1. Work and Machines

2. Machines Do Work  A Machine is a device that changes a force.  Ex. A Jack used to change a tire.  Machines make work easier to do.  They change the size of a force needed  The direction of a force  The distance over which a force acts

3. Machines Do Work  The more Force applied, the shorter the overall distance.  Ex. Carry 5 books at the same time from Mrs. Larose's class to Mrs. Hendee’s class.  The less force applied, the longer the overall distance.  Ex. Carry one book at a time from Mrs. Larose's class to Mrs. Hendee’s class

4. Machines Do Work  Change in direction of applied force.  Ex. Pulling back on the handle of a oar causes its other end to move in opposite direction.  If the oar is pushed father away from the boat, how will the force needed to pull the oar through the water change?

5. Work Input and Work Output  A rower pulls back on each oar handle and the other end of the oar pushes against the water.  Work is done on the oar (Machine) by pulling on them, and the oars do work on the water to move the boat.  Because of friction, the work done by a machine is always less than the work done on the machine

6. Work Input and Work Output  The force you exert on a machine is called input force.  The distance the input force acts through is called input distance.  The work done by the input force acting through the input distance is called work input.  Formula:  Work input= Input force x input distance

7. Work output of a Machine  The force that is exerted by a machine is called output force.  The distance the output force is exerted through is called output distance.  Formula:  Work output= output force x output distance

8. Work Output of a Machine  If there is no change in work input, there cannot be an increase in the work output.  You cannot get more work out of a machine than you put into it!

9. Mechanical Advantage  The mechanical advantage of a machine is the number of times that the machine increases an input force.  The relation of input force used to operate a machine and the output force exerted by the machine depends on the type of machine and how it is used.  Ex. A nutcracker

10. Actual Mechanical Advantage  The mechanical advantage determined by measuring the actual forces acting on a machine is the actual mechanical advantage  Formula:  Actual mechanical advantage= Output force Input force Ex. Long incline ramp

11. Ideal Mechanical Advantage  The ideal mechanical advantage of a machine is the mechanical advantage in the absence of friction.  Because friction is always present, the actual mechanical advantage of a machine is always less than the ideal mechanical advantage.  Formula:  Ideal mechanical advantage= Input distance Output distance

12. Calculating IMA  A woman drives her car up onto wheel ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the ideal mechanical advantage of the wheel ramp?

13. Efficiency  The percentage of the work input that becomes work output is the efficiency of a machine.  Because there is always friction, the efficiency of any machine is always less than 100%.  The work output of a machine is always less than the work input.  Formula:  Efficiency= (Work input / Work output) x 100%

14. Calculating Efficiency  You have just designed a machine that uses 1000J of work from a motor for every 800J of useful work the machine supplies. What is the efficiency of your machine?