Mechanical Systems

Mechanical systems is the study of forces and the application of forces



Mechanical Systems Overall Expectations
By the end of this course, students will:

The focus of this strand are forces and the application of forces via simple machines. The main headings of study are as follows:

  1. To start the program I will spent the openning sessions reviewing elementary math skills. Without these skills it will constantly be a struggle to achieve high marks! You will be expected to complete and hand worksheets provided and from this site. A hand in homework assignment   Due date : September 28 or February 25

  2. Friction and the Inclined Plane
  3. Identifying simple machines: ramp, lever, pulley will be studied in some detail
  4. Torques, Force and Displacement; how they are related to each other
  5. The Law of the Lever
  6. Simple machines and Mechanical Advantage; single, double and moveable pulley systems
  7. End-of-Unit Task: Construct a Compound Machine; eg a block and tackle

Coefficient of Friction

  1. The force of friction appears to be approximately the same different low speeds.
  2. The force of friction does not depend strongly on the area of contact between clean uniforn surfaces.
  3. The force of friction is directly proportional to the perpendicular force (the normal) pressing the two surfaces together.
  4. Starting friction is somewhat greater than moving friction. Remember that inertia must be overcome to start an object moving.

If the plane is not in the horizontal position but is inclined then the acting forces on the object interact with each other in a more complex manner.
In this situation the force of gravity Fg will influence the motion of the object down the inclined plane.
Here's a diagram of the forces acting on an object sitting on an inclined plane.

The dynamics of this object will be a function of the angle of inclination, the surfaces in contact with each other as measured by the coefficient of friction, the mass of the object, that is Fg and any applied force, Fm. These relations will be discussed in class, with an appropriate board note.
If the object is just about to move the angle θ is called the angle of repose
Information relating to our experiments

In the experiment investigating a block sliding down an inclined plane the relationships studied are Since the experiment was done qualitatively (no numbers or measurements), of the three variables it appeared that only type of material effected the ability of the block to slide down an inclined plane. Both surface area and mass did not change the angle of repose.

In the second experiment, with the same block will be pulled on a horizontal surface. In the class experiment the force required to overcome static friction and the force required to overcome kinetic friction (& keep the block) moving will be measured for four different masses. Mass must be changed to force in the calculations and graphings.
The starting force required to just get the block moving is related to static friction and the force to keep the block moving is related to kinetic friction is to be plotted against the different masses (Fg) and the slope of the line is to be determined. This slope is the coefficient of friction. The slope will be non zero and usually less than 1. There is no units to this slope.

In the third experiment the coefficient of Sliding Friction will be determined.
This relates directly to the diagram above seen here as a thumbnail.


Section 2

Machines, Work, & Mechanical Advantage

A machine is a device designed to make "work" easier or in some cases possible for a preson to do. If you have a flat tire you cannot lift your car, however using a car-jack or screwjack enables you to raise the car and change the flat. Also, you cannot remove the wheel nuts without a tire wrench. These are examples of machines. Here are pictures of 6 simple machines.

Some machines change one force into another, others transfer force from one place to another or cause an applied force to have its direction changed. Machines can amplify a force or decrease its size. You should and remember examples of each of these different machines.
One thing a machine cannot do: make energy. Machines change amplify but do not produce more energy than what can be inputed.
How is work and force related?

Work and Force

Two additional notes that are required readings
  1. Click Here A short note is give for each of the simple machines
  2. Click Here Other machines are shown here that you should be familiar with.

The Inclined Plane or Ramp

The inclined plane is a simple machine that allows large objects to be raised against the force of gravity, with less work than needed to directly lift the object.
An experiment will be done to confirm this concept. A mass will be lifted a given distance and the work calculated to do so. The same mass will be pulled up a ramp and the work required determined. The two values will be compared.
This leads to the concept of mechanical advantage. In each case the amount of energy is the same, however the force or effort can be manipulated. This is what a machine does best. The actual mechanical advantage of any machine is the ratio of the magnitude of the force required to lift the load vertically (the load force) to the magnitude of the force necessary to move the object by the machine doing the work.
Determining the efficiency of a machine, use the formula

               load force          Fg
       MA    =   ----------   =     -------
               applied force        Fa
        

Levers and Pulleys

A lever consists of a rigid bar that can rotate about a fixed point called a fulcrum There are three classes of levers which depent on the position of the load to applied force to fulcrum. Make sure you can define each. Pictures are shown below

The turning effect of a force on a lever is called Torque. It is defined to be the product of the force and the perpendicular distance from the axis of rotation to the application of the applied force or the load. This perpendicular distance is called the lever arm (l) . Torques has units of newton-meters but is not really considered to be work. The torque applied to one side of a lever must equal the torque on the other side of the lever. Since torque equals F x l, determing an applied force for a lever can be computed. Problems to be done in class.

Class picture of lever lab

Pulleys are another of the simple machines. Pulleys trade distance for effort so as to make lifting a heavy object easy that is, the applied force can be reduced. Examples of pulley systems are shown below.

For You: determine the mechanical advantage of each of these systems

To see pulleys in action where you select the number of pulleys, either two, four, or six Click Here From the ThinkQuest Library

A simple lab activity is carried out, in which you build the pulley system and compare the effort force to the load and the distance the load is raised to the distance pulled by the effort. Machanical advantage can then be determined.

Two note on machines and there properties. The second note is an extention of the first note.

NOTE #1 DIV 5
NOTE #2 DIV 6

Homework from the text book
   Page 65 # 7
   Page 66 #9, 10
   Page 68 #13, 14
   Force & Motion work sheet; requires motion equations to complete all the questions.

DIV 1

Hints when working with motion equations

These problems tend to be layered, that is multiple steps are required, several formulae are to be used in conjunction with one another.
A few common sence points


Test Information


Homework Assignment #3

Homework based on Inclined Planes and Coefficient of Friction
Third hand in Assignment
  1. A 20 kg pile of books is resting on a plank so that it makes an angle of 20o with the ground. What is the normal force?
  2. How much force is needed to keep a 62 kg crate moving across a floor at a constant speed if the kinetic coefficient of friction between the floor and the crate is 0.18?
  3. A 45 kg table can be pushed along the floor at a constant speed with a force of 170 newtons. What is the coefficient of friction (μ) between the table and the floor?
  4. A force of 40.0 N is needed to push a wagon up a 35o slope. What is the mass of this wagon? (Case 1 neglect friction.)
    Bonus: Case 2: The coefficient of friction between the wagon and the ramp (slope) is 0.0872
  5. You have a rope attached to a cart the weighs 450 newtons,and you are lowering the cart down a 25o slope. If the friction is 75 N, how hard do you have to pull on the rope to prevent the cart from running away from you? (Make sure you know what the physics of "run away from" means.)
    Also, find the angle of repose for this cart.
  6. A 45 kg load is on a ramp inclined at 38o to the horizontal, and the coefficient of friction between the load and the ramp is 0.26. Find the following
    1. the force that would be needed to push the load uphill if there were no friction;
    2. the normal force that the load exerts on the ramp;
    3. the force of friction;
    4. the actual force that would be needed to move the load up the ramp.
Answers to the above question may be found at Click Here



DIV 5

The Lever and Mechanical Advantage

A lever consists of a rigid bar that can rotate about a fixed point called a fulcrum. There are three classes of levers depending upon where the load, effort and fulcrum are positioned relative to each other. (See above diagrams and those sketched in class).
In this picture the effort is on the left and the load is on the right. Note the various ratios of effort to load, distance ratios and arc length ratios.

The turning effect of a force on a lever is called torque. When a lever is in the balanced position, the clockwise torque will equal the counterclockwise torque. This is called the law of the lever.
The magnitude of the torque symbol τ is calculated by multiplying the force applied (either load or effort) by the perpendicular distance of the force from the fulcrum. The perpendicular distance is called the lever arm l. The unit for torque are newton-meters (N-m)
Experiments are to be done in class to verify this law and to measure the Mechanical Advantage MA of a lever system.

There are two types of mechanical advantage:

Which one do you use: read the problem, it will tell you. Sometimes you calculate IMA and then calculate AMA
Both IMA or AMA are the ratio of the load force to the applied force or Fg to Fa.

The efficiency of a machine is the ratio of useful work done to actual work done. In most machines incolving motion effort muct be applied to overcome friction. This causes the machine's useful work to be less than the applied work. In the example of pushing a crate up a ramp the ramp makes the effort easier but frictional forces require extra effort.

For more information go to DIV 6

Simple Machines Group Project

Since there are 25 students and seven simple machines the groups will be divided into groups of three or two.
This semester Fall 2008 there will be no presentation or model building so ignor all references to either presentations or model building. Instead of the model a large diagram will be required showing all forces acting as the machine is in action and how these forces are related in appropriate equations.
The following is a suggested student designations:
  1. A team captain or coordinator
  2. A chief writer
  3. A major presentor
  4. The model builder or diagram artist who preforms the actual use of the machine showing all measurements related to the action of the machine. Mechanical Advantage

Introduced on Sept 23 with groups being assembled
Due on Sept 29. You cannot be late or expect and extention.
Class research time will NOT be provide

Requirements

Further Instructions

The Groups:

Have been assigned in class

Report any group member that is a "slacker". I'll try to help motivate this individual.


Independent Research Project
In this assignment you will be asked to investigate how science (in our case the theory and appliction of simple machines) is used in technologies to improve society and human development.
Write a short essay relating machine technology to societal improvement under these headings:
  1. Name three machines (inventions) that changed the course of history and the societal developements that ensued.
  2. How do machines make life easier for humanity in general.
  3. Describe the role of machines in everyday domestic life and in industry.
  4. Give examples where the use of a machine may not be benificial or has actual detrimental value to society.
  5. Identify simple machines that are part of a device used at home and explain the function of each machine.
    You must discuss three examples.
  6. Explain the benefits and detriments of "automation". (The use of machines to do everything).

Done in conjuntions with your machine essay hence date due is the same
A Making Connections activity with a communication mark.

Hints:

  1. Start right away
  2. Use the Internet
  3. Make sure you actually select a "machine". For example, the microscope would not be defined as a machine, it does no work.
  4. Ask questions in class

The following site may be helpful in getting started and/or reviewing simple machinesClick Here

DIV 6

Machines

Fundamental Types of Machines

The six (seven) simple machines can be catorgized into two modes:
  1. the lever which include the pulley, wheel and axle and gears
  2. the inclined plane which include ramps, the wedge, and screw

Moments or Torque

The turning effect produced by a force acting on an object free to rotate about a point of support is called its moment about that point or torque. The magnitude of this moment depends on two factors : the magnitude of the applied force (load or effort) and its distance from the point of support. This means that the torque or moment about a point is the product of magnitude and distance: Torque = F x d . This torque may be in either a clockwise or counterclockwise direction. The units for torque is the newton-meter (N-m).
Torque will occur on both sides of the fulcrum of a lever, but in opposite directions. Motion will occur on the side with the greatest torque and if the torque is equivalent on both sides of the fulcrum motion will not occur and all forces will be balanced or in equilibrium.

Mechanical Advantage

The force applied to a machine is understood to be the force the the operator applies to make the machine work. This force is frequently called the effort.
The force moved at the "other end " of the machine is refered to as the load or resistance. Remember, a mass when moved becomes a force (F = ma)
During the operation of a machine both the applied force, effort, and the load are displaced. The ideal mechanical advantage of a machine is the ratio of the distance that the effort moves to the distance theat the load moves. Since friction always occurs from the contact of moving parts,the actual mechanical advantage is somewhat less than the ideal mechanical advantage and can be determined as the ratio of the load to the applied
force FL to FE.

Efficiency of Machines

The fundamental purpose of any machine is to do useful work. In order ot do work the machine must be supplied with energy. If all this energy is converted into useful work this would be an ideal machine with an efficiency rating of 1 or 100%. Since energy is always lost in a machine to other nonuseful forms of energy such as sound, heat, or friction, no machine actually converts all its inputed energy or work to useful or actual work done. The efficiency of a machine is evaluated as the ratio of useful work done by the machine to the work or energy supplied to operate the machine. There are several versions of efficiency equations which will be discussed in the lesson.
Efficiency may be determined as the ratio of actual mechanical advantage to ideal mechanical advantage.

© R. Warren Sept 2002
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