Ch. 12 – Energy & Work

1. Ch. 12 – Energy & Work I. Energy and Work A. Work B. Simple Machines C. Energy

2. A. Work • Work • transfer of energy through motion • force exerted through a distance W = Fd W: work (J) F: force (N) d: distance (m) 1 J = 1 N·m Distance must be in direction of force!

3. W d F A. Work • Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W = ? WORK: W = F·d W = (30 N)(1.5 m) W = 45 J

4. W d F A. Work • A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W = ? WORK: W = F·d F = m·a F =(40kg)(9.8m/s2)=392 N W = (392 N)(1.4 m) W = 549 J during lift No work after lift. “d” is not in the direction of the force.

5. B. Simple Machines • A machine that does work with only one movement is a simple machine. • Lever – bar that is free to pivot about a fixed point called a fulcrum. • Effort arm is part of the lever on which effort force is applied. • Resistance arm is part of the lever that exerts the resistance force. • Three classes of levers based on positions of effort force, resistance force, and fulcrum.

6. B. Simple Machines • First-class lever – fulcrum is located between the effort and resistance force; multiplies and changes direction of force. • Second-class lever – resistance force is located between the effort and fulcrum; always multiplies force. • Third-class lever – effort force is between the resistance force and fulcrum; doesn’t multiply force but does increase distance over which force is applied

7. B. Simple Machines • Grooved wheel with a rope, simple chain, or cable running along the groove is a pulley, which is a modified first-class lever. • A fixed pulley is attached to something that doesn’t move; force is not multiplied but direction is changed • A movable pulley has one end of the rope fixed and the wheel free to move; multiplies force • Block and tackle – system of pulleys consisting of fixed and movable pulleys

8. B. Simple Machines • Wheel and axle – machine with two wheels of different sizes rotating together; modified lever form • Gears are a modified form of the wheel and axle

9. B. Simple Machines • Inclined Plane – sloping surface that reduces the amount of force required to do work • Less force is required if a ramp is longer and less steep.

10. B. Simple Machines • Screw – inclined plane wrapped in a spiral around a cylindrical post. • Inclined plane with one or more sloping sides is a wedge. • Compound machine – uses a combination of two or more simple machines.

11. THERMAL The ability to cause change. MECHANICAL NUCLEAR ELECTRICAL CHEMICAL joules (J) C. Energy internal motion of particles ENERGY motion of objects changes in the nucleus motion of electric charges bonding of atoms

12. Which has the most KE? • Which has the least KE? 80 km/h 50 km/h 80 km/h C. Energy • Kinetic Energy (KE) • energy in the form of motion • depends on mass and velocity 80 km/h truck 50 km/h motorcycle

13. C. Energy • Potential Energy (PE) • stored energy • depends on position or configuration of an object • Which boulder has greater gravitational PE? • What other ways can an object store energy?

14. C. Conservation of Energy • Law of Conservation of Energy • Energy may change forms, but it cannot be created or destroyed under ordinary conditions. • EX: • PE  KE • mechanical  thermal • chemical  thermal

15. C. Conservation of Energy PE  KE View pendulum animation. View roller coaster animation.

16. C. Conservation of Energy Mechanical  Thermal View rolling ball animations. View skier animation.