1. Work & Energy Work & Energy Explaining the Causes of Motion in a Different Way

2. Which of the following is a measure of the ability to do work? 1.Power 2.Energy 3.Photosynthesis 4.Joules

3. What the heck is energy anyway? lEnergy lEnergy- the ability to do work lIf an object or organism does work (exerts a force over a distance to move an object) lthe object or organism uses energy. l Work = force x distance ex. When stretching a sling shot, you do work, and energy is transferred to the sling shot lMeasured in the unit joules (honors Mr. Joule) lb/c work is a force x distance it is measured in Newtons x meters. l These units are called joules. l lWork = force x distance 1 N x m = 1 J= 1 kg x m 2 /s 2

4. Energy and Work are closely related lWork is a transfer of Energy l VERY simple: Work has been done when one object transfers energy to another. (actually more complicated than that, but its OK for now) l SI unit for work is Joules (J) lEnergy is the capacity to do Work l We recognize it by the changes it causes We use energy to walk upstairs. (work)

5. Calculating WORK lThe change in Energy of an object lSO WORK DONE = CHANGE IN KINETIC ENERGY lInvolves an object moving in the direction of the FORCE applied lWork = Force x Distance

6. Try it… l Involves an object moving in the direction of the FORCE applied l A Force in physics- is a “push or a pull” l Ex- Let’s try it- 3 volunteers: l1- Hold 2 book out to your sides for 2 minutes l2- Carry 10 books around the room for 2 minutes l3-Constantly lift up a box of tissues for 2 minutes Who did the most work?

7. Work lThe product of force and the amount of displacement along the line of action of that force. Units: ft. lbs (horsepower) Newtonmeter (Joule)

8. Work = F x d To calculate work done on an object, we need: The Force ¬The average magnitude of the force ­The direction of the force The Displacement ¬The magnitude of the change of position ­The direction of the change of position

9. Calculate Work lDuring the ascent phase of a rep of the bench press, the lifter exerts an average vertical force of 1000 N against a barbell while the barbell moves 0.8 m upward lHow much work did the lifter do to the barbell?

10. Calculate Work Table of Variables: Force = +1000 N Displacement = +0.8 m Force is positive due to pushing upward Displacement is positive due to moving upward

11. Calculate Work Table of Variables: Force = +1000 N Displacement = +0.8 m Select the equation and solve:

12. Work performed climbing stairs lWork = Fd lForce l Subject weight lFrom mass, ie 65 kg lDisplacement l Height of each step lTypical 8 inches (20cm) lWork per step l 650N x 0.2 m = 130 Nm (Joules) lMultiply by the number of steps

13. Work on a stair stepper lWork = Fd lForce l Push on the step l???? lDisplacement l Step Height l8 inches l“Work” per step l ???N x.2m = ???Nm (Joules)

14. Energy lEnergy (E) is defined as the capacity to do work (scalar)Energy l Many forms lNo more created, only converted lchemical, sound, heat, nuclear, mechanical lMechanical Energy l Kinetic Energy (KE): l energy due to motion l Potential Energy (PE): lenergy due to position

15. Kinetic Energy Energy due to motion reflects l the mass l the velocity of the object KE = 1/2 mv 2

16. Calculate Kinetic Energy How much KE in a 5 ounce baseball (0.145 kg) thrown at 80 miles/hr (35.8 m/s)?

17. Calculate Kinetic Energy Table of Variables Mass = 0.145 kg Velocity = 35.8 m/s

18. Calculate Kinetic Energy Table of Variables Mass = 0.145 kg Velocity = 35.8 m/s Select the equation and solve: KE = ½ m v 2 KE = ½ (0.145 kg)(35.8 m/s) 2 KE = ½ (0.145 kg)(1281.54 m/s 2 ) KE = ½ (185.8 kg m/s 2 ) KE = 92.9 kg m/s 2, or 92.9 Nm, or 92.9J

19. Work - Energy Relationship lWork is the change in the mechanical energy of the object lFd =  KE +  PE

20. 1.You are asked for kinetic energy and stopping distance 2.You are given mass, speed and force of brakes. 3.Use E k = 1 / 2 mv 2 and W= fd 4.Solve for E k = ½ (1,300 kg) ( 30 m/s) 2 = 585,000 J l To stop the car, work done by brakes = E k of car, so W = E k l Solve for distance = W ÷ f = 585,000J ÷ 9,500 N = 62 m Calculating kinetic energy A car with a mass of 1,300 kg is going straight ahead at a speed of 30 m/s (67 mph). The brakes can supply a force of 9,500 N. Calculate: a) The kinetic energy of the car. b) The distance it takes to stop.

21. Work - Energy Relationship lIf more work is done, greater  energy l greater average force l greater displacement

22. Total Mechanical Energy lThe mechanical energy of an object can be the result of its motion (i.e. kinetic energy) and/or the result of its stored energy of position ( i.e. potential energy). lThe total amount of mechanical energy is merely the sum of the potential & kinetic energy (total mechanical energy lMechanical energyMechanical energy lTME = PE + KE