# Lect02 - Physics 211 Lecture 2 Today's Concepts: a) Vectors...

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Unformatted text preview: Physics 211 Lecture 2 Today's Concepts: a) Vectors b) Projectile motion c) Reference frames Reminder: HW, Due-dates, grade cutoffs Physics 211 Lecture 2, Slide 1 Physics Vectors Ay A Ax Think of a vector as an arrow. (An object having both magnitude and direction) (An having The object is the same no matter how we chose to describe it 5 Physics 211 Lecture 2, Slide 2 Physics Vectors A A θ Think of a vector as an arrow. (An object having both magnitude and direction) (An having The object is the same no matter how we chose to describe it 6 Physics 211 Lecture 2, Slide 3 Physics Vector Addition: 7 Physics 211 Lecture 2, Slide 4 Physics ACT ACT A Vectors A and B are shown to the right. Vectors Which of the following best describes A + B A 9 B C D B E Physics 211 Lecture 2, Slide 5 Physics ACT ACT A Vectors A and B are shown to the right. Vectors Which of the following best describes A - B A 11 B C D B E Physics 211 Lecture 2, Slide 6 Physics ACT ACT A Vectors A and B are shown to the right. Vectors Which of the following best describes A +2 B A 13 B C D B E Physics 211 Lecture 2, Slide 7 Physics Vectors in 3D A vector can be defined in 2 or 3 (or even more) dimensions: 14 Physics 211 Lecture 2, Slide 8 Physics Kinematics in 3D 15 Physics 211 Lecture 2, Slide 9 Physics Preflight 1 A physics demo launches one marble horizontally while at the same instant dropping a second marble straight down. Which one hits the ground first? Preflight Results A) The launched marble hits first. B) The dropped marble hits first. C) They both hit at the same time DEMO 19 Physics 211 Lecture 2, Slide 10 Physics Projectile Motion Horizontal 21 Vertical Boring Physics 211 Lecture 2, Slide 11 Physics Train Demo Act A flatbed railroad car is moving along a track at constant velocity. A passenger at the center of the car throws a ball straight up. Neglecting air resistance, where will the ball land? A. Forward of the center of the car correct B. At the center of the car C. Backward of the center of the car x- direction: Ball and car start with same position and velocity, a=0, so always have same position 25 Demo - train Physics 211 Lecture 2, Slide 12 Physics 25 25 Physics 211 Lecture 2, Slide 13 Physics Monkey Troubles You are a vet trying to shoot a tranquilizer dart into a monkey hanging from a branch in a distant tree. You know that the monkey is very nervous, and will let go of the branch and start to fall as soon as your gun goes off. In order to hit the monkey with the dart, where should you point the gun before shooting? A Right at the monkey B Below the monkey C Above the monkey 30 39 Shooting the Monkey... Dart x = v0 t y = -1/2 g t2 31 Monkey x = x0 y = -1/2 g t2 Physics 211 Lecture 2, Slide4015 Physics Shooting the Monkey... Still works even if you shoot upwards! y = v0y t - 1/2 g t2 32 y = y0 - 1/2 g t2 Dart hits the monkey Physics 211 Lecture 2, Slide 41 16 Physics Projectile Motion & Frames of Reference 33 Physics 211 Lecture 2, Slide 17 Physics ACT (Preflight 2) A destroyer simultaneously fires two shells with the same initial speed at same at enemy ships. If the shells follow the parabolic trajectories shown, which ship gets hit first? Destroyer Enemy 1 Enemy 2 The shell fired at Enemy 1 will be in the air longer than the shell fired at A) Enemy 1 enemy 2 because the height is higher for Enemy 1 than Enemy 2 B) Enemy 2 Since the inital speed is the same, the speed C) They are both hit at the same time vertical motion is the same for both shells. 35 Physics 211 Lecture 2, Slide 18 Physics Preflight 2 A destroyer simultaneously fires two shells with the same initial speed at same at enemy ships. If the shells follow the parabolic trajectories shown, which ship gets hit first? Destroyer Enemy 1 Enemy 2 A) Enemy 1 B) Enemy 2 C) They are both hit at the same time 36 Physics 211 Lecture 2, Slide 19 Physics ACT (Preflight 3) A battleship simultaneously fires two shells with different initial speeds at different at enemy ships. If the shells follow the parabolic trajectories with same same maximum height as shown below, which ship gets hit first? maximum as Destroyer Enemy 1 Enemy 2 Since their vertical distance is the same, enemy one should get hit Since A) Enemy 1 first because there is less horizontal distance. B) Enemy 2 The vertical component of both motions C) They are both hit at the same time are the same, so they will be hit at the same time. 38 Physics 211 Lecture 2, Slide 20 Physics Preflight 3 A battleship simultaneously fires two shells with different initial speeds at different at enemy ships. If the shells follow the parabolic trajectories with same same maximum height as shown below, which ship gets hit first? maximum as Destroyer Enemy 1 Enemy 2 A) Enemy 1 B) Enemy 2 C) They are both hit at the same time 40 Physics 211 Lecture 2, Slide 21 Physics Field Goal Example • Our field goal kicker can kick the ball 30 m/s at an angle of 30 degrees w.r.t. the ground. If the crossbar of the goal post is 3m off the ground, from how far away can he kick a field goal? y x 3m D Y-direction x-direction vy0 = v sin(30) = 15 m/s y vx0 = v cos(30) = 26 m/s = y0 + vyt + ½ a t2 D = x0 + vx0 t + ½ a t2 3m = 0m + 15m/s t – ½ (9.8 m/s2) t2 = 0m + (26m/s)(2.8 s) + 0 m/s2 (2.8 s )2 t = 2.8 s or t = 0.22 s. = 72.8 m 50 Illini Kicks 70 yard Field Goal Physics 211 Lecture 2, Slide 22 Physics ...
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## This note was uploaded on 09/22/2011 for the course PHYSICS 211 taught by Professor Selig during the Fall '10 term at University of Illinois, Urbana Champaign.

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