Chapter 11 ENERGY

11.1 A Model of the Work-Energy Theorem Throwing a ball: –Force and motion are in the same direction, therefore work is positive –Work is positive and the energy of the ball increases by an amount equal to W Catching a ball: –Force is opposite the direction of motion, therefore work is negative –Work is negative and the energy of the ball decreases by an amount equal to W

Kinetic Energy: –K = ½ m v 2 –Measured in Joules Stored Energy –mass is a form of stored energy! E = mc 2

Gravitational Potential Energy –Choosing a Reference Level At the reference level, the potential energy is defined to be zero The reference level is arbitrary Only changes in energy can be measured

Elastic Potential Energy –Energy stored by distorting an object Ex: string in a bow and arrow, rubber ball, rubber band, slingshot, trampoline, bending a pole vaulting pole

11.2 Conservation of Energy Choosing a system –Conservation of mechanical energy TME = K + U K before + U before = K after + U after –Loss of mechanical energy Energy can leave the system in a number of ways.

Analyzing Collisions –Elastic: kinetic energy is conserved –Inelastic: kinetic energy is not conserved

Carefully identify the system. Make sure it is closed. Identify the initial and final states of the system. Is the system isolated? –If there are no external forces acting on the system, then the total energy of the system is constant. E before = E after –If there are external forces, then E before + W = E after PSS

Identify the forms of energy in the system. –If mechanical energy is conserved, then there is no thermal energy change. If it is not conserved, then the final thermal energy is larger than the initial thermal energy. –Decide on the reference level for the potential energy and draw bar graphs to showing initial and final energy. Check your answer.