Transcript Slide 1
Chapter 5 Pgs. 167-192 1. Identify several forms of energy. 2. Calculate Kinetic energy for an object. 3. Apply the work-energy theorem to solve problems. 4. Distinguish between kinetic and potential energy 5. Classify different types of potential energy. 6. Calculate the potential energy associated with an object’s position. Energy Ability to do work • Kinetic Energy – Energy of motion = ability to do work because of motion • Potential Energy – Energy of position = ability to do work because of position within a field force Kinetic Energy (KE) • • • • Depends on mass and velocity Inversely proportional KE is a scalar quantity Unit is the joule (J) A 7.00 kg bowling ball moves at 3.00 m/s. How much kinetic energy does the ball have? How fast must a 2.45 g ping-pong ball move in order to have the same kinetic energy as the bowling ball? The Work-Energy Theorem • The net work due to all forces equals the change in the kinetic energy of a system. • Wnet = DK – Wnet: work due to all forces acting on an object – DK: change in kinetic energy (Kf – Ki) Work – Kinetic Energy Theorem ∆KE = KEf – KEi A 65 kg crate is pushed along a horizontal surface by two students. After the crate is pushed a distance of 6.0 m, starting from rest, its speed is 4.5 m/s. Find the magnitude of the net force on the crate. Potential Energy (PE) • Gravitational Potential Energy – Energy due to the object’s position relative to a zero level Gravitational Potential Energy • Depends on mass, height, and acceleration due to gravity • Unit is the joule (J) A spoon is raised 21.0 cm above a table. If the spoon and its contents have a mass of 30.0 g, what is the gravitational potential energy associated with the spoon at that height relative to the surface of the table? Potential Energy (PE) • Elastic Potential Energy – Energy due to the object’s position when it is stretched or compressed Elastic Potential Energy (PEe) Elastic Potential Energy (PEe) •k is the spring constant •units of newtons divided by meters (N/m) •x is the distance compressed or stretched •usually the difference between unstretched and stretched lengths Springs • When a spring is stretched or compressed from its equilibrium position, it does negative work, since the spring pulls opposite the direction of motion. • Ws = - ½ k x2 – Ws: work done by spring (J) – k: force constant of spring (N/m) – x: displacement from equilibrium (m) • The force doing the stretching does positive work equal to the magnitude of the work done by the spring. • Wapp = - Ws = ½ k x2 A stuntman is attached to a bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the elastic potential energy? Other Forms of Energy Thermal energy – heat c = Q/(m•Δt) Electrical energy – electricity Ue = K[(q1•q2)/r] Chemical energy – stored in bonds, vander waals forces Body uses all types of energy converting one into another food energy stored in bonds Calories 1 Calorie = 4180 Joules Can convert mass to energy by fusion – combining mass fission – breaking down mass