1. Work, Machines, and Energy HS Physical Science Hull Middle School

2. Work and Power  Work-The transfer of an energy to an object by using a force that causes the object to move in the direction of the force.  W=F x d  Work (in joules)= Force (in newtons) times distance (in meters)

3.  Work involves a transfer of energy.  Force and work are not the same thing!  Applying a force does not always mean work. For example, you can push on a large boulder. If the boulder does not move, then work has not been done.  If the boulder were to move, then work has been done.

4. Work, defined…  In order for work to have been done, the object has to move in the same direction as the force applied.  See p. 211 in textbook. Discuss the examples.

5.  Different amounts of force might be needed to do the same amount of work. See diagram on p. 212 (discuss).

6. Vocabulary  Joule- Unit used to express energy; equivalent to the amount of wonk done by a force of 1 N acting through a distance of 1 m in the direction of the force (symbol, J)

7. Vocabulary  Power-rate at which work is done, or energy is transferred.  Watt-unit used to express power; equivalent to joules per second (W).

8.  See p. 214. Solve the power problem and discuss.

9. Calculating Power  P=W/t  Power is measured in watts. One watt is 1J/s.  Work is measured in Joules.  Time is measured in seconds.

10. Summarize  1. In scientific terms, work is done when a force causes an object to move in the _____________ of the force.  2. Work is calculated as _________ times __________.  3. The unit of work is the newton-meter, or _______.  4. _______ is a measure of how fast work is done.  5. Power is calculated as ______ divided by ______.

11. PRACTICE  Use W=fd  1. A crane uses an average force of 5,200 N to lift a girder 25 m. How much work does the crane do on the girder?  2. An apple weighing 1 N falls a distance of 1 m. How much work is done on the apple by the force of gravity?  3. A bicycle’s brakes apply 125 N of frictional force to the wheels as the bike moves 14.0 m. How much work do the brakes do?  4. A mechanic uses a hydraulic lift to raise a 1,200 kg car 0.50 m off the ground. How much work does the lift do on the car?

12. Answers  1. 1.3 x 10 (fifth) J  2. 1 J  3. 1,750 J  4. 5,900 J

13. Machines  Machine- A device that makes work easier by changing the size or direction of the force.  Work Input-work done on a machine.  Work Output-work done by a machine.

14. Simple Machines Video  http://www.youtube.com/watch?v=jGkQ64zn1j0 http://www.youtube.com/watch?v=jGkQ64zn1j0

15. Calculating Work Input and Output  Work Input-The product of the input force and the distance through which force is exerted.  Work Output-The product of the output force and the distance through which force is exerted.  W=F x d

16.  What does a machine do to forces?  Machines can either change the size or direction of the force, but not both…  A MACHINE DOES NOT CHANGE THE AMOUNT OF WORK THAT YOU DO!

17. FORCE AND DISTANCE TRADE- OFF  If a machine changes the size of the force, it also has to change the distance through which the force is exerted.  Force or distance will increase, but not both.

18. Mechanical Advantage  Mechanical Advantage-a number that tells how many times a machine multiplies force.  MA= output force/input force.  Note that this is force, not work.  What is the mechanical advantage of a machine that puts out 25N of force, when you only put in 5N of force?

19.  Look at the illustration on p. 219.  How does a nutcracker work?  How does a hammer work?  How does a pulley work?  How does a screwdriver work?

20. Mechanical Efficiency  Mechanical Efficiency-A percentage, that measures work output to work input.  ME= (work output/work input) x 100  An ideal machine would have 100% efficiency, but this just doesn’t happen…  Output will always be less than input, because some friction must be overcome.

22. Summarize!  1. A machine makes work easier by changing the _____ or ______ of the force.  2. A machine can increase _____ or _______, but not both.  3. ________ _________ tells how many times a machine multiplies force.  4. ______ _______ is the comparison of a machine’s work output compared to work input.  5. Why are machines never 100% efficient?