Honors&Physics&First!&
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Energy&Change&in&Collisions&
Kinetic'Energy'
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Kinetic&Energy&is&the&energy&of&motion.&
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Sample&Problem:&&What&is&the&kinetic&energy&of&a&456Bkg&racecar&moving&at&42.4&m/s?&
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Practice&Problem:&&What&is&the&kinetic&energy&of&a&97.4Bkg&man&on&a&bike&moving&at&18.7&
m/s?&
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• Practice&Problem:&&What&is&the&kinetic&energy&of&a&0.45Bkg&soccer&ball&moving&at&24&m/s?&
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Energy'Change'in'Collisions'
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• While&momentum&is&conserved&in&collisions,&kinetic&energy&may&not&be&conserved.&
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• Collisions&are&described&as:&
– Perfectly&Inelastic:&Objects&stick&together&after&a&collision.&&Momentum&is&
conserved,&but&Kinetic&Energy&is&not&conserved.&
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– Elastic:&Objects&bounce&off&each&other.&&Both&Momentum&and&Kinetic&Energy&are&
conserved.&
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– Inelastic:&&Objects&deform&somewhat,&absorbing&kinetic&energy,&but&separate&after&
the&collision.&&Momentum&is&conserved,&but&Kinetic&Energy&is&not&conserved.&
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• Sample&Problem:&A&0.017Bkg&BB&moving&at&38&m/s&hits&a&stationary&0.015Bkg&soda&can,&and&
gets&stuck&inside.&
– What&is&the&speed&of&the&BB&&&can&after&the&collision?&
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– What&is&the&change&in&kinetic&energy&of&the&system?&
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Honors&Physics&First!&
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Energy&Change&in&Collisions&
Practice&Problem:&In&a&“Wipeout”&contest,&a&79Bkg&man&running&at&6.4&m/s&hits&a&stationary&
125Bkg&hanging&pad&and&clings&on&as&it&moves.&
– What&is&the&speed&of&the&man&&&pad&after&the&collision?&
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What&is&the&change&in&kinetic&energy&of&the&system?&
Practice&Problem:&Trying&to&get&it&through&a&hanging&tire,&a&boy&throws&a&0.42–kg&football&
at&12&m/s.&&Instead&it&hits&the&14–kg&tire&and&bounces&back&at&3.6&m/s.&
– What&is&the&speed&of&the&tire&after&the&collision?&
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What&is&the&change&in&kinetic&energy&of&the&system?&