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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
Midterm exam 1 Distribution
Mechanics Lecture 4, Slide 3
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
Mechanics Lecture 4, Slide 4
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
Mechanics Lecture 4, Slide 5
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
Discussion Sections?
Mechanics Lecture 4, Slide 8
http://doodle.com/r5h452cvbd3h8hhb Enter your available times
Mechanics Lecture 4, Slide 15
Acceleration is caused by force.
A bigger mass makes this harder
Newton’s 2nd Law
Mechanics Lecture 4, Slide 16
Force is a vector. Acceleration vector is parallel to (same direction) as (Net) force vector.
Force is a Vector Quantity
Mechanics Lecture 4, Slide 17
Acceleration is caused by force.
A bigger mass makes this harder
Newton’s 2nd Law
Newton’s 3rd Law
Mechanics Lecture 4, Slide 26
The motion of an object is affected
by only the forces acting
upon it.
Checkpoint 1A. B. C. D.
Mechanics Lecture 4, Slide 29
0% 0%0%0%
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
Mechanics Lecture 4, Slide 30
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.
Mechanics Lecture 4, Slide 31
0% 0%0%
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
Mechanics Lecture 4, Slide 32
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.
Mechanics Lecture 4, Slide 35
0% 0%0%0%
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.
Mechanics Lecture 4, Slide 37
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.
Mechanics Lecture 4, Slide 38
* 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.
Checkpoint Momentum
Mechanics Lecture 4, Slide 42
0% 0% 0%0%0%
You are driving a car with constant speed around a horizontal circular track. The momentum of your car
A. B. C. D. E.
Mechanics Lecture 4, Slide 43
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
Mechanics Lecture 4, Slide 46
Ice-puck Fake Forces: Coriolis (YouTube)
Newton’s 1st Law
Mechanics Lecture 4, Slide 48
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.
Mechanics Lecture 4, Slide 49
0% 0%0%
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.
Midterm “Take-home” due on Friday Feb 20
Mechanics Lecture 4, Slide 54
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
Mechanics Lecture 4, Slide 55
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
Mechanics Lecture 4, Slide 56
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
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.
Mechanics Lecture 4, Slide 69
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How much time should we have had to complete all problems?
A. 50 minutesB. 75 minutesC. 90 minutesD. 120 minutes
Mechanics Lecture 4, Slide 70
0% 0%0%0%
Given sufficient time I could have correctly completed “most” of the
problemsA. TrueB. False
Mechanics Lecture 4, Slide 71
0%0%