WORK,+POWER+AND+MACHINES

**Work**
 * Work - A force acting through a distance.**
 * The force must cause motion in the direction of the force for work to be done.**


 * Ex: If a student pushes a wall with all of his strength - he has done no work on the wall (if the wall does not move) [[image:http://www.glynn.k12.ga.us/%7Epmcveigh/Resources/Image35.gif width="141" height="128" align="top"]]**
 * Also a student carrying a book does NO work on the book because the force and motion are NOT in the same direction.[[image:http://www.glynn.k12.ga.us/%7Epmcveigh/Resources/Image36.gif width="138" height="112" align="middle"]]**
 * Any time a mass is lifted upward work is done. The force is the weight of the object.**
 * If a person goes up a ladder or a flight of stairs, the force is the weight of the person and the distance is the vertical distance when calculating work**

**Work is calculated:**


 * W = Fd - remember the d must be in the same direction as the force.**
 * The unit for work is the newton -meter. This is called the Joule named after James Prescott Joule.**

=Fd= (10N)(5m) = 50 J =(80 n)(5 m)= 400 J
 * Ex 1. What is the work when a force of 10 N pulls a 80 N box a distance of 5.0 m across the floor?**
 * W**
 * Ex 2: What is the work done if the same box is lifted 5 m?[[image:http://www.glynn.k12.ga.us/%7Epmcveigh/Resources/Image39.gif width="124" height="164" align="top"]]**
 * W**
 * Ex 3: What is the work done on the box if a 10 N force applied at an angle of 30 degrees pulls the box 5.0 m across the floor?**
 * This time the force is not in the same direction as the movement. Therefore, you must find the horizontal component of the F (Fx) to determine the work. **



=(8.66 N)(5.0m)= 43.3 J
 * Fx = (cos 30)(10 N) = 8.66 N **
 * W **

**Power** =Fd= (70N)(25 m) = 1750 J =1750 J/3.5 s= 500 W or .5 kW **Machines** 1. changing the size of the force 2. changing the direction of the force 3. Increase the speed or distance an object is moved**
 * Power is the rate of doing work or how fast the work is done.**
 * To calculate power**
 * P = W/t**
 * The unit of power is J/s which is renamed the watt after James Watt.**
 * Ex. If a student weighing 70 N runs up a flight of steps 25 m high in 3.5 seconds, what is his power?**
 * W**
 * P**
 * Machines make jobs easier by:

**Mechanical Advantage (MA) - the number of times a machine multiplies the effort force** **Actual Mechanical Advantage (AMA)** **If you have perfect machine then you would have Ideal Mechanical Advantage (IMA)** **The efficiency of a machine can be found in two ways** **The Six Simple Machines** **Inclined Plane The IMA of inclined plane is found by dividing its length by height.** because your force acts through a longer distance.** 1500 n onto the back of a truck 2 m off the ground. A force of 900n was needed to push the piano up the inclined plane. (A) What was the work in?**
 * Effort force (E) - the force a person puts into a machine**
 * Resistance force - the force that must be overcome to do the work**
 * AMA = R/E**
 * eff = AMA/IMA x 100%**
 * eff = work out/work in x 100%**
 * Work in = Effort X effort distance**
 * Work out = Resistance X R distance**
 * A simple machine do work with one movement.**
 * There are two groups of similar machines**
 * 1. Inclined plane, wedge, screw**
 * 2. Lever, pulley, wheel and axle**
 * The longer it is the more MA you get, but you have to do more work,
 * Example: An inclined plane 4 m long is used to lift a piano weighing


 * W in = 900n x 4 m = 3600 J**


 * (B) What was the work out?**


 * Wout = 1500 n x 2 m = 3000 J**

plane? IMA = length/height = 4/2=2 (D) What was the AMA of the inclined plane?**
 * (C) What was the IMA of the inclined


 * AMA = R/E = 1500 n/900n = 1.67**


 * (E) What was the efficiency?**


 * eff = 1.67/2 = 83.5% or**
 * eff = 3000 J/ 3600 J = 83.3 %**

**Lever The IMA of a lever is determined by dividing the effort arm by the resistance arm. The resistance arm is the distance from the location of the resistance to the fulcrum.

The effort arm is the distance from the location of the effort to the fulcrum**


 * IMA = EA/RA**

**Pulley The pulley is just a round lever with a fixed fulcrum in the middle. The effort arm and the resistance arm are equal in length so the MA of a round fixed pulley is always 1. The fixed pulley does not increase your MA but it does change the direction of the force making it easier to lift objects. A movable pulley can give you a MA of 2 because two ropes support the resistance.** pulleys) The IMA of any pulley system can be determined by counting the number of ropes supporting the resistance. You can also divide the distance the rope is pulled by the distance the resistance rises.**
 * Pulley systems (Combinations of

**Wheel and Axle A wheel and axle is similar to a pulley but the resistance arm and the effort arm are not equal lengths. The radius of the wheel is the effort arm. The radius of the axle is the resistance arm. Since the radius of the wheel is always larger than the radius of the axle the MA is always greater than 1.** by dividing the radius of the wheel by the radius of the axle.**
 * The IMA of a wheel and axle is found
 * IMA =r of wheel/r of axle

[|Refresh your memory here] [|Practice for your test]**