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Stuff you asked about:
Love it
I like turtles
I really need a sheet that has all of the equations that we will need.
It would help me to focus on specific topics.
Can we discuss how the force is used in star wars.
I'm really curious as to how he came up with these laws!
I hope we can at least touch on it in lecture
None because I'm amazing
The concept of why I am doing this at 3:17AM.
The explanation of the Newton's first law was a bit confusing, mostly the part
explaining the inertial reference frame.
The circular race track was a tricky question for me. If we could go over that in
lecture tomorrow, that would be AWESOME. :)
I feel like I understood both checkpoint questions, but I wouldn't mind
reinforcement on both topics.
I beasted through this stuff in high school, but I think it would help to further
explain the concept of centripetal acceleration and the dynamics of centripetal
force and tangential velocity.
Also, why does every physics course always start teaching Newton's laws with the
Second law?
“These videos are a great idea, and I love the SmartPhysics system.
A small side note- in lecture you always say "You guys did really
good" or something along those lines. Not to be a grammar teacher,
but objects can't "do good" they "do well." I'm sorry for pointing
this out but to be honest it is kind of annoying. I say that with
constructive criticism in mind. No hard feelings.”
Clicker Tips
Use frequency AA (old clickers default to this)
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should say PHYS 211 when you
turn it on in this classroom.
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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 do not like the way this pre-lecture tried to change my view of
F=MA to A=F/M... LONG LIVE F=MA!!!!!!!!!”
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) Force and acceleration are always in the
same direction, however the velocity doesn't
have to be in the same direction..
C) Its acceleration is pointing the same way as
the net force, which means the velocity is also
pointing in that direction.
Concerns you raised:
“I was pretty confused about the first question on the
prelecture. Could we go over it in lecture?”
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 radically inward toward the center of the circular track
B) Points radically 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 points inward
radially so the force should.”
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
Tablecloth & Dishes
Students’ Momentum Concerns
“Really, I could benefit from a clarification
of the concept of momentum.”


p  mv
Oooomf
Checkpoint
You are driving a car with constant speed around a horizontal circular track. The
momentum of your car
A) Points radically inward toward the center of the circular track
B) Points radically 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 velocity points forward so the
momentum should point forward.”
Checkpoint Responses
Quite 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
“Can we talk more about momentum and about forces acting on a "car" going in a circle.”


p  mv


F  ma
“Can we talk more about momentum and about forces acting on a "car" going in a circle.”
Newton’s 1st Law
Ice-puck
“Even though the example of the phone in the car was a given example in the
prelecture, I do not understand why accelerating reference frames are not
reference frames in which newton's laws are valid.”
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
Earth’s Rotation important over large distances
And makes hurricanes rotate CCW (CW) in the Northern
(Southern) hemisphere.
Fake Forces:
Coriolis (YouTube)
Newton’s 3rd Law
Fire-cart
Forces come in pairs!
Newton’s 3rd Law Concerns
“The most difficult concept i found was the man pushing on the box and
how the two forces acting on each other make for an acceleration”
“why do the equal and opposite forces "cancel
eachother out" and yet the objects still move?”
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.
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
2
acentripetal
v

r
Homework Problem
a
ay
ax
ax  a cos( )
All of the other parts are similar..