Science Buddies: "Ask an Expert"

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I've been trying to alleviate some loudness in a garage door, and this seems to be an answer, but I don't know exactly why.

From the Manual, Chamberlain LiftMaster 3280 - page 11 . . . http://i.imgur.com/Mqg4J1s.gif

A 2x4 laid flat is convenient for setting an ideal door-to-rail distance.

The top of the door should be level with the top of the motor unit. Do not position the opener more than 4" (10 cm) above this point.

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Question:

What physics is going on here, with the angle of the garage door opener rail?

What are the options for the rail?

1.) angled down - as seen in the manual, this is how you want it, but specifically, why?

2.) horizontal - I think this would make it harder to pull, than if angled down, and wear out the motor faster?

3.) angled up - I think this would make it harder to pull, than if angled down, and wear out the motor faster?

Some explanations that I've gotten:

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Your opener appears to be installed almost level with the garage door, rather than angled backward.
That will cause the jerky/noisy operation when you close the door.
Go to your local home improvement store, and buy some angle iron, and lower the power head.
(See the section in your manual titled "position the opener")
When the door is fully opened, the opener rail should be max 2" above the edge of the top section.

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Friction - and the fact that when you reduce the normal force on the rollers / rods, yes the engine works harder but the friction is reduced. It's called the normal force.

Ff=uFN
Force friction = coefficient of friction x Force normal
http://www.mrfizzix.com/weightlifting/frict.htm

The greater the angle, the less the normal force, and thus the less friction, and less noise, unless you have a loud motor.

When angled down, the normal force is reduced, which in turn reduces the friction, and sound. The equation above shows that the friction is reduced.

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Inclined Plane

http://en.wikipedia.org/wiki/Inclined_plane

I wonder why they don't say to put the header bracket 1 foot higher than where the motor will be, maybe a greater angle would be a benefit.

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Pulley

http://en.wikipedia.org/wiki/Pulley

Do you have a link to the door opener manual, or a link showing a diagram of what you are describing?

Yes, on this page . . . the LiftMaster® Model 3280 . . . please see page 11

http://chamberlain.custhelp.com/app/answers/detail/a_id/1033/~/liftmaster%C2%AE-model-3280,-3280-267-owners-manual

I also uploaded a screenshot of page 11 to imgur.com, link is above

I think it has to do with Force, friction, and the pulley. Springs are also involved.

I don't think the Inclined Plane has anything to do with it. That's about moving an object from lower to higher, or the reverse.

It's a task just to get your mind wrapped around simply what elements are involved.

Another question would be, how do you determine the best angle for the belt or chain / rail? Or, is there a range that works? And why?

I think this must be like a pulley.

Why, exactly, does angling it down make it work better than, say, having the force pull parallel to the ground.

It's a 1/2 horsepower motor, so the Force equals 1/2 horsepower.

What equation uses Force, and an angle, to determine . . . ? . . . what . . . work? Is that it? This is not an easy question to answer. I wonder if this page could do an example with a garage door. Maybe someone else has it.

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http://www.physicsclassroom.com/class/energy/Lesson-1/Calculating-the-Amount-of-Work-Done-by-Forces

"In a previous part of Lesson 1, work was described as taking place when a force acts upon an object to cause a displacement. When a force acts to cause an object to be displaced, three quantities must be known in order to calculate the work. Those three quantities are force, displacement and the angle between the force and the displacement. The work is subsequently calculated as force•displacement•cosine(theta) where theta is the angle between the force and the displacement vectors. In this part of Lesson 1, the concepts and mathematics of work will be applied in order to analyze a variety of physical situations."