* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download MotionEnergyRevised0..
Survey
Document related concepts
Transcript
15 Part 2, A: THERMODYNAMICS 6 WORK Definition (Young): Energy is the capacity to do work. 16 Part 2, A: THERMODYNAMICS 56 WORK Definition (Young): Energy is the capacity to do work. W = Fd W F d is the work done (Joules) is the applied force (Newtons) is the distance moved as a result of the applied force (meters) Part 2, A: THERMODYNAMICS KINETIC ENERGY Quantity of Motion KE = ½ m v 2 10 9 Part 2, A: THERMODYNAMICS KINETIC ENERGY An object of mass 10 kg has a speed of 10 m/sec. What is the kinetic energy? 11 Part 2, A: THERMODYNAMICS KINETIC ENERGY An object of mass 10 kg has a speed of 10 m/sec. What is the kinetic energy? KE = ½ (10 kg) (10 m/sec) (10 m/sec) 500 Joules 12 Part 2, A: THERMODYNAMICS KINETIC ENERGY An object of mass 10 kg has a speed of 10 m/sec. By how much will the kinetic energy increase if the speed is doubled? 13 2 Part 2, A: THERMODYNAMICS 14 KINETIC ENERGY An object of mass 10 kg has a speed of 10 m/sec. By how much will the kinetic energy increase if the speed is doubled? 2 2 2 2 KE = ½ m (2v) = 2 (½ m v ) = 4 (½ m v ) The kinetic energy is 4 times greater. Energy Energy is Conserved KE = W Energy Energy is Conserved KE = W Work-Energy Theorem Motion – Newton’s Laws Special Case: Friction Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2. 15 Motion – Newton’s Laws Special Case: Friction Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2. NOTE: We already solved this problem in our discussions about Newton’s second Law. John E. Erdei, SCI190 Lecture Slides – Newton’s Second Law, Slide 18, University of Dayton (Unpublished) . 16 Motion – Newton’s Laws Special Case: Friction Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2. ΔKE = W = F d = µk m g d ½ m v2 = µk m g d v2 = 2 µk g d d = v2 / ( 2 µk g) = 0.25 m 17 Motion – Newton’s Laws Special Case: Friction Example: A force is applied to an object, causing the object to slide on a table (with friction) at a constant velocity. The speed is 2 m/sec. If the force is removed, how far will the block slide before it stops? The coefficient of kinetic friction is 0.8 and g = 10 m/s2. Note: This is the same result that we got using the constant acceleration equations d = ½ a t2 Δv = a t 18 Energy Potential Energy Potential energy comes in a variety of forms. Energy Potential Energy Gravitational Potential Energy PEGRAV = mgh 36 Part 2, A: THERMODYNAMICS Power Power is defined to be the rate at which energy is used. P = Energy/Time 37 Part 2, A: THERMODYNAMICS Power P = Energy/Time Energy Time Power measured in measured in measured in Joules Seconds Joules/sec = Watt Energy Potential Energy Football coach Jones becomes angry with a player, and in order to get the players attention, coach Jones makes the player go from playing field level up to the top row of seats in the stadium. Will the player do more work if he walks to the top row or runs to the top row? Assume the player starts from rest and stops when he reaches the top. Energy Potential Energy Football coach Jones becomes angry with a player, and in order to get the players attention, coach Jones makes the player go from playing field level up to the top row of seats in the statium. Will the player do more work if he walks to the top row or runs to the top row? Assume the player starts from rest and stops when he reaches the top. There is actually not enough information to determine the work from first principles. However, since the change in kinetic energy is 0, the work done by the player must be used to increase his potential energy (Conservation of Energy). The amount of potential energy (PE = mgh) is the same, independent of running or walking, and therefore the amount of work done by the player is the same if he runs or walks! Energy Potential Energy Football coach Jones becomes angry with a player, and in order to get the players attention, coach Jones makes the player go from playing field level up to the top row of seats in the stadium. Will the player do more work if he walks to the top row or runs to the top row? Assume the player starts from rest and stops when he reaches the top. If this is true, why do they make you run as punishment, ie, why not punish the player by making him walk at a leisurely pace to the top of the stadium???? Energy Potential Energy Football coach Jones becomes angry with a player, and in order to get the players attention, coach Jones makes the player go from playing field level up to the top row of seats in the stadium. Will the player do more work if he walks to the top row or runs to the top row? Assume the player starts from rest and stops when he reaches the top. The punishment is related to Power, and not directly to work. Since P=W/t Walking will result in the work to be expended over a longer time period, therefore requiring less power. Running expends the work over a shorter time period, therefore requiring more power.