1. Ch. 5.2 Efficiency

2. Efficiency efficiency – measurement of the useful work done by a machine compared to the work needed to operate the machine useful output work – work that a machine is designed to perform

3. Bicycle Example a bicycle is designed to move forward the bicycle’s useful output work is determined by measuring the bicycle’s forward motion the input work is the work done by the person moving the pedals for mechanisms like the bicycle, the useful output work is always less than the input work WHERE DOES THAT EXTRA ENERGY GO?

4. Work Done by Friction whenever a machine is used to do work, parts of the machine are moving these moving parts produce friction since this force of friction is applied to a distance of motion, work is done by the force of friction

5. Work Done by Friction work done by the force of friction transforms input energy into heat therefore, extra work must be input into the machine to compensate for the work done by friction for this reason, the useful output work of a machine is ALWAYS less than the input work

6. Efficiency and Friction highly efficient machines have less friction and therefore produce less heat from friction – more of the input work is changed into useful output work An ideal machine would have no friction, allowing all the input work to be converted to output work – this does not exist, so we aim to make out machines as efficient as possible

7. Calculating Efficiency Efficiency = OR Efficiency = useful output work (joules) x 100% input work (joules) W out x 100% W in

8. Calculating Efficiency For example, if a machine is capable of doing 35 J or work when 50 J is put into the machine, the efficiency of that machine is: Efficiency = Efficiency = = 70% W out x 100% W in (35 J) x 100% (50J) So, 70% of the work put into the machine goes into doing work that the machine was designed for. The other 30% of the input work goes into other forms of energy.

9. Calculating Efficiency If a 500-N crate is moved up a 5.0-m-long ramp, what is the efficiency of that ramp if the person pushes the crate with a force of 400 N in order to raise the crate a vertical distance of 2.0 m? – we must first calculate the useful output work and the input work: W out = Fd = (500 N)(2.0 m) = 1,000 J W in = Fd = (400 N)(5.0 m) = 2,000 J Efficiency = = Efficiency = 50% W out x 100% W in (1,000 J)x 100% (2,000 J)

10. Efficiency of Common Mechanisms when a mechanism does work, its energy is transformed from one form to another or from one object to another car example: – a car transforms the chemical energy stored in its fuel into several other forms of energy, such as kinetic energy, sound energy, light energy, and thermal energy – since the main purpose of a car is transportation, the useful output work of the car would be the work done to provide motion (kinetic energy)

11. Efficiency of Common Mechanisms MechanismEfficiency (%) electric generator99 Olympic track bike98 mountain bike85 hydrid-diesel car45 electric car44 hydrid-gasoline car36 conventional gas-powered car22 solar cell10

12. Increasing Efficiency to increase efficiency in most mechanisms, we try to decrease the friction produced by that mechanism – this is often done by lubricating the parts of the mechanism with oil or grease – lubricants fill the gaps between two surfaces, making it easier for those surfaces to slide past each other

13. Increasing Efficiency efficiency cannot always be increased by a lubricant – example: thermal energy produced by a light bulb an incandescent bulb operates at 175 o C, but only 5% of the electrical energy is transformed into light energy (very inefficient) a compact fluorescent lamp (CFL) operate at 30 o C, converting less electrical energy to heat