Physics 101: Lecture 7, Pg 1 More Constant Acceleration and Relative Velocity Physics 101: Lecture 07 l Today’s lecture will cover more material from Textbook Chapters 3&4

Physics 101: Lecture 7, Pg 2 Last Time l X and Y directions are Independent! (poor Zip) è Position, velocity and acceleration are vectors  F = m a applies in both x and y direction l Projectile Motion è a x = 0 in horizontal direction è a y = g in vertical direction

Physics 101: Lecture 7, Pg 3 Today l More examples of 2D motion l Newton’s 3 rd Law Review l Relative Motion

Physics 101: Lecture 7, Pg 4

Physics 101: Lecture 7, Pg 5 Pulley, Incline and 2 blocks y x 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? y x

Physics 101: Lecture 7, Pg 6 Newton’s Third Law è For every action, there is an equal and opposite reaction. Finger pushes on box F finger  box = force exerted on box by finger F finger  box F box  finger Box pushes on finger F box  finger = force exerted on finger by box Third Law: F box  finger = - F finger  box

Physics 101: Lecture 7, Pg 7 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 Newton’s 3 rd 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, F S, to the magnitude of the force exerted by the spacecraft on you, F A, while you are pushing: 1. F A = F S 2. F A > F S 3. F A < F S

Physics 101: Lecture 7, Pg 8

Physics 101: Lecture 7, Pg 9 Newton’s 3 rd Example 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? M1M1 M2M2 T x y

Physics 101: Lecture 7, Pg 10 Relative Motion

Physics 101: Lecture 7, Pg 11 Relative Velocity (review) l Sometimes your velocity is known relative to a reference frame that is moving relative to the earth. è Example 1: A person moving relative to a train, which is moving relative to the ground. è Example 2: a plane moving relative to air, which is then moving relative to the ground. l These velocities are related by vector addition: »v ac is the velocity of the object relative to the ground »v ab is the velocity of the object relative to a moving reference frame »v bc is the velocity of the moving reference frame relative to the ground

Physics 101: Lecture 7, Pg 12

Physics 101: Lecture 7, Pg 13 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 Preflight 7.1 A B C x y

Physics 101: Lecture 7, Pg 14 A B C x y 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 !

Physics 101: Lecture 7, Pg 15 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 ACT A B C x y

Physics 101: Lecture 7, Pg 16

Physics 101: Lecture 7, Pg 17 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? Swimmer Example A B C x y

Physics 101: Lecture 7, Pg 18 Summary of Concepts l X and Y directions are Independent! è Position, velocity and acceleration are vectors l F = m a applies in both x and y direction l Newton’s 3 rd Law l Relative Motion (Add vector components)

Physics 101: Lecture 7, Pg 19