Transcript Work
Work and Energy Come in and get out your notes Journals and a paper off the blue bookshelf ! Energy is the ability of a physical system to cause change All Energy is measured in Joules (J) Energy Energy is anything that can be con-verted into work; i.e., anything that can exert a force through a distance. Energy is the capability for doing work. Types of Energy * Kinetic energy – the energy of motion * Potential Energy – the energy of position * Chemical Potential Energy – energy stored in molecules * Thermal Energy – heat energy * Nuclear Energy – energy related to matter itself * Radiant Energy – light, microwaves, radio waves, x rays Work Work energy required to make something move. WORK is ENERGY! Force (N) Work = F d Distance (m) Work Energy (J) Work is measured in Joules What is a Joule? Work Force Distance 1 Nm = 1 Joule (J) Mass is NOT Force!! Many times the Force that an object exerts is not known. We can find that Force by using the Formula F = mg (g =9.8) Example: W = m = 8 kg d=2m W = (mg) * d W=? F * d W = (8* 9.8) * 2 W = 156.8 J Three things are necessary for the performance of work 1. Force “F”. 2. distance “d”. 3. The force and distance need to be parallel If a force does not affect displacement, it does no work. F W The force F exerted on the pot by the man does work. The earth exerts a force W on the pot, but does no work even though there is displacement. 3 things to have Work! 1. Force 2. Distance 3. A component of force in the direction of distance F W When you lift something your force is in the upward direction and your distance is going up F d WORK DONE When you carry something, you move to the side, but your force is up NO WORK d F Power Power is defined as the rate at which work is done: Work (J) Force (N) Distance (m) P = W = Fd Power (watts) t t Time (sec) Power is measured in Watts Power 1. Two physics students, Bob and Sam are in the weightlifting room. Bob lifts the 50-kg barbell a distance of .5 meters one minute; Sam lifts the 50-kg barbell the same distance in 10 seconds. A). Which student does the most work? B). Which student delivers the most power? Work of Bob = Fd Wb = mgd Wb = (50 kg) (9.8 m/s2) (.5m) Wb = 245 J Work of Sam = mgd Ws = (50 kg) (9.8 m/s2) (.5m) Ws = 245 J Power 1. Two physics students, Bob and Sam are in the weightlifting room. Bob lifts the 50-kg barbell a distance of .5 meters one minute; Sam lifts the 50-kg barbell the same distance in 10 seconds. A). Which student does the most work? SAME FOR BOTH! B). Which student delivers the most power? Power of Bob =W/t P of bob= 245 J / 60 sec Power = 4.08 WATT Power of Sam = W/t P of sam = 245 J/ 10 sec Power = 24.5 WATT Potential Energy: Energy of Position. PE = mgh Potential Energy (J) Mass (Kg) Height of object (m) Gravity (9.8 m/s2) Potential Energy measured in Joules If h = 0 , then PE = 0 Kinetic Energy: Energy in motion KE = ½ mv2 Kinetic Energy (J) Mass (kg) Velocity (m/s) Kinetic Energy measured in Joules If v = 0 , then KE = 0 Kinetic and Potential Energy are EQUAL!! Total PE = Total KE There is height but no velocity There is height and velocity There is velocity Height is 0 P.E. = max K.E. = 0 P.E. = K.E. K.E. = max P.E. = 0 Energy is converted but not lost! Total Energy in this system = 100 J P.E. = 100 J P.E. = 50 J K.E. = 100 J K.E. = 0 K.E. = 50 J P.E. = 0 Examples: What is the Potential energy of a 1000-kg car resting at the top of a 20m hill? P = mgh = (1000 kg)(9.8)(20 m) P = 196,000 J What is the kinetic energy of a 1000-kg car traveling at 14.1 m/s? K mv (1000 kg)(14.1 m/s) 1 2 2 1 2 K = 99,405 J 2 Beware of the evil force! Sometimes the problem will tell you the force of the object. This refers to the WEIGHT. F = W = mg PE = F h You can also use the Weight to find the mass. m = W/g or m = F/g How are P.E. and K.E. related? There is height but no velocity P.E. = max K.E. = 0 There is height and velocity There is velocity Height is 0 P.E. = K.E. K.E. = max P.E. = 0 Law of Conservation of Energy @ highest point P.E. = max K.E. = 0 @ midpoint P.E. = K.E.