Coefficient of Restitution: Tennis Ball Today’s Topics: Coefficient of Restitution 2D collisions A regulation tennis ball has to bounce between a height of 53 to 56 inches when dropped from a height of 100 inches onto a concrete floor. Determine the coefficient of restitution. Final Relative Velocity Type of Collision Perfectly Inelastic Zero; objects move together Real World Between two limits; defined by __________________________ Perfectly Elastic Same magnitude as original relative velocity; opposite sign A D e v1 v2 v1 v2 e=0 e=1 Perfectly ________ Perfectly ________ B EF 151 Fall, 2014 Lecture 3-7a 1 Car Crash Two cars – same mass – collide head-on. After the collision the cars skid with their brakes locked as shown. Determine the speed of car B and the effective coefficient of restitution. vA = 5 km/hr (1.4 m/s) right uk = 0.3 C EF 151 Fall, 2014 Lecture 3-7a 2 Collisions in 2 dimensions A vA v A A v B B A plane of _______ 1m 4m B Which car had greater speed before collision? vB B y x line of _______ x motion - ________ v AX EF 151 Fall, 2014 Lecture 3-7a v1 v2 v1 v2 e 3 EF 151 Fall, 2014 Lecture 3-7a vBX y motion: Forces and impulse m A v AY mB v BY m A vAY mB vBY e vAY vBY v AY vBY 4 Example: Shooting Pool I Example: Shooting Pool I The cue ball A is given an initial speed of 5 m/s. It makes direct perfectly elastic impact with ball B, giving ball B a speed of 5 m/s. Ball B then makes contact with cushion C (e = 0.6). Each ball has a mass of 0.4 kg. Neglect friction and the size of each ball. e=0.6 Determine the speed of ball B and the angle θ after ball B hits cushion C. If e=1, will θ be: A. >30° B. =30° C. <30° EF 151 Fall, 2014 Lecture 3-7a If e=0, will θ be: A. =30° B. 30°< θ < 90° C. =90° If e=0.6, will θ be: A. >30° B. =30° C. <30° 5 EF 151 Fall, 2014 Lecture 3-7a 6

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