Work, Energy, and Power

1. Work, Energy, and Power

2. Identify each of the situations below that represent work being done. • A book in free fall • Pushing on a wall • Lifting a box • Carrying a box across the room • Pushing a box across the room • Holding a weight over your head • Reading a book • Writing notes

3. Work • When a force acts upon an object to cause a displacement of the object, it is said that work was done upon the object. • There are three key components to work - force, displacement, and cause. • In order for a force to qualify as having done workon an object, there must be a displacement and the force must cause the displacement. (the force must be in the same direction as the displacement)

4. Identify each of the situations below that represent work being done. • A book in free fall • Pushing on a wall • Lifting a box • Carrying a box across the room • Pushing a box across the room • Holding a weight over your head • Reading a book • Writing notes

5. Vector or Scalar? • Work is a vector quantity- it has a magnitude and a direction. • Since work is a vector quantity, it can have a negative value. • Negative workoccurs when the force doesn't cause the displacement but rather hindersit or when the force moves the object in the negative direction.

6. Calculating Work • The SI unit for work is the joule, J. • A joule is equal to a force of 1 newton applied over a distance of 1 meter, or 1 newton-meter. • Formula: work = force x distance • W = F x d

7. Sample Problem #1 • How much work is done by a person who uses a force of 27.5N to move a grocery buggy 12.3m? • W = 338 J

8. Sample Problem #2 • 55, 000J of work is done to move a rock 25m. How much force was applied? • F = 2200 J

9. Sample Problem #3 • You and 3 friends apply a combined force of 489.5N to push a piano. The amount of work done is 1762.2J. What distance did the piano move? • d = 3.6 m

10. Sample Problem #4 • John pulls a 4.5 kg sled across level snow with a force of 225 N on a rope that is 35o abovethe horizontal. If the sled moves a distance of 65.3 m, how much work does John do?

11. Sample Problem #5 • Sharon is pushing a lawn mower with a force of 88.0 N along a handle that makes an angle of 41.0o with the horizontal. How much work is done in moving the lawn mower 1.2 km to mow the lawn?

12. Sample problem #6 • A 4200 N piano is to be slid up a 3.5 m frictionless ramp at a constant speed. The plank makes an angle of 30.0o with the horizontal. Calculate the work done by the person sliding the piano up the ramp.

13. Sample Problem #7 • Marcus slides a 60.0 kg crate up a ramp that is 2.0 m long and attached to a platform 1.0 m above the floor level. A 400.0 N force, parallel to the ramp, is needed to slide the crate up the ramp at a constant speed. a) How much work does Marcus do in sliding the crate up the ramp?  b) How much work would be done if Marcus simply lifted the crate straight up from the floor to the platform?

14. Power • Power is the rate of doing work. • The SI unit for power is the watt, W. • A watt is equal to 1 Joule/second. • Formula: power = work / time • P = W / t

15. ReVIEW

16. In physics, work is defined as force 10 • times time • divided by time • times distance • divided by distance

17. The unit of work is the 10 • Newton • Watt • meter • Joule

18. ___ is the rate at which work is done. 10 • Power • Energy • Force • Friction

19. The unit of power is the 10 • Newton • Watt • meter • Joule

20. When we carry an object across a room, without lifting it or setting it down, we do no physical work on it. 10 • True • False

21. How much work is done on a 10 N block that is lifted 5 meters off the ground? 10 • 2 J • 5 J • 50 J • 500 J

22. You did 170 J of work lifting a 140 N backpack. How high did you lift it? 10 • 30 m • 0.82 m • 1.2 m • 310 m

23. You lift a 45 N bag of mulch 1.2 m and carry it a distance of 10 m to the garden. How much work was done? 10 • 54 J • 504 J • 450 J • 540 J

24. A mouse carries a 1.0 g ant on his back across the floor for a distance of 10 m. How much work was done? 10 • 10 J • 98 J • 0.098 J • None of the above

25. What power is expended if you lift a 10 kg boulder 1 m in 2 s? 10 • 5 W • 20 W • 50 W • 500 W

26. Is work a scalar or vector quantity? 10 • Scalar • Vector • Neither

27. It is not possible for a person to do negative work. 10 • True • False

28. Energy • Energy is the ability to produce change in itself or the environment. • The energy can take many forms including thermal, chemical, energy of motion. • When the position of a moving object changes over time, the change in position indicates that the object has energy. The energy resulting from a change in motion is called kineticenergy.

29. Kinetic Energy • The equation for kinetic energy is: KE = ½ mv2 m is the mass of the object and v is the velocity of the object. • What is the relationship between kinetic energy and velocity? Between kinetic energy and mass?

30. Work • Work is the process of changing the energy of the system. • When work is done on an object, a change in kineticenergy results. • Two conditions must be met for work to occur. a. The object must move through a distance b. A force must act upon the object in the direction the object moves.