Exam I Physics 101 Lecture 07 More Constant

Exam I Physics 101: Lecture 07 More Constant Acceleration and Relative Velocity -Check gradebook to be sure your iclicker scores are entered correctly. -James scholars need to fill out HCLA and get me a topic by Fri. Feb 19. -Hour Exam 1 is Mon Mar 1, review session is 8 pm+ on Sun Feb 28. Physics 101: Lecture 27, Pg 1

Some Charts Physics 101: Lecture 27, Pg 2

Top Ten Comments… Never any seats I still don't like physics, so I will never enjoy this class, sorry. Go over what the variables represent in lecture. slow down / more time for notes Do not show so much algebra / explain algebra better Demos are great. /too many demos/hard to see demos/animal cruelty uncool …how will discussion sections/quizzes be graded… practice problems… Physics 101: Lecture 27, Pg 3

Last Time l. X and Y directions are Independent! èPosition, velocity and acceleration are vectors l SF = m a applies in both x and y direction l Projectile Motion èax = 0 in horizontal direction èay = g in vertical direction Physics 101: Lecture 27, Pg 4

Today l More 2 -D Examples l Newton’s 3 rd l Relative Law Review Motion Physics 101: Lecture 27, Pg 5

Pulley, Incline and 2 blocks A block of mass m 1 = 2. 6 kg rests upon a frictionless incline as shown and is connected to mass m 1 via a flexible cord over an ideal pulley. What is the acceleration of block m 1 if m 2 = 2. 0 kg? X – direction SFx = m ax: Combiney x y Block 1: T – m g = m N 2 2 a 2 y T T – m 1 g sin(30) = m 1 a 1 x m 1 g sin(30) + m 1 T a 1 x – m 2 g = m 2 a 2 y T = m 1 g sin(30) + m 1 a 1 x – m 2 g = -m 2 a 1 x Y – direction Fy = m ay: Block 2: T – m 2 g = m 2 a 2 y Note: a 1 x = - a 2 y m 1 a 1 x +m 2 a 1 x= m 2 g - m 1 g sin(30) (m 1+m 2) a 1 x= g (m 2 - m 1 sin(30)) mg m 1 g 1. 49 m/s 2 2 Physics 101: Lecture 27, Pg 6 x

Newton’s Third Law è For every action, there is an equal and opposite reaction. Ffinger box • Finger pushes on box • Ffinger box = force exerted on box by finger Fbox finger • Box pushes on finger • Fbox finger = force exerted on finger by box • Third Law: Fbox finger = - Ffinger box Physics 101: Lecture 27, Pg 7

Newton’s rd 3 Law Suppose you are an astronaut in outer space giving a brief push to a spacecraft whose mass is bigger than your own. 1) Compare the magnitude of the force you exert on the spacecraft, FS, to the magnitude of the force exerted by the spacecraft on you, FA, while you are pushing: 1. FA = FS 2. FA > FS 3. FA < FS correct Third Law! 2) Compare the magnitudes of the acceleration you experience, a. A, to the magnitude of the acceleration of the spacecraft, a. S, while you are pushing: 1. a. A = a. S 2. a. A > a. S 3. a. A < a. S correct a=F/m F same lower mass give larger a Physics 101: Lecture 27, Pg 8

y Newton’s 3 rd Example x A rope attached to box 1 is accelerating it to the right at a rate of 3 m/s 2. Friction keeps block 2 on top of block 1 w/o slipping. What is the tension in the rope? M 2 X-direction: F = ma M 1 Block 2: f 21 = m 2 a 2 N 1 Block 1: T – f 12 = m 1 a 1 N 3 L says |f 12| = |f 21| Combine: T - m 2 a 2 = m 1 a 1 T = m 1 a 1+ m 2 a 2 = (m 1+m 2) a T f 21 NT f 12 T W 2 W 1 N 2 • Same as if had one block M = m 1+m Physics 101: Lecture 27, Pg 9 2 !!!!

Relative Velocity You are on a train traveling 40 mph North. If you walk 5 mph toward the front of the train, what is your speed relative to the ground? A) 45 mph B) 40 mph C) 35 mph 40 mph N + 5 mph N = 45 mph N 40 45 5 Physics 101: Lecture 27, Pg 10

Relative Velocity You are on a train traveling 40 mph North. If you walk 5 mph toward the rear of the train, what is your speed relative to the ground? A) 45 mph B) 40 mph C) 35 mph 40 mph N - 5 mph N = 35 mph N 40 35 5 Physics 101: Lecture 27, Pg 11

Relative Velocity You are on a train traveling 40 mph North. If you walk 5 mph sideways across the car, what is your speed relative to the ground? A) < 40 mph B) 40 mph C) >40 mph N + 5 mph W = 41 mph N 5 40 Physics 101: Lecture 27, Pg 12

Relative Velocity l Sometimes your velocity is known relative to a reference frame that is moving relative to the earth. èExample 1: A person moving relative to a train, which is moving relative to the ground. èExample 2: a plane moving relative to air, which is then moving relative to the ground. l These velocities are related by vector addition: » vac is the velocity of the object relative to the ground » vab is the velocity of the object relative to a moving reference frame » vbc is the velocity of the moving reference frame relative to the ground Physics 101: Lecture 27, Pg 13

Tractor Demo 1 Which direction should I point the tractor to get it across the table fastest? A) 30 degrees left B) Straight across C) 30 degrees right 1 2 3 Physics 101: Lecture 27, Pg 14

Tractor Demo (moving table) l Which direction should I point the tractor to get it across the table fastest? A) 30 degrees left B) Straight across C) 30 degrees right 1 2 3 Physics 101: Lecture 27, Pg 15

y Preflight 7. 1 x Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth (B) swims perpendicular to the flow, Ann (A) swims upstream, and Carly (C) swims downstream. Which swimmer wins the race? A) Ann B) Beth C) Carly correct t = d / vy Ann vy = v cos(q) Beth vy = v Carly vy = v cos(q) A B C Physics 101: Lecture 27, Pg 16

y ACT Three swimmers can swim equally fast relative to the water. They have a race to see who can swim across a river in the least time. Relative to the water, Beth (B) swims perpendicular to the flow, Ann (A) swims upstream, and Carly (C) swims downstream. Who gets across second Ann or Carly? A) Ann B) Same C) Carly t = d / vy Ann vy = v cos(q) Beth vy = v Carly vy = v cos(q) x A B C q Physics 101: Lecture 27, Pg 17

y Swimmer Example x What angle should Ann take to get directly to the other side if she can swim 5 mph relative to the water, and the river is flowing at 3 mph? VAnn, ground = Vann, water+Vwater, ground q x-direction 0 = Vx, Ann, Water + 3 0 = -VAnn, Water sin(q) + 3 A B C 5 sin(q) = 3/5 Physics 101: Lecture 27, Pg 18

Think of a swimming pool on a cruise ship When swimming to the other side of the pool, you don’t worry about the motion of the ship ! y A B C x HW: swimmer Demo - bulldozer Physics 101: Lecture 27, Pg 19

Summary of Concepts l. X and Y directions are Independent! èPosition, velocity and acceleration are vectors l. F = m a applies in both x and y direction l Newton’s 3 rd l Relative Law Motion (Add vector components) Physics 101: Lecture 27, Pg 20