Post on 11-Jun-2022
Mechanics Lecture 3, Slide 1
Classical Mechanics Lecture 3
Today's Concepts: Newton’s Lawsa) Accelera6on is caused by forcesb) Force changes momentumc) Forces always come in pairsd) Good reference frames
Midterm
D100: 11:30—13:20 (A&B) SUR2975, (C—F): SUR 5360
D200: 15:30—17:20 SUR 3090
10 MulGple Choice, 4 or 5 WriMen ! (2 short,3 long or 4 long)
You may use your acGvity guides.
No other notes or books.
Bring pen, pencil, scienGfic calculator, ruler! Problems wriMen in pencil cannot be regraded
Your comments
Mechanics Lecture 3, Slide 3
AcceleraGon is caused by force.
A bigger mass makes this harder
Newton’s 2nd Law
Mechanics Lecture 3, Slide 4
CheckPoint
The net force on a box is in the posiGve x direcGon. Which of the following statements best describes the moGon of the box :
A) Its velocity is parallel to the x axisB) Its acceleraGon parallel to the x axisC) Both its velocity and its acceleraGon are parallel to the x axisD) Neither its velocity or its acceleraGon need be parallel to the x axis
Mechanics Lecture 3, Slide 5
B) F = ma so force in the x direc6on will cause an accelera6on in that direc6on but the velocity is not stated so it may have a y velocity and/or a nega6ve star6ng x velocity.
C) The direc6on of accelera6on is the same as the direc6on of the force. And the direc6on of velocity is the same as that of accelera6on.
CheckPoint
The net force on a box is in the posi6ve x direc6on. Which of the following statements best describes the mo6on of the box :
A) Its velocity is parallel to the x axisB) Its acceleraGon parallel to the x axisC) Both its velocity and its acceleraGon are parallel to the x axisD) Neither its velocity or its acceleraGon need be parallel to the x axis
Mechanics Lecture 3, Slide 6
Concerns you raised:
! I did not clearly understand the relaGon between force and momentum and I hope we go over it in class.
! when to take into consideraGon two forces acGng on an object or when to only take into account one force.
Mechanics Lecture 3, Slide 3
AcceleraGon is caused by force.
A bigger mass makes this harder
Newton’s 2nd Law
Net Force
Mechanics Lecture 3, Slide 6
more Concerns you raised:
! The bug ge]ng smashed by the car concept. ! Later we will smash carts together and measure the forces.
! Can we go over the example given in the pre-‐lecture, where they express that according to Newton's third law, only the force that the man is exerGng on the box is taken into account and not the force back on the man?
! Try a demo with two ppl on chairs pushing off each other.
! it was kind of lame to have circular moGon show up when we haven't covered it yet.
! OK, there are a few dislocaGons between SmartPhysics and Workshop Physics but we do circular moGon soon.
Mechanics Lecture 3, Slide 7
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.
A) FM < F5MB) FM = F5MC) FM > F5M
MFa
Net ForceSame acceleraGon, so larger mass has larger net force.
5M
Mechanics Lecture 3, Slide 10
* They can have also have tangenGal acceleraGon if their speed is not constant
Aside: Centripetal acceleration and force
1) Objects moving in a circle always have a component of acceleraGon, called centripetal, which is toward the center of the circle.*
2) Centripetal acceleraGon must be caused by a force:! FricGon, gravity – whatever force keeps it moving in a circle.! This force is ogen called the “centripetal force”
3) There is no “new” kind of force here.
4) There is no such thing as centrifugal force.
Midterm
D100: 11:30—13:20 (A&B) SUR2975, (C—F): SUR 5360
D200: 15:30—17:20 SUR 3090
Coverage: Up through last Friday.
10 MulGple Choice, 4 or 5 WriMen ! (2 short,3 long or 4 long)
You may use your acGvity guides.
No other notes or books.
Bring pen, pencil, scienGfic calculator, ruler! Problems wriMen in pencil cannot be regraded
Mechanics Lecture 3, Slide 11Tablecloth & Dishes
Momentum & Force
Mechanics Lecture 3, Slide 12
Oooomf
Students’ Momentum Concerns
! Need more feedback on the topic of momentum! The concept of momentum was very confusing to me. ! Concept of momentum is bit confusing..
Mechanics Lecture 3, Slide 13
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 direcGon your car is moving
D) Points backward, opposite to the direcGon your car is moving
E) Is zero.
Mechanics Lecture 3, Slide 14
A) The momentum is in the same direcGon as the force.
C) It points in the same direcGon as v, which is forward
CheckPoint ResponsesYou 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 direcGon your car is movingD) Points backward, opposite to the direcGon your car is moving
E) Is zero.
Mechanics Lecture 3, Slide 15
Mechanics Lecture 3, Slide 16
Mechanics Lecture 3, Slide 17
Ice-‐puck Fake Forces: Coriolis (YouTube)
Newton’s 1st Law
Mechanics Lecture 3, Slide 18Physics 211 Lecture 3, Slide 18
This effect can make cannons miss their targetif you don’t takeit into account.
Hurricane Earl
And makes hurricanes rotate CCW in the Northern hemisphere —CW in Southern.
Mechanics Lecture 3, Slide 19
Forces come in pairs!
Fire-‐cart
Newton’s 3rd Law
Mechanics Lecture 3, Slide 20
Newton’s 3rd Law Concerns
I would ask you to briefly explain why when the person is pushing the box does his force on the box not per se cancel with the force the box exerts on him and the box remain moGonless?
Fbm and –Fmb are on different objects; they don’t add.
Mass Comparisons
How does one weight oneself in a space station when “weightless.”