3. Work is only done by a force on an object if the force causes the object to move in the direction of the force. Objects that are at rest may have many forces acting on them, but no work is done if there is no movement. W = F·d Work is a scalar quantity.

4. Work, by definition, is the product of the force exerted on an object and the distance the object moves in the direction of the force. W = F·d Work is a scalar quantity.

5. The SI unit of work is the joule, named in honor of James Prescott Joule James Prescott Joule James Prescott Joule James Prescott Joule. One joule, J, of work is the work done when 1.0 N of force is applied through a distance of 1.0 m.

6. Graphically, work is the area under a “Force vs. Displacement” graph. displacement, m

7. Rules For Work: If the force and displacement are in the same direction – the work done is POSITIVE If the force and displacement are in the opposite direction – the work done is NEGATIVE If the force and displacement are perpendicular – the work done is equal to ZERO

8. Practice Work Problem 1: You are pushing a very heavy stone block (200 kg) across the floor. You are exerting 620 N of force on the stone, and push it a total distance of 20 m in 1 direction before you get tired and stop. How much work did you just do? W = (620N)*(20m) W = 12,400 N*m or 12,400 J Notice that the force and displacement are in the same direction so work is positive!

9. Practice Work Problem 2: Austin lifts a 200 N box 4 meters. How much work did he do? W = (200N)*(4m) W = 800 J

10. Practice Work Problem 3: Now, Austin lowers the same 200 N box 4 meters, how much work did he do? W = (200N)*(-4m) W = -800J Notice that the force and displacement are in opposite directions, so the work is NEGATIVE

11. Practice Work Problem 4: A waiter is carrying a tray full of meals with one arm (F = 20N) straight across a room (d = 10 m) at a constant speed. W = (20N)*(10m) ****but the force and displacement are perpendicular!!!! W = 0J The waiter does not do work upon the tray as he carries it across the room

12. Practice Work Problem 5: A weightlifter lifts a barbell with a mass of 280 kg a total of 2 meters off the floor. What is the minimum amount of work the weightlifter did? The barbell is “pulled” down by gravity with a force of (280kg)*(9.8m/s 2 ) = 2,744N So the weightlifter must exert at least 2,744 N of force to lift the barbell at all –If that minimum force is used, the work done will be: –W = (2744N)*(2m) –W = 5488 J

13. Meaning of Negative Work On occasion, a force acts upon a moving object to HINDER a displacement –A car skidding to a stop on a roadway –A baseball player sliding to a stop on the infield dirt In such cases, the force acts in the direction opposite the objects motion in order to slow it down The force doesn’t cause the displacement but opposes the displacement, resulting in negative work.

14. Which athlete is more powerful?

15. Power, by definition, is the time rate of doing work; or the time rate transfer of energy. P = W / tP = W / tP = W / tP = W / t Power is a scalar quantity.

16. The SI unit of power is the watt, named in honor of James Watt James Watt James Watt James Watt. One watt, W, of power is the power achieved when 1.0 J of work is done or 1.0 J of energy is transferred in a time of 1.0 s.

17. Click here to explore energy, work, and the here Work-Energy Theorem in more depth.

18. Practice Power Problem 1: A woman lifts a 250 Newton child a distance of 2.5 meters in 0.5 seconds. What is her power output in lifting the child? –P = W/t, so solve for Work first –W = F*d…….(250N)*(2.5m) –W = 625 J –P = (625J)/(0.5s) –Power = 1250 Watts

19. Practice Power Problem 2: A teacher pushed a 68 N desk across a floor a distance of 6 meters. He exerted a horizontal force of 20 N for 4 seconds. How much Power did he use? Power = W/t or (F*d)/t (20 N)*(6 m) = 120 J of work P = (120J)/(4s) = 30 Watts of Power

20. Practice Power Problem 3: What is the power output of a crane that lowers 2,000 N of material a distance of 250 meters in 45 seconds? (2000N)*(-250m) = W = -500000 J P = (-500000J)/(45s) = P = -11111.11 Watts

21. Energy the ability (capacity) to do work Energy comes in many forms: mechanical, electrical, magnetic, solar, thermal, chemical, etc... thermal, chemical, etc... The SI unit of energy is the joule. Energy, like work and power, is a scalar.

22. Kinetic Energy energy of motion All moving objects that have mass have kinetic energy. KE = 1/2 mv 2 m - mass of the object in kg v - speed of the object in m/s KE - the kinetic energy in J

23. Work-Energy Theorem the net work done on an object is equal to its change in kinetic energy

24. Learn more about the Work-Energy Theorem herehere and here. here A net force causes an object to change its KE because a net force causes an object to accelerate, and acceleration means a change in velocity, and if velocity changes, KE changes.

25. Potential Energy energy of position or condition gravitational potential energy PE g = mgh m - mass of object in kg m - mass of object in kg g - acceleration of gravity in m/s 2 g - acceleration of gravity in m/s 2 h - height of object, in m h - height of object, in m PE g – gravitational potential energy in J

26. Potential Energy energy of position or condition elastic potential energy PE elastic = ½ kx 2 k – elastic constant in N/m k – elastic constant in N/m x - elongation or compression in m x - elongation or compression in m PE elastic – elastic potential energy in J PE elastic – elastic potential energy in J Click here to investigate elastic constants. here

27. Law of Conservation of Energy “Energy can be neither created nor destroyed. It may only change forms.”  all types of energy before the event =  all types of energy after the event =  all types of energy after the event Examples: A dropped object loses gravitational PE as it gains KE.A dropped object loses gravitational PE as it gains KE. A block slides across the floor and comes to a stop.A block slides across the floor and comes to a stop. A compressed spring shoots a ball into the air.A compressed spring shoots a ball into the air.