# Machines Feedback

by Ron Kurtus

Readers have sent in a total of 29 comments and questions on Machines issues. They are listed according to date.

## List of most recent 15 letters

### Country

Class of lever of pressure cooker Three Lever Classes USA

When is change significant Mechanical Advantage USA

Materials for levers Levers India

Efficiency of a screw jack Efficiency Ghana

Increasing efficiency through use Efficiency Nigeria

Finding center of gravity of a small machine General Australia

What is the physics behind force and a lever? Levers USA

Need correction on Class 3 lever Levers USA

Formula for calculating efficiency of a machine Efficiency Nigeria

Why is a class one lever more efficient? Levers Pakistan

Using a lever in dental braces Levers USA

Next 15 letters

## Class of lever of pressure cooker

January 5, 2018

### Question

Pressure cooker is which type of class lever? Can we say pressure cooker simple machine?

Bhima - USA (28376)

Since a pressure cooker has no moving parts,, it could be considered a form of simple machine, where heat creates an internal pressure.

However, it is not a lever and really has no moving parts. But the heat can create pressure and a force.

## When is change significant

September 19, 2017

### Question

Is there a point at which a change in mechanical advantage is considered to be empirically significant? +/-5%? +/-7%?

Greg - USA (28245)

The actual mechanical advantage of a given configuration can have a tolerance variation according to how accurate it is measured. This would have to be taken into consideration when seeing how significant a change would be. I really don't have any numbers, because it depends so much on the configuration.

## Materials for levers

March 8, 2017

### Question

With what material mechanical lvers are made up of?
And what are the operation of mechanical levers?

Thank you :)

Chinmoy - India (27933)

A lever can be made of any material that is solid and strong enough to withstand a force. This includes some board used to lift an object on a pair of pliers. Look at the examples on the lever pages.

## Efficiency of a screw jack

January 5, 2017

### Question

A screw jack is being used to support the axle of a tractor,the load on it being 2.4 KN.The screw jack has an effort of effective radius 200 mm and a single-start square thread,having a load of 5 mm.Determine the efficiency of the jack if an effort of 60 N is required to raise the tractor axle.

DAASI - Ghana (27787)

Efficiency is the output work divided by the input work. Work is defined as the force times the distance moved.

The input force is 60 N and the output force is 2.4 KN. According to your configuration, you need to determine how far the input and output forces move to determine the work ratio.

## Increase mechanical advantage in simple machine

January 4, 2017

### Question

How can mechanical advantage of simple machines be increased

mercy sharleen - Uganda (27782)

It depends on the type of simple machine. In a lever, you can change the distance from the fulcrum to change the mechanical advantage. See Mechanical Advantage Relationships

## Increasing efficiency through use

December 26, 2016

### Question

Can the efficiency of a machine increases through consistent use or frequently ?

Felix - Nigeria (27581)

Factors that can reduce the efficiency of a machine are friction and heat build-up. In some machines, friction may be increased if it is not used often. This can be due to oxidation of the surfaces. In such a case, using the machine more often can reduce or eliminate this type of friction, thus increasing its mechanical efficiency.

## Finding center of gravity of a small machine

October 31, 2016

### Question

Hi School of Champions

I've built a small machine in my home workshop that is a little heavy to rotate about an axis, before I redesign it in cad I need to calculate the center of gravity & force required to rotate it based on the current design and compare it to a revised cad design.

I can send a simple image that shows the problem.

Thank you, your help here would be much appreciated.

Darryl - Australia (27428)

The Center of Gravity article shows ways to find the COG. However, that might not be practical in your situation. But it might give you some ideas.

If you could hang the machine from a wire, you could find the COG. Rotating it from the wire with no friction would depend on its mass.

## What do you lose with force advantage?

September 28, 2016

### Question

When you gain a force advantage you usually lose what?

Sphilile - SA (27334)

The force mechanical advantage requires a greater input distance than the resulting output distance.

## What is the physics behind force and a lever?

September 17, 2016

### Question

Can you explain the physics behind the lever ? I know how to calculate, and understand it is the product of force*length, but I don't understand the force distribution(?) of why the force is increased due to the length ?
What is the mechanism that makes it works ?

Thank you,
Roy A

Roy - USA (27294)

That is a very good question, and it is not often mentioned in textbooks.

The relationship between forces and lengths of the lever arms is dependent on the Law of Conservation of Work and Energy. Work is defined as the force times the distance moved.

I updated several pages to show the derivation of the equation. See Increasing Force with a Lever.

I hope that helps your understanding.

## Need correction on Class 3 lever

July 24, 2016

### Question

Please reverse the location of the fulcrum for the 3rd class lever. It should be above the lever, rather than below it. The lever will not work as illustrated. Unlike a 2nd class lever, there's no support there.

Clyde - USA (27167)

Thanks for pointing that out to me. It was very perceptive of you to notice that. Many resources on Class 3 levers simply place the fulcrum at the bottom.

I updated Levers are Simple Machines and to show requirements for the Class 3 lever fulcrum.

I hope that corrects the issue.

## When is mechanical advantage equal to 1?

July 18, 2016

### Question

in what condition MA<1 MA>1 and MA=1 ?

bhabin - Nepal (27154)

The mechanical advantage (MA) equals 1 when there is no increase in the output compared to the input. An example is a pulley, where you simply change the direction of the object when you pull on the rope.

## Formula for calculating efficiency of a machine

May 29, 2016

### Question

Formular for calculating efficiency of a machine

fatosa - Nigeria (27035)

The formula for the efficiency of a machine is E = Wo/Wi where Wo is the output work and Wi is the input work.

The output work is always less than the input work, because of losses due to friction and heat.

The work equals force times distance Wo = F*d.

## Mechanical advantage greater than 1

April 26, 2016

### Question

When machanical advantage of a machine is greater than 1,what is the implication ? Is the machine a good machine or a bad machine ?

jimoh - Nigeria (26966)

You want the machine to have the force mechanical advantage to be greater than 1. That way you can lift a greater load than your effort.

## Why is a class one lever more efficient?

January 10, 2016

### Question

why class one lever is more efficient

sneha - Pakistan (26676)

Efficiency is related to the distance that the object is moved divided by the distance the force moves. A Class 1 lever is arranged to move the object further for a given force than the other classes of levers.

## Using a lever in dental braces

December 6, 2015

### Question

I am an orthodontist and will be speaking to a high school group soon on applied physics in orthodontics.

My question: attaching a longer wire in front of and behind a displaced tooth (say high up in the gum) certainly places less force on the displaced tooth than if the wire is shorter between the wire attached to the teeth in front and behind:( i.e, the wire is attached to the upper central incisor, bypasses the lateral incisor, attaches to a high displaced canine, bypasses the first bicuspid and attaches to the second bicuspid, thereby bypassing the teeth in front of and behind the high displaced canine to lessen the force to bring down the displaced canine).

What is considered the fulcrum, the load and the force? I presume that the displaced tooth is the load and the adjoining teeth are the fulcrums and the wire is the force? Are both arms of the portions of the wire in front of and behind the tooth considered effort arms vs. load arms?

Also, is this an example of a class one lever or a class 2 or 3 lever? Is there a better way to describe the physics property as to why a longer wire to the displaced tooth exerts lesser force than a shorter wire? Thanks so much for your help! Dr M Wintner, Pittsburgh

Dr. Morton - USA (26592)

I am not sure I can visualize how the wires are placed and the orientation of the lever. Perhaps the following two articles would help to answer your questions:

Torque control in lingual orthodontics with lever arm mechanics: a case report.

Locating the center of resistance of maxillary anterior teeth retracted by Double J Retractor with palatal miniscrews.

I hope that helps. Best wishes for success in your presentation.

## Misc

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