Work and Machines

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?