Stuff you asked about:
Instrumental illness
Can you go over the change in momentum with respect to the change in time being used to
calcuate the net force in depth more during the lecture. We never really learned that in my
phyics class in high school, and if we did, we never used it all.
I would like to hear more about omega. it still confuses me.
I LOVE PHYSICS!
I really like the setup of phys 211, with the prelectures, checkpoints, and online homework to
guide us
Everything should be discussed in lecture, all the time.
The guy reciting the prelectures is fine; whoever complained earlier is just looking for something
to be upset about. I could use some help relating momentum and Newton's Second law.
if you are on a Ferris wheel, do you have a reference frame? Isn't the velocity changing so it
wouldn't have one?
I started taking notes along with the prelecture slideshows and I found that it really helped me
focus and grasp the concepts.
I hope one day I write something witty enough to grace the slide show.
Momentum in circlesss. :,(
I would like to discuss and go into detail about anything we need to know about homework. It
seems like every time I do homework I am unprepared and have to study more on my own. In
that case, what is the point of class?
What the heck is an inertial reference frame?
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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
Clarification
“Sometimes in the Prelecture, there is three horizontal lines for an
equation, yet other times there is only two horizontal lines, like a
normal equal sign. Do these things mean the same thing? If so, why
are there two different things that have the same meaning.”
“equals”
“defined to be”
Newton’s 2nd Law
Acceleration is caused by force.
A bigger mass makes this harder
“I get that force is related to acceleration but I
don't get how velocity relates to force.”
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
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 net force is in the positive x direction, so
the acceleration must also be in that direction. the
velocity can be whatever the heck it wants to be..
C) Acceleration is the change in velocity. Since the
force is causing an acceleration, the velocity and
acceleration must be going in the same direction.
Last Semester
Concerns you raised:
“Question #1 of the prelecture.... what??”
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
A) FM < F5M
B) FM = F5M
C) FM > F5M
Net Force
Same acceleration, so larger
mass has larger net force.
a
F
 ma
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.
“Acceleration is inward as shown in
the circular motion. Hence, the
force is inward.”
Aside: 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
“…it was good to see how to derive momentum from F=ma”
Tablecloth & Dishes
Lots of Momentum questions…
“P=mv P/t=F So the direction of momentum is ?”


p  mv
Oooomf
Checkpoint
You are driving a car with constant speed around a horizontal circular track. The
momentum of 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.
“The momentum is pointed forward
because the momentum is in the same
direction as the velocity.”
Checkpoint Responses
A few people said something like this (which is wrong):
“The momentum is in the same direction as the force.”
The correct statement should be:
“The change in momentum is in the same
direction as the force.”

v

a
“The directions of acceleration/velocity/change in velocity from the equation a = F/m
when a force is applied in a particular direction are confusing.”


p  mv


F  ma
“The directions of acceleration/velocity/change in velocity from the equation a = F/m
when a force is applied in a particular direction are confusing.”
Newton’s 1st Law
Ice-puck
“I find it amusing that Newton's laws state they must be in an inertial reference frame,
and an inertial reference frame is defined as a reference frame that holds true to
Newton's laws. So, essentially Newton's laws only work when Newton's laws work..”
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
“Explain more about third law and when to ignore certain forces.”
“Newton's third law confuses me on how things are still able to
move even if objects are pusing back the same force”
We will discuss friction next week
Could you go over how force and acceleration differ
upon impact, such as with the bug hitting the
windshield, I don't quite get how it works.
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.
Newton's third law is a little confusing. Why are the effects different than the forces?
Clicker Question
A small guy moving at a high speed collides with a
stationary large football player. Now, which person
experiences the larger force during the collision?
A) The small guy experiences the larger force.
B) The football player experiences the larger force.
C) Both experience the same force.
Homework Problem
a centripetal 
v
2
r
Homework Problem
a
ay
ax
a x   a cos( )
All of the other parts are similar..