mechanics lecture 4, slide 1 classical mechanics lecture 4 homework 3 and midterm exam 1...

Mechanics Lecture 4, Slide 1

Classical Mechanics Lecture 4

Homework 3 and Midterm Exam 1Today's Concepts: Newton’s Laws

a) Acceleration is caused by forcesb) Force changes momentumc) Forces always come in pairsd) Good reference frames

Homework 3


Average=84.5%

Midterm exam 1 Distribution


Average=9.2

Take-home exam due Friday Feb 20

Only need to do problems that were missed!

Will receive ¾ credit for corrected problems%100*

1543

....

HTCI NNScore

Midterm exam 1 Problem Difficulty


p01 p02 p03 p04 p05 p06 p07 p08 p09 p10 p11 p12 p13 p14 p15 p16 p17 p18 p19 p20

26 26 30 23 23 7 16 15 10 19 13 17 17 11 24 18 11 10 12 31

Midterm “Take-home” due on Friday Feb 20


Meet with Instructor or TA if desired Prepare notes/Review before taking exam Practice solving homework/sample exam

problemsTake exam in timed situation Answer questions that were incorrect:

Letter response in spaces provided. Written response for each answer.

Brief justification for answer to conceptual questions (1,2,3,7,9,15,20)

Show all steps in responses requiring calculations. State input data and relevant equations.

Simulate Real Exam Situation

Course Schedule: February


Course Schedule: March


Discussion Sections?


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Sir Isaac Newton and his 3 Laws


Main Points


Lecture Thoughts



Let’s start with Newton’s 2nd Law


Acceleration is caused by force.

A bigger mass makes this harder

Newton’s 2nd Law


Force is a vector. Acceleration vector is parallel to (same direction) as (Net) force vector.

Force is a Vector Quantity


Acceleration is caused by force.

A bigger mass makes this harder

Newton’s 2nd Law

Pre-lecture Question 1 A. B. C. D. E.


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Pre-lecture Question 1


Unit of Force


Kinematics units

Dynamics unit

Momentum


Spaceship example


Newton’s First law


Inertial Reference Frame


Newton’s 3rd Law


Newton’s 3rd Law


The motion of an object is affected

by only the forces acting

upon it.

Prelecture Question 2

A. B. C. D. E.


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Prelecture Question 2


Checkpoint 1A. B. C. D.


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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 axisB) Its acceleration parallel to the x axisC) Both its velocity and its acceleration are parallel to the x axisD) Neither its velocity or its acceleration need be parallel to the x

axis


B) Net force causes acceleration, but it does not necessarily say anything about the direction of the velocity.

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 axisB) Its acceleration parallel to the x axisC) Both its velocity and its acceleration are parallel to the x axisD) Neither its velocity or its acceleration need be parallel to the x axis

Force on Two Masses A. B. C.


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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

5M


Force on Two Masses

MFa

Net ForceSame acceleration, so larger

mass has larger net force.

amF

5M

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

CheckpointA. B. C. D.


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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.


CheckPoint


A) Force is in the same direction as acceleration (in this case, centripetal).

CheckPoint Responses

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.


* They can have also have tangential acceleration if their speed is not constant

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.


Oooomf

Momentum

vmp


Momentum

vmp


Momentum & Force

Checkpoint Momentum


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You are driving a car with constant speed around a horizontal circular track. The momentum of your car

A. B. C. D. E.


C) It points in the same direction as v, which is forward

CheckPoint ResponsesYou are driving a car with constant speed around a horizontal circular track. The momentum of your

car


v

a

Momentum & Centripetal Acceleration


amF

vmp

Momentum & Centripetal Acceleration


Ice-puck Fake Forces: Coriolis (YouTube)

Newton’s 1st Law

http://www.youtube.com/watch?v=_36MiCUS1ro


Forces come in pairs!

Fire-cart

Newton’s 3rd Law


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 motionless?

bmF

Is not acting on the box

Clicker Question A. B. C.


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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.

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http://www.iclipart.com/search.php?keys=165143&id=104_10_3_17

Main Points


Midterm “Take-home” due on Friday Feb 20


Prepare notesMeet with Instructor or TA if desiredTake exam in timed situationAnswer questions that were incorrect:

Letter response in spaces provided. Written response for each answer.

Brief justification for answer to conceptual questions (1,2,3,7,9,15,20)

Show all steps in responses requiring calculations. State input data and relevant equations.

Midterm exam 1 Distribution


Average=9.2

Take-home exam due Friday Feb 20

Only need to do problems that were missed!

Will receive ¾ credit for corrected problems

Midterm exam 1 Problem Difficulty


p01 p02 p03 p04 p05 p06 p07 p08 p09 p10 p11 p12 p13 p14 p15 p16 p17 p18 p19 p20

26 26 30 23 23 7 16 15 10 19 13 17 17 11 24 18 11 10 12 31

Exam Problems


26/39 correct

26/39 correct

30/39 correct

23/39 correct

Exam Problems


23/39 correct

Exam Problems


6/39 correct

Exam Problems


16/39 correct

Exam Problems


15/39 correct

10/39 correct

Exam Problems


19/39 correct

13/39 correct

Exam Problems


17/39 correct

Exam Problems


17/39 correct

11/39 correct

Exam Problems


24/39 correct

Exam Problems


18/39 correct

Exam Problems


11/39 correct 10/39 correct

Exam Problems


12/39 correct

31/39 correct

How difficult was the exam overall?

A. Most difficult exam that you ever encountered.

B. Moderately DifficultC. Fairly EasyD. Easiest Exam that you ever

encountered.


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How much time should we have had to complete all problems?

A. 50 minutesB. 75 minutesC. 90 minutesD. 120 minutes


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Given sufficient time I could have correctly completed “most” of the

problemsA. TrueB. False


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What percentage of the questions were “fair”?

A. 100%B. >80%C. >60%D. >30%E. 0%


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mechanics lecture 4, slide 1 classical mechanics lecture 4 homework 3 and midterm exam 1...

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