Simple Machine (Pulley)

Fig. 1

        Pulley is a simple machine made up of wheel and rope. The rope fits into the wheel and one end of the rope is connected to the load. When one end of the rope is pulled, the wheel turned so thus the load. In the diagram, (Fig. 1) a man is pulling one end of the rope to lift the load upward. The Actual Mechanical Advantage (AMA) of a machine can be computed by the formula:

                        AMA= Fo/Fi

wherein Fo is the output force and Fi is the input force or the force that you exert. The IMA or the Ideal Mechanical Advantage could be computed by the formula:
                       IMA= Si/So  
wherein Si is the input distance and So is the output distance. Efficiency(e)= AMA/IMA and no machine can have a 100% efficiency.

            This simple machine (pulley) makes work easier and effortless. Pulley really saves a lot of effort or energy when you have more than one pulley working together.(Fig. 2) It is designed to reduce the amount of force needed to lift a load. Pulleys are used in our everyday life, from vehicles to moving equipments such as of anchors in ships, etc.

 

About the Pulley

       Pulleys are wheels on an axle that have a groove around their outer edge on which a rope or belt can fit. The wheel is designed in such a way to support the movement of the cable or belt that fit along the circumference of the wheel. The application of pulleys can be for many different functions; lifting loads, applying forces or transmitting power. For simple, single fixed pulleys, the load is attached to one end of the rope while the wheel is secured at a higher position with the rope running through it and the force being applied to one end of the rope to lift the load on the other end. This is one of the simplest models of a pulley system demonstrating how it works. There are several different, more complex models of pulley systems that work in different ways to serve different functions.

    
      The wheel can be secured at the top in some pulley systems and can be movable in some. It is easier to lift a load when the wheel is secured at the top and the rope is pulled downwards to lift the load rather than having to pull the rope upwards to lift a load in some movable pulley systems. The output force or work done by a pulley system can be calculated by multiplying the effort required to pull the rope to lift a load with the distance the that the rope moves. Some pulley systems can make use of more than one or more pulleys which are linked. The advantage of using such systems is that they reduce the amount of effort required to get work done.






TYPES OF PULLEY

Fig.2

           

•         The simplest type of pulleys is the fixed pulley systems. These pulleys are the only pulley systems though which, if used individually, require an equal amount of effort to the load to lift it off the ground. In this system, the wheel is secured at a fixed place and does not move. What this system does is that it changes the direction of the force in order to complete a task. The advantage with this is that one does not have to push or pull a load to be able to move the load as it allows for easy displacement of the load. The disadvantage being that more effort is required to move the load as compared to other pulley systems.

•             Unlike a fixed pulley system, in the movable pulley systems, the wheel used in the pulley moves along with the load that is being displaced. This function of the pulleys allows it to use lesser effort to be able to move the load. Unlike fixed pulley systems that exert only as much force on the load as that of which is applied on the rope, movable pulley systems are able to multiply the force that a user applies to the machine to carry out a task, in turn making the job seem more easier. This way, lesser force is required by the user to carry out the same task if using a fixed pulley system. This pulley also acts as a second class lever, whereby the load is placed in between the fulcrum and the effort. The disadvantage with these systems is that one has to pull or push to displace a load and the main advantage is that it requires lesser effort to be able to move the load.

•              The third type of pulley systems present today are the compound pulley systems. These are a combination of fixed and movable pulleys. These systems have the advantages of both the fixed and movable pulley systems as one would not require pushing or pulling a load to be able to move it while using only minimal force. Several pulleys can be combined to make this system. The more pulleys that are used, the lesser the force required to move the load. However, these systems can take up a lot of space and they involve the object moving over a longer distance.

Interesting Experiment in Physics Lab! :)

          

             Our most interesting lab experiment in Physics was experiment no. 5 which is Friction.It was were we were able to learn that friction is a force resisting motion of an object when in contact with another. This resistive force is caused by the surface roughness of the contact area of the materials, molecular attraction or adhesion between materials, and deformations in the materials. The cause of friction may be any or all of these items and this applies to sliding, rolling and fluid frictions.

Surface roughness

Most friction results because the surfaces of materials being rubbed together are not completely smooth. If you looked at what seems to be a smooth surface under a high-powered microscope, you would see bumps, hills and valleys that could interfere with sliding motion. Of course, the rougher the surface, the more the friction.

Close-up view of surface roughness

Treads add to friction

Treads or grooves on one or both sliding surfaces can increase the friction, especially if the treads have sharp edges and are not parallel with the line of motion. The most common use if treads are seen in automobile and bicycle tires, as used in rolling friction. You also may see them on pads intended to keep surfaces from sliding.

Sharp edges of treads add to sliding friction

The number and types of grooves or treads is an added factor to the friction equation.

Molecular adhesion

Another factor in friction can be caused by molecular adhesion or attraction. Ultra-smooth materials and "sticky" materials fall in this category.

Ultra-smooth

If both surfaces are ultra-smooth and flat, the friction from surface roughness becomes negligible, but then friction from molecular attraction comes into play. This can often become greater than friction if the surfaces where relatively rough.

Sticky materials

Rubber is an example of a material that can have friction caused by molecular attraction. Discounting resistance due to deformations with rubber, it is its stickiness factor that causes it to grip so well and have so much friction.

Fluids

Fluids often exhibit molecular adhesion, increasing the friction. This adhesion force is often seen in the capillary effect. This is where water will be pulled up a glass tube by the forces of molecular adhesion. That same force can slow down fluid motion.

One example is how a coin will easily slide down a ramp. But if you wet the coin, it will stay in place. That is because of the molecular friction of the fluid on the hard surfaces.

The motion of two fluids or two sections of a fluid against each other is also slowed down by the molecular attraction factor. This type of fluid friction is usually not considered as friction and is studied under the complex field of fluid dynamics.

Deformations

Soft materials will deform when under pressure. This also increased the resistance to motion. For example, when you stand on a rug, you sink in slightly, which causes resistance when you try to drag your feet along the rug's surface. Another example is how rubber tires flatten out at the area on contact with the road.

When materials deform, you must "plow" through to move, thus creating a resistive force.

Pushing object on soft surface

When the deformation becomes large, such that one object sinks into the other, streamlining can affect the friction, similar to what happens in fluid friction.

In conclusion:

Friction is a force resisting motion of an object when in contact with another. It is caused by the surface roughness of the contact area of the materials, deformations or the molecular attraction. The standard friction equation holds for hard objects being held back by surface roughness.

 Upon performing the experiment in friction, we were able to observe the kinds of friction, namely: static, kinetic and rolling friction.

Static Friction

Static frictional forces from the interlocking of the irregularities of two surfaces will increase to prevent any relative motion up until some limit where motion occurs. It is that threshold of motion which is characterized by the coefficient of static friction. The coefficient of static friction is typically larger than the coefficient of kinetic friction.
In making a distinction between static and kinetic coefficients of friction, we are dealing with an aspect of "real world" common experience with a phenomenon which cannot be simply characterized. The difference between static and kinetic coefficients obtained in simple experiments like wooden blocks sliding on wooden inclines roughly follows the model depicted in the friction plot from which the illustration above is taken. This difference may arise from irregularities, surface contaminants, etc. which defy precise description. When such experiments are carried out with smooth metal blocks which are carefully cleaned, the difference between static and kinetic coefficients tends to disappear. When coefficients of friction are quoted for specific surface combinations are quoted, it is the kinetic coefficient which is generally quoted since it is the more reliable number.

Kinetic Friction

When two surfaces are moving with respect to one another, the frictional resistance is almost constant over a wide range of low speeds, and in the standard model of friction the frictional force is described by the relationship below. The coefficient is typically less than the coefficient of static friction, reflecting the common experience that it is easier to keep something in motion across a horizontal surface than to start it in motion from rest.    

Rolling Friction

A rolling wheel requires a certain amount of friction so that the point of contact of the wheel with the surface will not slip. The amount of traction which can be obtained for an auto tire is determined by the coefficient of static friction between the tire and the road. If the wheel is locked and sliding, the force of friction is determined by the coefficient of kinetic friction and is usually significantly less.
Assuming that a wheel is rolling without slipping, the surface friction does no work against the motion of the wheel and no energy is lost at that point. However, there is some loss of energy and some deceleration from friction for any real wheel, and this is sometimes referred to as rolling friction. It is partly friction at the axle and can be partly due to flexing of the wheel which will dissipate some energy. Figures of 0.02 to 0.06 have been reported as effective coefficients of rolling friction for automobile tires, compared to about 0.8 for the maximum static friction coefficient between the tire and the road.

1. Advantages of Friction 

   In Meteorology:

   • Meteorologists found that friction decreased surface wind speeds, making them less volatile. Friction also encourages surface air masses to merge and rise, which aids the rain cycle. Rough terrain, trees and buildings create the friction that acts on wind speeds.
     

   For Animals:

   • Friction between animal's feet and the ground makes running and walking easier. In fact, without friction, animals would have difficulty standing. It's like the comic scenario of someone slipping on a banana peel. If there's no friction, people and animals can't walk, because they can't plant their feet firmly on the ground. There's no friction (i.e. traction) to keep their feet from slipping and sliding all over the place.
     

    In Everyday Life:

   • In everyday life, friction between the road and a car's tires help the driver control the speed of travel, and applying breaks slows the car to a stop. Friction also makes writing on paper possible. When using a pencil, the paper's friction causes the pencil lead to rub off. When using a ballpoint pen, friction triggers the ball to roll, thereby releasing the pen's ink.
     

   In Space Sciences:

   • While in space, meteors and comets have no force to slow them down. As they hit the Earth's atmosphere, however, not only does the atmosphere's friction slow them down, it tends to break them apart into smaller pieces, thereby lessening their impact on the Earth's surface.
     

   In Friction Welding:

   • Friction welding works by using a compressive force along with friction-induced heat to join two surfaces together. The friction-induced heating softens the metal components to make them moldable, at which point the motion needed to create the friction is stopped and the compression force is applied until the joint between the two components cools. This method of welding allows different materials to be joined together (for instance, wood and metal) and increases productivity in mass production industries.
     

   In Camping:

   • In a camping or survivalist setting, friction can been use to start friction fires. Friction created by rubbing two pieces of wood together heats the wood until it reaches combustion temperature (around 800 degrees Fahrenheit) and catches fire. For this to work, the wood must be quite dry and have little or no resin present.
     
     
     

     Disadvantages of Friction

     
     

     1. The disadvantages of friction are as follows.
     
     

     
     (a) Slows down or stops the movement of objects
     A bigger force is needed to overcome the friction so that an object can move faster.
     For example, a bicycle will eventually stop if it not cycled consistently. if you want make the bicycle speed up, you have to cycle it faster. That means more energy is needed to overcome the frictional force.
     
     (b) Causes the surface of an object to wear out - for example, soles of shoes and surfaces of tires.
     (c) Produces unnecessary heat - Car engines becomes hotter because of friction.