CONTENT: fixed and movable pulleys, hoists, single and multiple
TOOLS USED: multimedia presentation (Microsoft PowerPoint), video projector
PLACE: multimedia lab
DURATION : 3 hours
As already indicated for the previous lesson, we are in the computer lab, where there's the teacher's computer station and projector needed for the presentation of the contents of the lesson.
initially with the first images will immediately focus the topic today (pulleys) and, for students be easy to refer to the many applications in everyday life of these machines (the pulley using the builders to raise the vessel containing the lime, rather than the applications in the domestic sector shutters ....) At this stage I try
references examples of objects commonly used in order to clarify the role and use of those objects.
The slides I try to present the theoretical concepts by focusing only on the arguments strictly necessary, preferring the use of images that among other things greatly facilitate the teacher's life: Think about how long it would take to achieve with the chalk hoist a multiple!
's comment to the presentation by the teacher clarifies and integrates the flow of the slides guide the student to the understanding of the content offered.
Once the existing information-theoretic pass to the most practical and interactive lesson, for which students can use the PC using the internet connection you can use a couple of applets on the and pulley systems pulleys on which they can try working individually or in groups.
In a separate section of the blog are some possible applets that will be used during the lessons.
The argument concludes by presenting Numerical applications to simple class with an explanation of why the front chalkboard.
Let the content ....
derived directly from the draft, seen daily in many applications, we now consider a new machine simple: the pulley or pulley.
The pulley is a wheel with the outer edge with a groove called the "throat" can be lodged for a rope or chain.
The wheel is free to turn around a hub supported on a stand called "bar" passing through a hole in the center of itself.
The pulleys are, in effect, the mechanical levers, which are mainly used for lifting operations.
fixed pulley in the bracket is fixed to a support, the resisting force is applied at one end of the rope or chain passing through the throat, and the driving force is applied to the other end.
The machine is like a lever of the kind with the distances between the fulcrum and application points of the two forces are identical and equal to the radius of the crank.
The advantage is the reduction of power but that could change the direction of the applied force, having to raise weight instead of doing so directly is more convenient to do this by pulling a rope from the top down because it allows us to leverage the weight of our body.
The fixed pulley is thus a kind of first lever arm when the power is equal to the arm of resistance, both being equal to the radius of the pulley. This lever is therefore neither advantageous nor disadvantageous.
Scheme of a fixed pulley
From equilibrium to the rotation around the center 0 of the pulley is obtained
simplifying and F = Q
In the mobile pulley bracket is the point of application of the drag force, one end of the rope is attached to a support while the other is applied to the driving force.
If the two lengths of rope are parallel to the machine tends to rotate around the axis of the pulley, but not around the focal point of the section of fixed rope with the pulley.
The distance between the fulcrum and application point of power is therefore the diameter of the pulley, while that between the fulcrum and application point of the drag force is equal to the radius. It follows that the balance is sufficient that the intensity of the power is half that of the resisting force. The pulley is mobile
then a lever of the second kind (ie profitable) in which the arm of the resistance is equal to the radius of the pulley, while the arm of power is equal to twice the radius.
diagram of a movable pulley
Similarly to what we saw above, if you can ignore the weight of the pulley and the rope which surround it, the equilibrium condition becomes:
simplifying and F = Q / 2 from which, recalling the definition advantage of a simple machine, we get K = 2 or the use of a movable pulley leads to a power equal to 50% of the resistant.
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