Exam I Physics 101: Lecture 08 Centripetal Acceleration and Circular Motion http://www.youtube.com/watch?v=zyf5wsmxrai Today s lecture will cover Chapter 5 Exam I is Monday, Oct. 7 ( weeks!) Physics 101: Lecture 8, Pg 1
Circular Motion Act B A C v Answer: B A ball is going around in a circle attached to a string. If the string breaks at the instant shown, which path will the ball follow (demo)? Physics 101: Lecture 8, Pg
Acceleration in Uniform Circular Motion v v a v R v 1 R R v 1 v R 1 a ave = v / t Acceleration inward Centripetal Acceleration Directed radially inward! Acceleration is due to change in direction, not speed. Since the object turns toward center, there must be a force toward center: Centripetal Force Physics 101: Lecture 8, Pg 3
Prelectures Consider the following situation: You are driving a car with constant speed around a horizontal circular track. On a piece of paper, draw a Free Body Diagram (FBD) for the car. How many forces are acting on the car? 1% 8% 39% 31% 1% F N A) 1 B) C) 3 correct D) 4 E) 5 f W Friction, Gravity, & Normal R F Net = ma = mv /R Centripetal Force is NOT an additional force! Draw your FBD as normal, and one of the forces will be the Centripetal Force! Physics 101: Lecture 8, Pg 4
Prelecture Consider the following situation: You are driving a car with constant speed around a horizontal circular track. On a piece of paper, draw a Free Body Diagram (FBD) for the car. The net force on the car is F N 19% 60% 1% A. Zero B. Pointing radially inward C. Pointing radially outward W correct f R F Net = ma = mv /R Physics 101: Lecture 8, Pg 6
ACT Suppose you are driving through a valley whose bottom has a circular shape. If your mass is m, what is the magnitude of the normal force F N exerted on you by the car seat as you drive past the bottom of the hill A. F N < mg a=v B. F N = mg /R C. F N > mg correct R SF = ma F N - mg = mv /R F N = mg + mv /R F N mg v Physics 101: Lecture 8, Pg 7
Roller Coaster Example What is the minimum speed you must have at the top of a 0 meter roller coaster loop, to keep the wheels on the track? Y Direction: F Net = ma -N mg = m a = m v /R Let N = 0, just touching -mg = -m v /R g = v / R v = sqrt(g*r) = 9.9 m/s N +y -y mg Physics 101: Lecture 8, Pg 8
Circular Motion Angular displacement q = q -q 1 How far it has rotated Units radians (p = 1 revolution) Angular velocity w = q/ t How fast it is rotating Units radians/second Period =1/frequency T = 1/f = p / w Time to complete 1 revolution Physics 101: Lecture 8, Pg 9
Circular to Linear Displacement s = r q (q in radians) Speed v = s/ t = r q/ t = rw Direction of v is tangent to circle v v 1 s Physics 101: Lecture 8, Pg 10
Merry-Go-Round ACT Bonnie sits on the outer rim of a merry-go-round with radius 3 meters, and Klyde sits midway between the center and the rim. The merry-go-round makes one complete revolution every two seconds (demo). Klyde s speed is: Klyde Bonnie (a) the same as Bonnie s (b) twice Bonnie s (c) half Bonnie s V 1 Klyde V Bonnie Bonnie travels p R in seconds v B = p R / = 9.4 m/s Klyde travels p (R/) in seconds v K = p (R/) / = 4.71 m/s Physics 101: Lecture 8, Pg 11
Merry-Go-Round ACT II Bonnie sits on the outer rim of a merry-go-round, and Klyde sits midway between the center and the rim. The merry-goround makes one complete revolution every two seconds. Klyde s angular velocity is: Klyde Bonnie (a) the same as Bonnie s (b) twice Bonnie s (c) half Bonnie s The angular velocity w of any point on a solid object rotating about a fixed axis is the same. Both Bonnie & Klyde go around once (p radians) every two seconds. Physics 101: Lecture 8, Pg 1
Angular Acceleration Angular acceleration is the change in angular velocity w divided by the change in time. w f t w 0 If the speed of a roller coaster car is 15 m/s at the top of a 0 m loop, and 5 m/s at the bottom. What is the cars average angular acceleration if it takes 1.6 seconds to go from the top to the bottom? V 5 w 15 w f. 5 w0 1. 5 R 10 10.5 1.5 1.6 = 0.64 rad/s Physics 101: Lecture 8, Pg 13
Summary (with comparison to 1-D kinematics) Angular Linear c o n s ta n t a c o n s ta n t w w 0 t v v at 0 1 q q0 w 0t t 1 x x0 v 0t at And for a point at a distance R from the rotation axis: x = Rq v = wr a = R Physics 101: Lecture 8, Pg 14
CD Player Example The CD in a disk player spins at about 0 radians/second. If it accelerates uniformly from rest with angular acceleration of 15 rad/s, how many revolutions does the disk make before it is at the proper speed? w w 0 f 0 0 rad/s 15 rad/s q? w w q w f 0 f 0 w 15 0 0 q q q = 13.3 radians 1 Revolutions = p radians q = 13.3 radians =.1 revolutions Physics 101: Lecture 8, Pg 15
Summary of Concepts Uniform Circular Motion Speed is constant Direction is changing Acceleration toward center a = v / r Newton s Second Law F = ma Circular Motion q = angular position radians w = angular velocity radians/second = angular acceleration radians/second Linear to Circular conversions s = r q Uniform Circular Acceleration Kinematics Similar to linear! Physics 101: Lecture 8, Pg 16