Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 1 PHYS 1443 – Section 001 Lecture #3 Wednesday, June 8, 2011 Dr. Jaehoon Yu • One Dimensional Motion: Free Fall • Coordinate System • Vectors and Scalars and their operations • Motion in Two Dimensions: Motion under constant acceleration; Projectile Motion; Maximum ranges and heights • Newton’s Laws of Motion Today’s homework is homework #2, due 10pm, Monday, June 13!!Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 2 Announcements • Homework registration and submissions – 27/28 registered but only 9 completed the submission – The due for homework #1, the freebee, is 10pm tonight! • Quiz #2 coming Tuesday, June 14 – Covers: CH 1.1 – what we finish on Monday, June 13 • Reading assignment – CH3.9Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 3 Reminder: Special Problems for Extra Credit • Derive the quadratic equation for Bx2-Cx+A=0 5 points • Derive the kinematic equation from first principles and the known kinematic equations 10 points • You must show your work in detail to obtain full credit • Due at the start of the class, Thursday, June 9Wednesday, June 8, 2011 4 Special Project for Extra Credit • Show that the trajectory of a projectile motion is a parabola!! – 20 points – Due: next Tuesday, June 14 – You MUST show full details of your OWN computations to obtain any credit • Much beyond what was covered in page 40 of this lecture note!! PHYS 1443-001, Spring 2011 Dr. Jaehoon YuWednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 5 Free Fall • Free fall motion is a motion under the influence of the gravitational pull (gravity) only; Which direction is a freely falling object moving? – A motion under constant acceleration – All kinematic formula we learned can be used to solve for falling motions. • Gravitational acceleration is inversely proportional to the distance between the object and the center of the earth • The magnitude of the gravitational acceleration is g=9.80m/s2 on the surface of the earth, most of the time. • The direction of gravitational acceleration is ALWAYS toward the center of the earth, which we normally call (-y); where up and down direction are indicated as the variable “y” • Thus the correct denotation of gravitational acceleration on the surface of the earth is g=-9.80m/s2 when +y points upward Yes, down to the center of the earth!!Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 6 Example for Using 1D Kinematic Equations on a Falling object (similar to Ex. 2.16) Stone was thrown straight upward at t=0 with +20.0m/s initial velocity on the roof of a 50.0m high building, g=-9.80m/s2 (a) Find the time the stone reaches at the maximum height. What is the acceleration in this motion? What is so special about the maximum height? V=0 (b) Find the maximum height. vyi + ayt =Solve for t yi + vyit +12ayt2=Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 7 Example of a Falling Object cnt’d vyf = yf =Position vyf =Velocity (c) Find the time the stone reaches back to its original height. (d) Find the velocity of the stone when it reaches its original height. (e) Find the velocity and position of the stone at t=5.00s. vyi + ayt = vyi + ayt = yi + vyit +12ayt2Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 8 2D Coordinate Systems • They make it easy and consistent to express locations or positions • Two commonly used systems, depending on convenience, are – Cartesian (Rectangular) Coordinate System • Coordinates are expressed in (x,y) – Polar Coordinate System • Coordinates are expressed in distance from the origin ® and the angle measured from the x-axis, θ (r,θ) • Vectors become a lot easier to express and compute O (0,0) (x1,y1) r1 θ1"How are Cartesian and Polar coordinates related? y1 x1 +x +y tanθ1= x12+ y12( ) r1 r1= (r1,θ1) θ1= tan−1y1x1⎛⎝⎜⎞⎠⎟ cosθ1 sinθ1Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 9 Example Cartesian Coordinate of a point in the xy plane are (x,y)= (-3.50,-2.50)m. Find the equivalent polar coordinates of this point. y x (-3.50,-2.50)m r θ"θs θ= 180 +θs tanθs =∴θ= = −3.50( )2+ −2.50( )2( ) = 18.5 = 4.30(m) x2+ y2( ) −2.50−3.50=57 tan−157⎛⎝⎜⎞⎠⎟= 35.5 180 +θs= 180+ 35.5= 216Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 10 Vector and Scalar Vector quantities have both magnitudes (sizes) and directions Scalar quantities have magnitudes only Can be completely specified with a value and its unit Force, gravitational acceleration, momentum FTheir sizes or magnitudes are denoted with normal letters, F, or absolute values: F or FEnergy, heat, mass, time Normally denoted in normal letters, E Both have units!!!Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 11 Properties of Vectors • Two vectors are the same if their and the are the same, no matter where they are on a coordinate system!! x y Which ones are the same vectors? Why aren’t the others? sizes directionsWednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 12 Vector Operations = Since subtraction is the equivalent to adding a negative vector, subtraction is also commutative!!! ORWednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 13 Example for Vector Addition A car travels 20.0km due north followed by 35.0km in a direction 60.0o west of north. Find the magnitude and direction of resultant displacement. N E 60o"θ r 20 Do this using components!! Bcos60o Bsin60o A + B cosθ( )2+ B sinθ( )2Wednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 14 Components and Unit Vectors Coordinate systems are useful in expressing vectors in their components A= Ax2+ Ay2(Ax,Ay) θ"Ay Ax x y A=}Components (+,+) (-,+) (-,-) (+,-) } Magnitude Acosθ( )2+ Asinθ( )2 = A2cos2θ+ sin2θ( ) = A Acosθ AsinθWednesday, June 8, 2011 PHYS 1443-001, Spring 2011 Dr. Jaehoon Yu 15 Unit Vectors • Unit vectors are the ones that tells us the directions of the components • Dimensionless •
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