Unit 12 Competency 1 - Examine mechanical systems in engineering
Suggested Objective a: Examine Newton's Laws of Motion as related to mechanisms
1.3.1 Newton's First Law
When no force is exerted on a body, it stays at rest or moves in a straight line with constant speed. This principle of inertia is also known as Newton's first law. It is from this law that Newton was able to build up our present understanding of dynamics.
1.3.2 Newton's Second Law
From our daily life, we can can observe that:
- When a force F is applied on an object,
V, the change of the velocity of the object, increases with the length of time delta t increases; - The greater the force F, the greater V; and
- The larger the body (object) is, the less easily accelerated by forces.
It is convenient to write the proportionality between Ft and V in the form:
The proportionality constant m varies with the object. This constant m is refered to as the inertial mass of the body. The relationship above embodies Newton's law of motion (Newton's second law). As
in which a is the acceleration of the object. We have
If m = 1 kg and a = 1m/sec2, than F = 1 newton.
Forces and accelerations are vectors, and Newton's law can be written in vector form.
Information copied from http://www.cs.cmu.edu/~rapidproto/mechanisms/chpt1.html
Links to an external site. on December 10, 2014
The PowerPoint below covers several of our suggested objectives for this page. Click the PowerPoint to open it. Take notes as needed.
lecture4.2.ppt Download lecture4.2.ppt
PowerPoint was retrieved from bohr.winthrop.edu/faculty/mahes/link_to_webpages/.../lecture4.2.ppt on December 10, 2014
Newton's 3 (three) Laws of Motion
Links to an external site.
Suggested Objective b: Differentiate between mass and weight
Look at the attached pdf from http://www.uccs.edu/~rtirado/ch4.pdf Links to an external site.. It was retrieved on December 10, 2014. This is a chapter from a textbook and contains a wealth of information on weight, mass, force, work, and more. You should review this information and take notes as needed.
Mass is a measure of how much matter an object has. Weight is a measure of how strongly gravity pulls on that matter. Thus if you were to travel to the moon your weight would change because the pull of gravity is weaker there than on Earth but, your mass would stay the same because you are still made up of the same amount of matter.
Information copied from http://www.physlink.com/Education/AskExperts/ae321.cfm Links to an external site. on December 10, 2014
What Is the REAL Difference between Mass and Weight?? Explained
Links to an external site.
Suggested Objective c: Calculate force, rate, work, and mass using Newton's Laws of Motion
Force: an agent or influence that, if applied to a free body results chiefly in an acceleration of the body and sometimes in elastic deformation and other effects.
Every day we deal with forces of one kind or another. A pressure is a force. The earth exerts a force of attraction for all bodies or objects on its surface. To study the forces acting on objects, we must know how the forces are applied, the direction of the forces and their value. Graphically, forces are often represented by a vector whose end represents the point of action.
A mechanism is what is responsible for any action or reaction. Machines are based on the idea of transmitting forces through a series of predetermined motions. These related concepts are the basis of dynamic movement.
Torque: Something that produces or tends to produce rotation and whose effectiveness is measured by the product of the force and the perpendicular distance from the line of action of the force to the axis of rotation.
Consider the lever shown in Figure 1-1 Links to an external site.. The lever is a bar that is free to turn about the fixed point, A, called the fulcrum; a weight acts on the one side of the lever, and a balancing force acts on the other side of the lever.
Figure 1-1 A lever with balanced forces
To analyze levers, we need to find the torques of the forces acting on the lever. To get the torque of force W about point A, multiply W by l1, its distance from A. Similarly F x l2 is the torque of F about fulcrum A.
Work is a force applied over a distance. If you drag an object along the floor you do work in overcoming the friction between the object and the floor. In lifting an object you do work against gravity which tends to pull the object toward the earth. Steam in a locomotive cylinder does work when it expands and moves the piston against the resisting forces. Work is the product of the resistance overcome and the distance through which it is overcome.
Power is the rate at which work is done.
In the British system, power is expressed in foot-pounds per second. For larger measurements, the horsepower is used.
1horsepower = 550ft *lb/s = 33,000ft*lb/min
In SI units, power is measured in joules per second, also called the watt (W).
1hp = 746 W = 0.746kW
All object possess energy. This can come from having work done on it at some point in time. Generally, there are two kinds of energy in mechanical systems, potential and kinetic. Potential energy is due to the position of the object and kinetic energy is due to its movement.
For example, an object set in motion can overcome a certain amount of resistance before being brought to rest, and the energy which the object has on account of its motion is used up in overcoming the resistance, bring the object to rest. Fly wheels on engines both receive and give up energy and thus cause the energy to return more smoothly throughout the stroke.
Elevated weights have power to do work on account of their elevated position, as in various types of hammers, etc.
All information is copied from http://www.cs.cmu.edu/~rapidproto/mechanisms/chpt1.html Links to an external site. on December 10, 2014
Suggested Objective d: Identify and describe mechanisms used in daily life
The attached document is lengthy. It explains in detail about Newton's Third Law of Motion in mechanisms, force, load, how mechanisms are used in everyday life, and much more. Look at the document and take notes as needed.
Structures and Mechanisms Booklet.pdf Download Structures and Mechanisms Booklet.pdf
The Structures and Mechanisms Booklet was retrieved from http://www.t4.ie/Resources/Resources%20by%20Topic/Core/Structures%20&%20Mechanisms/Structures%20and%20Mechanisms%20Booklet.pdf Links to an external site. on December 10, 2014.
The following Mechanism pdf explains what a mechanism is and provides examples of mechanisms in life. Take a look.
mechanisms.pdf Download mechanisms.pdf
It was retrieved from http://www.engr.sjsu.edu/minicurric/images/lecture_powerpoints/mechanisms.pdf Links to an external site. on December 10, 2014.
Look at the mechanisms video from Education Portal Links to an external site.. A quiz and transcript are available as well.