1. Energy, Work and Mechanical Efficiency
How do you measure mechanical efficiency?

2. Mechanical Efficiency
Mechanical efficiency is a comparison of the useful work or energy that a mechanical system produces to the work or energy that was put into the system.
Efficiency is written as a percentage, and can be calculated as follows:
mechanical efficiency (%) = _output energy (or work)_ × 100 %
input energy (or work)
We want machines to have a high level of efficiency so that we waste less energy performing everyday tasks.

3. Mechanical Efficiency
No machine is 100 % efficient.
Even simple machines experience small amounts of friction and slippage.
Complex machines are made of many simple machines, so the friction and slippage accumulates, making the complex machine less efficient than its individual parts.
Energy can be lost from slippage or friction.
levers
pulley
Wheels and axles

4. Pulleys Many work sites use pulleys to move objects.
For example, a crane has a pulley system.
This pulley system can be used to lift heavy objects to great heights.
A pulley system that is old and rusty will not be very efficient. (friction)
A new pulley system that moves easily is more efficient.

5. Levers Levers are usually quite efficient.
When the bar is rigid and the fulcrum is hard and slightly pointed or rounded, there is minimal friction and little energy is lost.
However, sometimes even simple machines are inefficient.

8. CHECK YOUR LEARNING
Use a Venn diagram to compare the everyday meaning of efficiency from last lesson and its scientific meaning from this lesson.
How efficient is a pulley system if an operator has to pull 4.0 m of rope to raise a 250 N pail of water a distance of 2.0 m? The operator is pulling with a force of 150 N.
A block and tackle is a rope and pulley system that uses multiple pulleys. It multiplies a small force into a large one. If a motor has a weight of about 2500 N and people pull 4.0 m of rope with a force of 700 N to raise the motor 1.0 m, how efficient is the block and tackle?
Name something you can do to increase the efficiency of your bicycle.