Chapter 5 Work and Energy

1. Chapter 5Work and Energy

2. Section 5.1 Work • Work is done on an object only when a net force acts on the object to displace it in the direction of a component of the net force. • Work = Force x displacement x cosΘ • W=fd(cosΘ) • Work is measured in Nm or Joules

3. Calculating Work When Force and Distance are in the same direction, cosΘ is 1 and W = Fd

4. Work vs. Effort : only the Second Box Requires “Work”.

5. Example Problem A box is dragged across a floor by a 100N force directed 60o above the horizontal. How much work does the force do in pulling the object 8m?

6. Example Problem #2 Decide if work is done and if so, the sign of the work for each case: a) A crane lifting a bucket of concrete b) The force of gravity on the bucket being lifted c) An athlete holds a weight up in a fixed position d) An athlete lowers a weight slowly e) A person pushes a book across the table.

7. Power • Power is the rate at which work is done. • Power = work/elapsed time • P = W/Δt • The SI Unit for power is the watt (W) which equals one Joule per second (J/s)

8. Power

9. Example Problem • A 50 kg girl climbs a flight of stairs that is 5.0 m high. Calculate the power output if she takes 10.0 s to do this. • Find the work done. (Recall that her force is equal to mg) • 2. Calculate the power.

10. Energy – Potential and kinetic • Energy: The ability to do Work • Potential Energy: Energy of position or stored energy • ΔPE = mghwhere “h” refers to height.

11. Other Forms of Stored Energy • Compressed spring • Bow pulled back in archery • Stretched rubber band

12. Kinetic Energy • Kinetic Energy: the mechanical energy of motion. It is how much work an object is currently doing. • KE = ½ mv2

13. Energy and Work • The SI unit for energy is the Joule. This is the same unit for work. • When work is done on an object, energy is transformed from one form to another. • The sum of the changes in PE, KE and heat energy are equal to the work done on the object. • Mechanical energy is transformed into heat energy when work is done to overcome friction.

15. Elastic Potential Energy • When a string is stretched or compressed, it gains elastic potential energy.

16. Elastic Potential Energy • The force that pulls it back and attempts to restore the spring to equilbriumis the restoring force. • PE = ½ kx2 • Elastic PE = ½ (spring constant)(distance compressed or stretched)2

17. Elastic Potential Energy

18. Example Problem • A spring with a force constant of 5.2 N/m has a relaxed length of 2.45 m. When a mass is attached to the end of a string, and allowed to come to rest, the vertical length of the spring is 3.57 m. Calculate the elastic potential energy of the spring (page 180 problem #1, Answer: 3.3 J)

19. Conservation of Energy • Law of Conservation of Energy: Energy cannot be created or destroyed. • Total amount of ME in a system remains constant if no work is done by any other force besides gravity. • Δ KE = ΔPE • KE and PE before an interaction equals all the KE and PE after the interactionl • KE0 + PEo = KEf + PEf

20. Mechanical Energy

21. Example Problem: • Bo flings a 0.20 kg pool ball off a 0.68 m high pool table and the ball hits the floor with a speed of 6.0 m/s. How fast was the ball moving when it left the pool table?