 1st Law
Ice-puck
Earth’s Rotation important over large distances
It is why hurricanes rotate CCW in the Northern hemisphere.
Fake Forces:
Coriolis (YouTube)
Newton’s 1st Law
By the way, now we can understand why the dice hang sideways
(a look ahead to the next lecture)
String
acceleration
Weight
Newton’s 3rd Law
Fire-cart
Forces come in pairs!
Newton’s 3rd Law Concerns
“You should have made it clearer that the man pushing the box is
also being pushed by the box. The only reason he doesn't go
backwards is because the friction force with his feet.”
We will discuss friction next week
Clicker Question
A small guy and a large football player moving at the same
speed collide head-on. Which person experiences the
larger force during the collision?
A) The small guy.
B) The football player.
C) They experience the same force.
Clicker Question
A small guy and a large football player moving at the same
speed collide head-on. Which person experiences the
larger acceleration during the collision?
A) The small guy.
B) The football player.
C) The accelerations are the same.
…How do you determine, through physics equations, when an object is damaged?)