Three Classes of Lever

First-class levers

First class lever

A first-class lever is a lever in which the fulcrum is located between the input effort and the output load. In operation, a force is applied (by pulling or pushing) to a section of the bar, which causes the lever to swing about the fulcrum, overcoming the resistance force on the opposite side. The fulcrum may be at the center point of the lever as in a seesaw or at any point between the input and output. This supports the effort arm and the load.

Examples:

1. Beam engine although here the aim is just to change the direction in which the applied force acts, since the fulcrum is normally in the centre of the beam (i.e. D1 = D2)
2. Bicycle hand brakes
3. Can opener and bottle opener
4. Crowbar
5. Curb bit
6. Hammer, when pulling a nail with the hammer's claw
7. Hand trucks are L-shaped but work on the same principle, with the axis as a fulcrum
8. Oars
9. Pliers (double lever)
10. Scissors (double lever)
11. Seesaw (also known as a teeter-totter)
12. Shoehorn
13. Spud bar (moving heavy objects)
14. Trebuchet, an upside down example of the above picture
15. Wheel and axle because the wheel's motions follows the fulcrum, load arm, and effort arm principle

Second-class levers

Second class lever

In a second class lever the input effort is located at one end of the bar and the fulcrum is located at the other end of the bar, opposite to the input, with the output load at a point between these two forces. Examples:

1. Dental elevator
2. Door
3. Nutcracker
4. Paddle
5. Springboard (diving board)
6. Wheelbarrow
7. Wrench
8. Diving Board

Third-class levers

Third class lever. For the lever in this diagram to work correctly, one must assume that the fulcrum is attached to the bar.

For this class of levers, the input effort is higher than the output load, which is different from second-class levers and some first-class levers. However, the distance moved by the resistance (load) is greater than the distance moved by the effort. Since this motion occurs in the same length of time, the resistance necessarily moves faster than the effort. Thus, a third-class lever still has its uses in making certain tasks easier to do. In third class levers, effort is applied between the output load on one end and the fulcrum on the opposite end.

Examples:

1. Arm
2. Baseball bat
3. Boat paddle
4. Broom
5. Chopsticks
6. Door
7. Fishing rod
8. Hockey stick
9. Tongs
10. Tweezers
11. Mandible
12. Mousetrap
13. Nail clippers, the main body handle exerts the incoming force
14. Shovel (the action of picking or lifting up sand or dirt)
15. Sling
16. Tools, such as a hoe or scythe
17. Stapler

source: http://www.wikipedia.org