1. ## Mechanical Principles

Below are some mechanical principles to help you on what many find difficult in the exam process

MECHANICAL PRINCIPLES

PULLEYS:

1. The more pulleys, the easier it is to pull or lift an object.
2. The more pulleys involved, the greater distance you must pull, but it is still easier to lift an object.
3. The thinner a windlass, the easier it is to turn.
4. In two different sets of pulleys, if the wheels are connected by a shaft, and the two wheels on one pulley are the same as the other two that they are connected to, then they both turn at the same speed.

BELTS:

1. Always determine in which direction one of the wheels in a diagram is turning, as the belt will be going in the same direction. Also, you can determine the direction of the belt and the wheel direction will be the same.
2. Wheels under a belt that is not twisted all turn in the same direction. Those on the outside of the same belt would turn in the opposite direction of those on the inside.

WHEELS:

1. If wheels are a different size on the same vehicle, then the smaller wheel will turn faster.
2. When wheels of different sizes are joined together by belts, the smallest wheel turns fastest, the largest wheel slowest.
3. When two gears of different sizes are locked together, the smaller gear turns faster than the shaft connected to the larger gear.

TURNING/DIRECTIONS:

1. The faster an object whirls around, the more it will pull from the center of rotation.
2. If a car or tractor or objects are turning, then the inside wheels or objects will turn less distance and more slowly than the outside ones.
3. When a car skids, its speed increases momentarily to the outside when turning.

CENTER OF GRAVITY: (referring to the point which weight is evenly distributed)

1. A solid object with a space drilled out will rest on the section that is solid.
2. The higher a vehicle is packed with materials, the easier it will turn over when on an incline.

VOLUMES AND AREAS OF SOLID OBJECTS:

1. If several solid figures have the same width and height but different shapes, then their weight and volumes are different. The lowest weight or least volume is a solid of triangular shape. Then comes a cylindrical solid (circular in shape) and then a cube (square shape).
2. Objects (cars) placed or parked parallel to each other and perpendicular to the side occupy less space.

SIX PRINCIPLES OF FLUID PRESSURE:

1. Liquid pressure is exerted in a perpendicular direction to any surface on which it acts.
2. At any given point beneath the surface of a liquid, the pressure is the same in all directions--downward, upward and sideways.
3. Pressure applied to a confined liquid from without is transmitted in all directions without diminution (reduction in intensity).
4. The pressure of a liquid in an open vessel is proportional to the depth of the liquid.
5. The pressure of a liquid in an open vessel is proportional to the density of the liquid.
6. Liquid pressure on the bottom of a vessel is unaffected by the size and shape of the vessel.

Assistant Chief Brent Collins, Cleveland Fire Dept.
Firehouse.com Entry Level Contributor Author since 2002
http://cms.firehouse.com/content/con.../bio.jsp?id=17

www.fireprep.com
1-800-989-FIRE

2. Thank you for posting, Chief Collins. Was wondering if you or anyone else could help me with the following one I always get hung-up on.

(I checked the entire post, especially the belts and wheels, but didn't see exactly what I am looking for)

You have a series of two sets of gears connected by a belt. The gears on the right (the drive gears) are typically larger than the gears on the left (the driven gears) Think of it like gears on a bicycle with the pedals driving the drive gears.

The question asks something to the effect of "which combination of gears will result in the fastest rate of speed?" I typically select the combination with the largest drive gear and the smallest driven gear since one revolution of the large drive gear may result in more revolutions of the smaller driven gear. However, for some reason I feel like I get it wrong (we never get to see the results).

Any and all help would be appreciated!

Also, does anyone have any recommendations on the good mechanical principles books that aren't firefighter exam test prep books (i.e. ASVAB prep, etc)?

3. i had some questions too, which cable supports the sign the best and why? http://ratpimples.homestead.com/files/cablesupport.jpg

how much weight is needed to lift in direction of the arrow and why (those are pullies in the picture)?

and which pulley is going the fastest and why?

there are two different size pullies the 4 small and the 2 big on the right..

thanks...

4. Originally Posted by BigBlackWoman
i had some questions too, which cable supports the sign the best and why? http://ratpimples.homestead.com/files/cablesupport.jpg
CABLE 'A' Because gravity wants to pull the sign down and this cable supports more in the opposite direction rather than in a lateral direction.

Originally Posted by BigBlackWoman
how much weight is needed to lift in direction of the arrow and why (those are pullies in the picture)?
100lbs. as there is no mechanical advantage.

Originally Posted by BigBlackWoman
and which pulley is going the fastest and why?

The upper left on. To explain with simple numbers and working counter-clockwise from the lower left...assuming the smaller pulleys have a circumference that is 1/2 of the larger pulleys and assuming the lower left one is turning at 10rpm and the smaller one attached via belt is turning the same rpm, then the larger one (lower right) is also turning at 10rpm becuase it is fixed to he smaller one; however, it covers 2 times the distance. That distance is translated, via belt, to the smaller pulley, which is spinning at 20rpm. Again, attached to the larger pulley, also at 20rpm, which translates via belt to the smaller puller spinning 40rpm to cover the same distance as the larger pulley at 20rpm.

Hope all this makes sense?!?

Great examples!!!

5. Originally Posted by goingcamping
100lbs. as there is no mechanical advantage.
you don't half the force if its going through two pulleys? why is there no mechanical advantage? thanks for your thoughts.

6. Originally Posted by BigBlackWoman
you don't half the force if its going through two pulleys? why is there no mechanical advantage? thanks for your thoughts.
No, because you're not using the pulleys to gain advantage only to change directions. Hard to explain easier with a picture, anyway, if you have access to ropes and pulleys, set it up like the picture and play around with it (doesn't need to be 100lbs to get the priciple right). It should make more sense then. Most pulleys used have 2 channels for the ropes...so attach the pulley to a fixed anchor, then run the rope through down to the weight. Should be exactly the same amount only in stead of pulling to weight directly up, you're pulling down on the rope to move the weight up, a 1:1 ratio so, for every foot you move the rope the weight will move the exact same foot in a different direction. Now throw a second pulley in the mix and attached it directly to the weight then run the rope through the first pulley down through and around the second, lower, pulley back to a fixed postition on the first pulley, upper, you've now established a 1:2 ratio for mechanical advantage for every foot you pull the rope the weight will move half the distance and the amount of force you must exert will be less (can't remember if it is half, but assuming all pulleys are the same diameter I think it is?!? of course to be really anal, one might take into account friction, however minor ir may be!)
Hope my memory serves me well in this one as far as numbers go.

SEE BELOW
http://i686.photobucket.com/albums/v...agePulleys.jpg

Ciao!

7. Originally Posted by BigBlackWoman
you don't half the force if its going through two pulleys? why is there no mechanical advantage? thanks for your thoughts.
That depends on the configuration of the pulleys. In this setup, there is no mechanical advantage.

If the cable is pulled up 1ft, the weight will move up 1ft. In order for there to be a mechanical advantage, the weight would have to move less than the cable is pulled. To do this, one of the pulleys would have to be connected to the weight itself.

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