1. Work and Energy

2. Work is force x displacement Only the force parallel to the displacement applies to work Guy is pulling a sled with a force of 50 N at an angle of 40°, what work does he do over 20 m? W = Fd(cos  ) W = (50 N)(20 m)(cos 40°) W = 766= 766 J Joules 50 N 40° Nm

3. I push a red ball up an 8 m slide, with a force of 20 N. The slide is frictionless and makes a 48° angle with the ground. What work do I do? 20 N 48 ° What am I working against? Gravity! W = Fd(sin  ) = (20 N)(8 m)(sin 48°)= 119 J

4. Kinetic Energy is energy of motion units: kg m 2 /s 2 = J

5. A girl throws a.8 kg softball with a speed of 20 m/s. What is the ball’s kinetic energy? KE = ½mv 2 KE = ½(.8 kg)(20 m/s) 2 KE = 160 J

6. Potential Energy is energy of position Gravitational Potential Energy PE = mgh units: kg (m/s 2 ) (m) = J Elastic Potential Energy PE = 1/2kx 2 units: (N/m)m 2 = J

7. A 15 kg white ball is raised 12 meters. How much potential energy is it given? 12 m PE = mgh= (15 kg)(9.81 m/s 2 )(12 m)= 1766 J

8. A spring stretches 5.0 cm from its rest position. The spring has a spring constant of 25 N/m. How much potential energy has it been given? rest 5 cm PE = ½kx 2 PE = ½(25 N/m)(.05 m) 2 PE =.031 J