Exams and Grades Exams There will be two
Exams and Grades • Exams: – There will be two midterm exams and one final exam. All exams are closed-book. The final exam will be cumulative. – Exam 1: Feb 16, 2011, 6: 30 – 7: 30 PM – Exam 2: Mar 29, 2011, 6: 30 – 7: 30 PM – Final: TBA (2 hours) • Grades: – The final grade will be determined on the following basis: • 300 points final exam • 300 points two evening exams (150 points each) • 150 points laboratory • 100 points CHIP homework • 50 points recitation exercises • 100 points lecture exercises – No excused i. Clicker exercises. – You must pass the lab in order to pass the course. Lecture 1 Purdue University, Physics 220 1
PHYSICS 220 Lecture 02 Motion, Forces, and Newton’s Laws Textbook Sections 2. 2 -2. 4 • Mechanics is the area of physics which is concerned with the motion of objects. • Two questions need to be answered to understand mechanics – What causes motion? – Given a particular situation, how will an object move? Lecture 2 Purdue University, Physics 220 2
Definition of Motion • Concepts needed – Position – Velocity – Acceleration • http: //connect. garmin. com/activity/53002661
Question x x t x t t • Which plot represents an object at rest? • Which plot represents an object with a uniform velocity in the -x direction? Lecture 2 Purdue University, Physics 220 4
Exercise Find the average velocity for the object during the period t=5 s and t=6 s Lecture 2 Purdue University, Physics 220 5
Question If the average velocity of a car during a trip along a straight road is positive, is it possible for the instantaneous velocity at some time during the trip to be negative? A - Yes correct The car might have reversed for a little while along the trip creating a negative instantaneous velocity at the point. If the overall displacement of the car is positive for that particular time interval, than the average velocity is positive as well. Lecture 2 B - No If the car is traveling in a straight path the velocity will always be positive. The car needs to travel in the opposite direction to get a negative velocity. Purdue University, Physics 220 7
From motion diagram to instantaneous velocity / time diagram Lecture 2 Purdue University, Physics 220 8
Dropped Ball A ball is dropped from a height of two meters above the ground. • Draw vy vs t 3 v A -2 3 4 t 3 B v 3 4 -2 4 v C t 3 D -2 Lecture 2 v v -2 t E t -2 Purdue University, Physics 220 4 4 t 9
Acceleration (shows how velocity changes with time) • The average acceleration is the change in velocity divided by the change in time. v(t) v t • Instantaneous acceleration is limit of average acceleration as t gets small. It is the slope of the v(t) plot. v(t) t t Lecture 2 Purdue University, Physics 220 10
Acceleration of a Sprinter Lecture 2 Purdue University, Physics 220 11
i. Clicker Is it possible for an object to have a positive velocity at the same time as it has a negative acceleration? A - Yes “Yes, the object could be moving B - No forward but decelerating or slowing down. ” If the velocity of some object is not zero, can its acceleration ever be zero? “An object can have a constant A - Yes velocity, which means that the B - No acceleration is zero. ” Lecture 2 Purdue University, Physics 220 13
Acceleration vs Time Plots • Gives acceleration at any time • Area gives change in velocity a (m/s 2) 3 Acceleration at t=4, a(4) = -2 6 m/s 2 24 Change in v between t=4 and t=1. v = +4 m/s t=1 -3: v = (3 m/s 2)(2 s) = 6 m/s t (s) -3 t=3 -4: v = (-2 m/s 2)(1 s) = -2 m/s Lecture 2 Purdue University, Physics 220 14
Newton’s First Law Objects at rest remain at rest and objects in motion remain in motion in a straight line unless acted upon by an external agent INERTIA! - external agents are called Forces - Forces change the state of motion of an object Lecture 2 Purdue University, Physics 220 15
Force • Quantifies the “interaction” between two objects • Fundamental Forces – Gravitational force – Electromagnetic force • Force is a vector – Has magnitude and direction – Be careful when you add two forces! Lecture 2 Purdue University, Physics 220 16
Newton’s Second Law The net force on a body is equal to the product of the mass of the body and the acceleration of the body F = ma 1 N = 1 kg x m/s 2 - This is a vector equation - The direction of the net force is the same as the direction of the acceleration - In 3 dimensions Fx = max Fy = may Fz = maz 2 N Lecture 2 Purdue University, Physics 220 17
Newton’s Third Law For every action there is an equal an opposite reaction N Forces in nature come in pairs mg Ff Lecture 2 F F Ff The object accelerates IF F > Ff Purdue University, Physics 220 18
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