Work, Power, & Machines

What is work ?  The  The product of the force applied to an object and the distance through which that force is applied.

Is work being done or not?  Mowing  Mowing the lawn  Weight-lifting  Moving  Moving furniture up a flight of stairs  Pushing  Pushing against a locked door  Swinging  Swinging a golf club  YES  NO  YES

Calculating Work All or part of the force must act in the direction of the movement.

Do you do more work when you finish a job quickly?  Work does NOT involve time, only force and distance.  No work is done when you stand in place holding an object.  Labeling work: w = F x d  Newton X meter (N m)  Which also = (kg x m 2 ) s 2 s 2

The Joule  1 newton-meter is a quantity known as a joule (J).  Named after British physicist James Prescott Joule.

 How quickly work is done.  Amount of work done per unit time.  If two people mow two lawns of equal size and one does the job in half the time, who did more work?  Same work. Different power exerted.  POWER = WORK / TIME

The watt  A  A unit named after Scottish inventor James Watt.  Invented  Invented the steam engine.  P  P = W/t  Joules/second  1 watt = 1 J/s

watts  Used to measure power of light bulbs and small appliances  An electric bill is measured in kW/hrs.  1 kilowatt = 1000 W

Horsepower (hp) = watts  Traditionally associated with engines. (car,motorcycle,lawn-mower)  The term horsepower was developed to quantify power. A strong horse could move a 750 N object one meter in one second. 750 N

Group Work  P. 416 Q 1-3

Group Work 1.What conditions must exist in order for a force to do work on an object?  Some of the force must act in the same direction as the object moves 2.What formula relates work and power?  Power = work/time 3.How much work is done when a vertical force acts on an object moving horizontally?  The supporting force does no work because the object it acts on does not move

Work – vs - Power  Write in your bound composition notebook the difference in work and power.  Dr. Skateboard video

Machines  A device that makes work easier.  A machine can change the size, the direction, or the distance over which a force acts.

Forces involved:  Input Force FIFIFIFI  Force applied to a machine  Output Force FOFO  Force applied by a machine

Two forces, thus two types of work  Work  Work Input work work done on on a machine =Input =Input force x the distance through which that force acts (input distance)  Work  Work Output Work Work done by by a machine =Output =Output force x the distance through which the resistance moves (output distance)

Can you get more work out than you put in?  Work output can never be greater than work input.

Group Questions p. 420 Q 1-4

p. 420 Questions 1.How can using a machine make a task easier to perform? 2. How does the work done on a machine compare to the work done by a machine? 3.What changes can a machine make to a force? 4. A machine produces a larger force than you exert to operate the machine. How does the input distance of the machine compare to its output distance?

p. 420 Answers 1.How can using a machine make a task easier to perform? By altering the size of force needed, the Direction of the force, or the distance over Which a force acts. 2. How does the work done on a machine compare to the work done by a machine? Because of friction, the work done by a machine Always less than the work done on the machine. 3.What changes can a machine make to a force? Size of force, direction of force, distance through which a force acts 4. A machine produces a larger force than you exert to operate the machine. How does the input distance of the machine compare to its output distance? Because the output force is greater than the input force, the input distance must be greater than the output distance

Mechanical Advantage (MA) – expressed in a ratio WITH NO UNITS!!  The number of times a machine multiplies the input force.

2 types of mechanical advantage  IDEAL  Involves no friction.  Is calculated differently for different machines  Usually input distance/output distance  ACTUAL  Involves friction.  Calculated the same for all machines

Different mechanical advantages:  IDEAL  MA  MA equal to one. (output force = input force)  Change  Change the direction of the applied force only.  ACTUAL  Mechanical advantage less than one  An  An increase in the distance an object is moved (d o ) Eureka Episode 13: Mechanical Advantage & Friction

Efficiency  Efficiency can never be greater than 100 %. Why?  Some work is always needed to overcome friction.  A percentage comparison of work output to work input.  work output (W O ) / work input (W I )

Independent Work  P. 426 Q 1-3  5 minutes  Work on Study Guide questions when done.

p. 426 Questions 1.Why is the actual mechanical advantage of a machine always less than its ideal mechanical advantage? 2.Why can no machine be 100% efficient? 3.You test a machine and find that it exerts a force of 5 N for each 1 N of force you exert operating the machine. What is the actual mechanical advantage of the machine?

p. 426 Answers 1.Why is the actual mechanical advantage of a machine always less than its ideal mechanical advantage?  Friction 2.Why can no machine be 100% efficient?  Because of friction 3.You test a machine and find that it exerts a force of 5 N for each 1 N of force you exert operating the machine. What is the actual mechanical advantage of the machine?  5N/1N= 5

Work Practice Problem   Renatta Gass is out with her friends. Misfortune occurs and Renatta and her friends find themselves getting a workout. They apply a cumulative force of 1080 N to push the car 218 m to the nearest fuel station. Determine the work done on the car.   235,440 J

1. The Lever  A bar that is free to pivot, or move about a fixed point when an input force is applied.  Fulcrum = the pivot point of a lever.  There are three classes of levers based on the positioning of the effort force, resistance force, and fulcrum.

First Class Levers  Fulcrum is located between the effort and resistance.  Makes work easier by multiplying the effort force AND changing direction.  Examples:

Second Class Levers  Resistance is found between the fulcrum and effort force.  Makes work easier by multiplying the effort force, but NOT changing direction.  Examples:

Third Class Levers  Effort  Effort force is located between the resistance force and the fulcrum.  Does  Does NOT multiply the effort force, only multiplies the distance.  Examples:

Levers!!!!!!!!!!!  Draw these

Mechanical advantage of levers. IIIIdeal = input arm length/output arm length iiiinput arm = distance from input force to the fulcrum ooooutput arm = distance from output force to the fulcrum Eureka Episode 12 – The Lever

2. The Wheel and Axle  A lever that rotates in a circle.  A combination of two wheels of different sizes.  Smaller wheel is termed the axle.  IMA = radius of wheel/radius of axle.

3. The Inclined Plane  A slanted surface used to raise an object.  Examples: ramps, stairs, ladders  IMA = length of ramp/height of ramp Can never be less than one.

4. The Wedge  An inclined plane that moves.  Examples: knife, axe, razor blade  Mechanical advantage is increased by sharpening it.

5. The Screw  An inclined plane wrapped around a cylinder.  The closer the threads, the greater the mechanical advantage  Examples: bolts, augers, drill bits Eureka Episode 14 – The Screw & The Wheel

6. The Pulley  A chain, belt, or rope wrapped around a wheel.  Can either change the direction or the amount of effort force  Ex. Flag pole, blinds, stage curtain

Pulley types FFFFIXED CCCCan only change the direction of a force. MMMMA = 1 MMMMOVABLE CCCCan multiply an effort force, but cannot change direction. MMMMA > 1

MA = Count # of ropes that apply an upward force (note the block and tackle!) Fe

 A combination of two or more simple machines.  Cannot get more work out of a compound machine than is put in.

 P. 435 Q 1-4 OK GO – Rube Goldberg

p. 435 Answers LeverWheel & axle Incline plane wedgepulleyscrew crowbarScrew driver rampaxeFlag poleDrill bits Input distance/ output distance Input radius /output radius Distance along plane/ change in height Distance along wedge / change in wedge thickness Number of ropes Distance rotated / distance advanced 1 & 2 3. Wheel and axle is like a lever where the fulcrum is located at the center of two concentric rotating cylinders 4. IMA= input distance / output distance; 4/.5=8