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PHY183-Lecture23

PHY183-Lecture23 - Physics for Scientists Engineers 1...

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February 24, 2006 Physics for Scientists&Engineers 1 1 Physics for Scientists & Physics for Scientists & Engineers 1 Engineers 1 Spring Semester 2006 Lecture 23

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February 24, 2006 Physics for Scientists&Engineers 1 2 Momentum Review Momentum Review Linear Momentum Newton’s 2 nd Law Impulse Average Force r p = m r v r F = d r p dt r J ! r Fdt t i t f " r J = ! r p r J = r F av ! t
February 24, 2006 Physics for Scientists&Engineers 1 3 Impulse and Car Safety Devices Impulse and Car Safety Devices Car collision: very large impulse (cannot be changed) Typical collision time is very short Since , this means a very large force acting on the driver All passive safety devices (seat belt, air bag, crash crumple zones) are designed to extend time interval and thus to reduce average force r F av = r J ! t

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February 24, 2006 Physics for Scientists&Engineers 1 4 Impulse and Sports Impulse and Sports Why can one hit for more power with less tension on the tennis racket strings? Answer: relationship between impulse, force, and time If average force is fixed, increasing the contact time between racket and ball increases impulse given to ball r J = r F av ! t
February 24, 2006 Physics for Scientists&Engineers 1 5 Quiz 22.1 Quiz 22.1 • Jack swings at a 0.2 kg ball that is moving west with a velocity of 40 m/s and hits a line drive. The ball leaves his bat with a velocity of 40 m/s due east. Assuming the ball is in contact with the bat for 0.010 s, what is the average force of the bat on the ball? a) 800 N East b) 1600 N East c) 1600 N West d) 800 N West

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February 24, 2006 Physics for Scientists&Engineers 1 6 Quiz 22.1 Quiz 22.1 • Jack swings at a 0.2 kg ball that is moving west with a velocity of 40 m/s and hits a line drive. The ball leaves his bat with a velocity of 40 m/s due east. Assuming the ball is in contact with the bat for 0.010 s, what is the average force of the bat on the ball? a) 800 N East b) 1600 N East c) 1600 N West d) 800 N West 0.2 (40 ( 40) / ) 16 kg m/s east 16 1600 N east .01 p kg m s p F t ! = " " = ! = = = !
February 24, 2006 Physics for Scientists&Engineers 1 7 Collisions Collisions Collisions of two (or more) objects, or one object with a “wall” Two cars in a traffic accidents Two billiard balls One billiard ball bouncing off the wall Asteroid hitting Earth Two air molecules bouncing off each other Question : If we know the momenta of the two objects before the collision, can we predict the momenta after the collision?

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February 24, 2006 Physics for Scientists&Engineers 1 8 Momentum Conservation in Collisions Momentum Conservation in Collisions During the collision object 1 exerts a force on object 2.
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