Relative Motion New content!
Task: Draw the speed vs time graphs for the six toy cars
2-D Kinematics Relative Motion Projectile Motion Angled Projectiles
Announcements Meet in the lab tomorrow (bring your flash drives) Homework: Read the Lab Report rubric posted on the class website and look over the two sample labs that I posted Sign up for Quest if you haven t already (7 people, first homework will be posted soon) [instructions on website] Bring a laptop, if you have one
Relative Motion
If while you were sleeping on a train, a team of physicist rushed into your train and covered up all of the windows, how would you be able to tell if the train was moving or not (assume it isn t making any stops)?
You can t! For inertial reference frames (non-accelerating reference frames), there is absolutely no difference between being at rest and moving at a constant velocity.
Relative Motion video (3:16) Frames of Reference: https://www.youtube.com/watch? v=imm9cg6vk1w
Frame of Reference Two requirements: 1. Origin (Starting point), 2. Set of directions (East-West, etc.) Who is measuring what? In order to understand a Frame of reference, ask yourself: What would you see?
What do you think our most common reference frame is?
Example 1: Completely difference experiences Tennis ball thrown up in the air, on a moving platform
Example 2 A stationary observer watches two cars drive by. Car A is traveling at 50 m/s. Car B is traveling at 30 m/s. Q1. What velocity would car B say that car A is traveling at? Q2. What velocity would car A say that car B is traveling at? Q3. Who is correct and how can we know?
Example 2 (adding vectors) A stationary observer watches two cars drive by. Car A is traveling at 50 m/s. Car B is traveling at 30 m/s. Q1. What velocity would car B say that car A is traveling at? 20 m/s Q2. What velocity would car A say that car B is traveling at? - 20 m/s Q3. Who is correct and how can we know? Both are correct
Example 3 A stationary observer now watches three cars drive by in the same direction. Car A is traveling at 50 m/s. Car B is traveling at 30 m/s. Car C is traveling at 10 m/s. Q1. What velocity would car A say that car C is traveling at? Q2. What velocity would car A say that car B thinks car C is traveling at? Q3. What velocity would car B say that car C thinks car A is traveling at?
Example 3 (adding vectors) A stationary observer now watches three cars drive by in the same direction. Car A is traveling at 50 m/s. Car B is traveling at 30 m/s. Car C is traveling at 10 m/s. Q1. What velocity would car A say that car C is traveling at? - 40 m/s Q2. What velocity would car A say that car B thinks car C is traveling at? - 20 m/s Q3. What velocity would car B say that car C thinks car A is traveling at? 40 m/s
2-D Relative Motion: Riverboat problems
2-D relative motion A rowboat traveling 2.5 m/s W encounters a current traveling 6 m/s S. a) What is the resultant velocity, relative to the Earth? b) What angle is it traveling at?
2-D relative motion A rowboat traveling 2.5 m/s W encounters a current traveling 6 m/s S. a) What is the resultant velocity, relative to the Earth? 6.5 m/s SW [Pythagorean theorem] b) What angle is it traveling at? 67.4 degrees south of west [use arctan] -> 247 degrees
2-D relative motion A rowboat traveling 2.5 m/s W encounters a current traveling 6 m/s S. c) If the width of the river is 40 meters wide, then how much time does it take the boat to travel shore to shore?
2-D relative motion A rowboat traveling 2.5 m/s W encounters a current traveling 6 m/s S. c) If the width of the river is 40 meters wide, then how much time does it take the boat to travel shore to shore? 16s [only use westward velocity (x component)]
2-D relative motion A rowboat traveling 2.5 m/s W encounters a current traveling 6 m/s S. d) What distance downstream does the boat reach the opposite shore?
2-D relative motion A rowboat traveling 2.5 m/s W encounters a current traveling 6 m/s S. d) What distance downstream does the boat reach the opposite shore? 96m [only downstream velocity matters (y component]