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Classical Mechanics 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 Mechanics Lecture 4, Slide 1 First Midterm Exam on Friday Covers Units 1-3 plus “Math” One-dimensional Kinematics Two-Dimensional Kinematics Relative and Circular Motion Unit conversion Trigonometry and Algebra 50 minute duration Multiple choice Review on Wednesday Unit 4 will resume on Monday Mechanics Lecture 4, Slide 2 Sir Isaac Newton and his 3 Laws Mechanics Lecture 4, Slide 3 Main Points Mechanics Lecture 4, Slide 4 Main Points Mechanics Lecture 4, Slide 5 Main Points Mechanics Lecture 4, Slide 6 Lecture Thoughts Mechanics Lecture 4, Slide 7 Let’s start with Newton’s 2nd Law Mechanics Lecture 4, Slide 8 Newton’s 2nd Law Acceleration is caused by force. A bigger mass makes this harder Mechanics Lecture 4, Slide 9 Force is a Vector Quantity Force is a vector. Acceleration vector is parallel to (same direction) as (Net) force vector. Mechanics Lecture 4, Slide 10 Newton’s 2nd Law Acceleration is caused by force. A bigger mass makes this harder Mechanics Lecture 4, Slide 11 Pre-lecture Question 1 A. B. C. D. E. 0% 0% 0% 0% 0% Mechanics Lecture 4, Slide 12 Pre-lecture Question 1 Mechanics Lecture 4, Slide 13 Unit of Force Kinematics units Dynamics unit Mechanics Lecture 4, Slide 14 Momentum Mechanics Lecture 4, Slide 15 Spaceship example Mechanics Lecture 4, Slide 16 Newton’s First law Mechanics Lecture 4, Slide 17 Inertial Reference Frame Mechanics Lecture 4, Slide 18 Newton’s 3rd Law Mechanics Lecture 4, Slide 19 Newton’s 3rd Law The motion of an object is affected by only the forces acting upon it. Mechanics Lecture 4, Slide 20 Prelecture Question 2 A. B. C. D. E. 0% 0% 0% 0% 0% Mechanics Lecture 4, Slide 21 Prelecture Question 2 Mechanics Lecture 4, Slide 22 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 D) Neither its velocity or its acceleration need be parallel to the x axis 0% 0% 0% 0% Mechanics Lecture 4, Slide 23 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 24 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) FM < F5M B) FM = F5M C) FM > F5M a F M 5M 0% 0% 0% Mechanics Lecture 4, Slide 25 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. F A) FM < F5M B) FM = F5M C) FM > F5M ma Net Force Same acceleration, so larger mass has larger net force. a F M 5M Mechanics Lecture 4, Slide 26 Mechanics Lecture 4, Slide 27 Mechanics Lecture 4, Slide 28 Checkpoint A. B. C. D. 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. 0% 0% 0% 0% Mechanics Lecture 4, Slide 29 CheckPoint Mechanics Lecture 4, Slide 30 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 32 Momentum & Force Mechanics Lecture 4, Slide 35 Checkpoint Momementum 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 36 Momentum & Centripetal Acceleration v a Mechanics Lecture 4, Slide 38 Momentum & Centripetal Acceleration p mv F ma Mechanics Lecture 4, Slide 39 Newton’s 1st Law Ice-puck Fake Forces: Coriolis (YouTube) Mechanics Lecture 4, Slide 40 This effect can make cannons miss their target if you don’t take it into account. Hurricane Earl And makes hurricanes rotate CCW (CW) in the Northern (Southern) hemisphere. Mechanics Lecture Slide 41 41 Physics 211 Lecture 4,4,Slide Newton’s 3rd Law Forces come in pairs! Fire-cart Mechanics Lecture 4, Slide 42 Clicker Question A. B. C. 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? 33% 33% 33% A) The small guy. B) The football player. C) They experience the same force. Mechanics Lecture 4, Slide 44 Mechanics Lecture 4, Slide 45 Main Points Mechanics Lecture 4, Slide 46 Main Points Mechanics Lecture 4, Slide 47 Main Points Mechanics Lecture 4, Slide 48