410 OPENER WORK POWER ENERGY WORK WORK IS
- Slides: 13
4/10 OPENER
WORK, POWER, & ENERGY
WORK • WORK IS DONE WHEN AFORCE ACTS UPON AN OBJECT CAUSING A DISPLACEMENT • W=F X D X COS� • REMEMBER DISPLACEMENT IS THE OVERALL CHANGE IN POSITION (VECTOR QUANTITY) • THETA IS DEFINED AS THE ANGLE BETWEEN THE FORCE AND THE DISPLACEMENT • VERTICAL FORCES (THETA = 90) CANNOT DO WORK ON HORIZONTALLY DISPLACED OBJECTS BECAUSE COS(90)=0, THEREFORE W=0 • NEGATIVE WORK REFERS TO FORCES THAT COUNTERACT DISPLACEMENT BY ANOTHER FORCE • IE AN APPLIED FORCE MOVES AN OBJECT RIGHTWARD HORIZONTALLY AND A FRICTIONAL FORCE SLOWS THE OBJECT IN THE LEFTWARD DIRECTION. (THETA = 180) • UNITS OF WORK ARE JOULES (J) ORNEWTON METERS (NM)
PRACTICE PROBLEMS • DETERMINE WHETHER THESE STATEMENTS REPRESENT EXAMPLES OF WORK.
PRACTICE PROBLEMS
POTENTIAL ENERGY THE STORED ENERGY OF POSITION POSSESSED BY AN OBJECT • GRAVITIATIONAL POTENTIAL ENERGY (PEGRAV) • THE ENERGY STORED IN AN OBJECT AS THE RESULT OF ITS VERTICAL POSITION OR HEIGHT (BASED ON THE GRAVITATIONAL ATTRACTION OF THE EARTH FOR THE OBJECT) • BASED ON MASS AND OBJECT HEIGHT (USUALLY FROM THE EARTH’S SURFACE) • MASS AND HEIGHT ARE DIRECTLY RELATED TOPEGRAV • PEGRAV=MASS X G X HEIGHT UNITS=JOULES • ELASTIC POTENTIAL ENERGY • THE ENERGY STORED IN ELASTIC MATERIALS BASED ON THEIR ABILITY TO STRETCH AND COMPRESS • PESPRING=. 5 XK X X 2 • K= SPRING CONSTANTD(EPENDS ON SPRING PROPERTIES SUCH AS STIFFNESS) • X= THE DISTANCE THE SPRING IS STRETCHED FROM IT’S EQUILIBRIUM POSITION • EQUILIBRIUM POSITION THE NATURAL POSITION OF THE SPRING WHEN NO FORCES ARE BEING APPLIED • GREATER STRETCH ABILITY = MORE STORED ENERGY
PRACTICE QUESTION • A CART IS LOADED WITH A BRICK AND PULLED AT CONSTANT SPEED ALONG AN INCLINED PLANE TO THE HEIGHT OF A SEAT-TOP. IF THE MASS OF THE LOADED CART IS 3. 0 KG AND THE HEIGHT OF THE SEAT TOP IS 0. 45 METERS, THEN WHAT IS THE POTENTIAL ENERGY OF THE LOADED CART AT THE HEIGHT OF THE SEAT-TOP?
KINETIC ENERGY THE ENERGY OF AN OBJECT IN MOTION • VIBRATIONAL KINETIC ENERGY • MOTIONS THAT REPEAT IN A CYCLE • ROTATIONAL KINETIC ENERGY • KEROT=. 5 IW 2 • TRANSLATIONAL KINETIC ENERGY • KE=. 5 MV 2 UNITS=JOULES=KG(M/S)2
PRACTICE QUESTIONS • DETERMINE THE KINETIC ENERGY OF A 625 -KG ROLLER COASTER CAR THAT IS MOVING WITH A SPEED OF 18. 3 M/S.
MECHANICAL ENERGY • AN OBJECT THAT POSSESSES MECHANICAL ENERGY HAS THE ABILITY TO DO WORK • TOTAL MECHANICAL ENERGY (TME): SUM OF THE POTENTIAL AND KINETIC ENERGY • TME=PEGRAV + PESPRING + KE
POWER THE RATE AT WHICH WORK IS DONE • POWER = WORK/TIMETHEREFORE POWER= (F X D X COS)/T • UNITS= WATTOR JOULE/SECOND • 1 HORSEPOWER = 750 WATTS
PRACTICE PROBLEM • A TIRED SQUIRREL (MASS OF APPROXIMATELY 1 KG) DOES PUSH-UPS BY APPLYING A FORCE TO ELEVATE ITS CENTER-OF-MASS BY 5 CM IN ORDER TO DO A MERE 0. 50 JOULE OF WORK. IF THE TIRED SQUIRREL DOES ALL THIS WORK IN 2 SECONDS, THEN DETERMINE ITS POWER.
1/22 FINISH UP STOPPING DISTANCE LAB • TO SETUP SENSORS GO TO EXPERIMENT SETUP SENSORS SHOW ALL • SELECT THE PHOTOGATE AND CHANGE TO G “ ATE TIMING” • THEN INPUT LENGTH OF OBJECT (YOUR CAR) • REMEMBER 1 M=100 CM
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