Forces in Machines(Chapter 4)

Mr. Leavings

What are we going to Learn?

Describe and Explain a simple machine

Apply the concept of input force and output force

Determine the mechanical advantage of different machines

Mechanical Systems and Machines

What is a machine?A Device with moving parts that work together to accomplish a task.

Input and Output

The input includes everything you do to make the machine accomplish a task, like pushing on the bicycle pedals. The output is what the machine does for you, like going fast or climbing a steep hill

Simple Machines

A simple machine is an unpowered mechanical device, such as a lever.

Mechanical Advantage

Mechanical Advantage (MA): the ratio of output force to the input force.

MA = ____Foutput

Finput

Mechanical Advantage

Lets do an example!

On the board

Levers

Input and output forces for levers are related to the positioning of the fulcrum.

Fulcrum

LeversThe Mechanical Advantage in a Lever is a ratio of the lengths of the input and output arms.

Input Force/Arm

Output Force/ Arm

If the input arm is 5 times the output then the MA is 5.

LeversThere are three “classes” of levers. These are based on the location of the input and output in relation to the fulcrum.

1st Class

2nd Class

3rd Class

Fout

Fout

Fout

Fin

Fin

Fin

Block and Tackle (pulleys)

Ropes and strings carry tension forces along their length. They always act in the direction of the pulling force, this would be obvious if you ever tried to push a rope!

The force in a rope is the same everywhere, if you cut a rope and insert a tension meter it would read the same at all points.

Block and Tackle (pulleys)

For each rope supporting the load it feels the force that you input.

In this case you have two supporting ropes. Your input force is 5 N, what would the output force be? What is the Mechanical Advantage (MA)?

Output: 10 NMA: 2

Block and Tackle (pulleys)

GearsWhy are gears used in machines?

The transmission of rotating motion from one place to another!

You can transmit more force between gears than two smooth wheels because the teeth help prevent slippage.

GearsThe rule for how gears turn depends on the number of teeth in the gears. If one gear has 36 teeth and it turns once then an attached 12 tooth gear would have to turn 3 times to match the 36 teeth of the first gear.

361 turn =36

12 teeth

36/12 = 3 turns

Gears

Gear Turn Ratio

To = NiTi No

Turns of output gear

Turns of input gear Number of teethon output gear

Number of teeth on input gear

Gears