Classical Mechanics Lecture 4 Homework 3 and Midterm Exam 1 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 Mechanics Lecture 4, Slide 1
Homework 3 Average=84.5% Mechanics Lecture 4, Slide 2
Midterm exam 1 Distribution Average=9.2 Take-home exam due Friday Feb 20 Score 3 N I. C. + N = 4 15 T. H. *100% Only need to do problems that were missed! Will receive ¾ credit for corrected problems Mechanics Lecture 4, Slide 3
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 Mechanics Lecture 4, Slide 4
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 problems Take 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 Mechanics Lecture 4, Slide 5
Course Schedule: February Mechanics Lecture 4, Slide 6
Course Schedule: March Mechanics Lecture 4, Slide 7
Discussion Sections? http://doodle.com/r5h452cvbd3h8hhb Enter your available times Mechanics Lecture 4, Slide 8
Sir Isaac Newton and his 3 Laws Mechanics Lecture 4, Slide 9
Main Points Mechanics Lecture 4, Slide 10
Main Points Mechanics Lecture 4, Slide 11
Main Points Mechanics Lecture 4, Slide 12
Lecture Thoughts Mechanics Lecture 4, Slide 13
Let s start with Newton s 2 nd Law Mechanics Lecture 4, Slide 14
Newton s 2 nd Law Acceleration is caused by force. A bigger mass makes this harder Mechanics Lecture 4, Slide 15
Force is a Vector Quantity Force is a vector. Acceleration vector is parallel to (same direction) as (Net) force vector. Mechanics Lecture 4, Slide 16
Newton s 2 nd Law Acceleration is caused by force. A bigger mass makes this harder Mechanics Lecture 4, Slide 17
Pre-lecture Question 1 A. B. C. D. E. 0% 0% 0% 0% 0% Mechanics Lecture 4, Slide 18
Pre-lecture Question 1 Mechanics Lecture 4, Slide 19
Unit of Force Kinematics units Dynamics unit Mechanics Lecture 4, Slide 20
Momentum Mechanics Lecture 4, Slide 21
Spaceship example Mechanics Lecture 4, Slide 22
Newton s First law Mechanics Lecture 4, Slide 23
Inertial Reference Frame Mechanics Lecture 4, Slide 24
Newton s 3 rd Law Mechanics Lecture 4, Slide 25
Newton s 3 rd Law The motion of an object is affected by only the forces acting upon it. Mechanics Lecture 4, Slide 26
Prelecture Question 2 A. B. C. D. E. 0% 0% 0% 0% 0% Mechanics Lecture 4, Slide 27
Prelecture Question 2 Mechanics Lecture 4, Slide 28
Checkpoint 1 A. B. C. D. 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 0% 0% 0% 0% D) Neither its velocity or its acceleration need be parallel to the x axis Mechanics Lecture 4, Slide 29
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) Net force causes acceleration, but it does not necessarily say anything about the direction of the velocity. Mechanics Lecture 4, Slide 30
Force on Two Masses A. B. C. 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) F M < F 5M B) F M = F 5M C) F M > F 5M F M 5M a 0% 0% 0% Mechanics Lecture 4, Slide 31
Force on Two Masses 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) F M < F 5M B) F M = F 5M C) F M > F 5M F = Net Force ma Same acceleration, so larger mass has larger net force. F M 5M a Mechanics Lecture 4, Slide 32
Mechanics Lecture 4, Slide 33
Mechanics Lecture 4, Slide 34
Checkpoint A. B. C. You are driving a car with constant speed around a horizontal circular track. The net force acting on your car D. 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. 0% 0% 0% 0% Mechanics Lecture 4, Slide 35
CheckPoint Mechanics Lecture 4, Slide 36
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. A) Force is in the same direction as acceleration (in this case, centripetal). Mechanics Lecture 4, Slide 37
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 Mechanics Lecture 4, Slide 38
Momentum p = mv Oooomf Mechanics Lecture 4, Slide 39
Momentum p = mv Mechanics Lecture 4, Slide 40
Momentum & Force Mechanics Lecture 4, Slide 41
Checkpoint Momentum You are driving a car with constant speed around a horizontal circular track. The momentum of your car A. B. C. D. E. 0% 0% 0% 0% 0% Mechanics Lecture 4, Slide 42
CheckPoint Responses You are driving a car with constant speed around a horizontal circular track. The momentum of your car C) It points in the same direction as v, which is forward Mechanics Lecture 4, Slide 43
Momentum & Centripetal Acceleration v a Mechanics Lecture 4, Slide 44
Momentum & Centripetal Acceleration p = mv F = ma Mechanics Lecture 4, Slide 45
Newton s 1 st Law Ice-puck Fake Forces: Coriolis (YouTube) Mechanics Lecture 4, Slide 46
Newton s 3 rd Law Forces come in pairs! Fire-cart Mechanics Lecture 4, Slide 47
Newton s 3 rd 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? F bm Is not acting on the box Mechanics Lecture 4, Slide 48
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. B. C. 0% 0% 0% A) The small guy. B) The football player. C) They experience the same force. Mechanics Lecture 4, Slide 49
Mechanics Lecture 4, Slide 50
Main Points Mechanics Lecture 4, Slide 51
Main Points Mechanics Lecture 4, Slide 52
Main Points Mechanics Lecture 4, Slide 53
Midterm Take-home due on Friday Feb 20 Prepare notes Meet with Instructor or TA if desired Take 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. Mechanics Lecture 4, Slide 54
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 Mechanics Lecture 4, Slide 55
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 Mechanics Lecture 4, Slide 56
Exam Problems 26/39 correct 26/39 correct 23/39 correct 30/39 correct Mechanics Lecture 4, Slide 57
Exam Problems 23/39 correct Mechanics Lecture 4, Slide 58
Exam Problems 6/39 correct Mechanics Lecture 4, Slide 59
Exam Problems 16/39 correct Mechanics Lecture 4, Slide 60
Exam Problems 15/39 correct 10/39 correct Mechanics Lecture 4, Slide 61
Exam Problems 19/39 correct 13/39 correct Mechanics Lecture 4, Slide 62
Exam Problems 17/39 correct Mechanics Lecture 4, Slide 63
Exam Problems 17/39 correct 11/39 correct Mechanics Lecture 4, Slide 64
Exam Problems 24/39 correct Mechanics Lecture 4, Slide 65
Exam Problems 18/39 correct Mechanics Lecture 4, Slide 66
Exam Problems 11/39 correct 10/39 correct Mechanics Lecture 4, Slide 67
Exam Problems 12/39 correct 31/39 correct Mechanics Lecture 4, Slide 68
How difficult was the exam overall? A. Most difficult exam that you ever encountered. B. Moderately Difficult C. Fairly Easy D. Easiest Exam that you ever encountered. 0% 0% 0% 0% Mechanics Lecture 4, Slide 69
How much time should we have had to complete all problems? A. 50 minutes B. 75 minutes C. 90 minutes D. 120 minutes 0% 0% 0% 0% Mechanics Lecture 4, Slide 70
Given sufficient time I could have correctly completed most of the problems A. True B. False 0% 0% Mechanics Lecture 4, Slide 71
What percentage of the questions were fair? A. 100% B. >80% C. >60% D. >30% E. 0% 0% 0% 0% 0% 0% Mechanics Lecture 4, Slide 72