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Chapter 19 Simple Machines Force – ¾ a push or pull ‘exerted’ on a body ¾ may cause a change in the body. (These changes are called force effects.) Motion – ¾ The effect of force on a body. No motion is possible without some sort of force. ¾ Once a body is in motion, it will continue to move at the same speed and in the same direction until other forces act upon it. ¾ Consider space probes. Once a space probe escapes earth’s gravitational field, it will continue to travel in the same direction unless something happens: o Firing of steering thrusters. 1 o Runs into another probe o Flies too near another planet and gets pulled into its gravitational field and crashes. Friction – A force acting in the opposite direction of a body’s velocity, due to a ‘rub’ between the surface of the body and some other material like: ¾ The road ¾ A table top ¾ Air etc. ¾ The more uneven the interacting (or ‘rubbing’) surfaces, the greater the amount of friction. Problem: EVERYTHING (at least on Earth) is uneven! This means everything is affected by friction. SO? ¾ Everything has to overcome the effects of friction in order to start moving. ¾ Everything will slow down and come to a stop because of friction. ¾ Consider hockey pucks. Why do hockey pucks travel faster and farther on the ice than they do on concrete? Because, even though the puck is uneven, the concrete is more uneven than the ice. 2 Gravity – ¾ The attractive effect that all bodies have on all other bodies. Whoa! Does that mean we all have our own gravity? Yes, indeedy! Check out the computer simulation! ¾ However, for these effects to be noticeable, at least one of the bodies must be massive. Forces on a body: ↓Gravity 3 Question: You are peddling uphill like crazy. You are going so fast that you stop pedaling. After a while you notice that the bike slows and eventually stops. Why? (2 reasons) Newtons (N) – ¾ A unit of force approximately equal to unit of force in the meter-kilogram-second system, or international system (SI), of units; equals that force which, if applied to an object having a mass of 1 kg, would give that object an acceleration of 1 m per second per second in a vacuum. Sounds complicated? ¾ Okay how about this: 1 N = 1kg ⋅ m 4 ¾ If you know how much something weighs you can convert that to 1.0 kg = 9.8 N (because of the force of gravity…but who cares anyway, right?) 1.0 kg = 9.8 N 19.2 Work In order for work to happen (scientifically) two things must be present ¾ A force acting on a body ¾ The body moving a distance. Technically, Work = force X distance W=FXd 10 N of force 5m Suppose a force of 10 N was ‘exerted’ on a block of wood moving it 5 meters. Q: How much work was done? A: 5 W=FXd W = 10 N X 5 m W = 50 N-m N-m is too cumbersome so we say 50 J (joules). 1J = 1 Nm Instant practice: In class p417 # 3, #4. Activity 19C 19.3 Machines machine – a device that helps people do work more easily. Functions of machines All machines do at least one of the following: ¾ transfer force from one place to another. (e.g. pedals Æ chain Æ rear wheel) 6 ¾ transform energy from one form to another. (e.g. mechanical Æ generator Æ electrical) ¾ change direction of the force. (e.g. flagpole pulley) ¾ multiply speed or distance. (e.g. one pedal revolution Æ one wheel revolution) ---> --------------> 7 ¾ multiply force. (e.g. car jack)…How does this work? Mechanical Advantage – ¾ The amount the machine can multiply Force. Mechanical advantage = ___Load force___ Effort force MA = ____LF____ EF Example: Suppose in the above jack I could lift a 1000 N car by applying 20 N of force. What would the mechanical advantage be? Load force = 1,000 N Effort force = 20 N MA = ___LF__ = ___1,000 N___ = 50 EF 20 N Review 19.3 ALL 19.4 The Six Simple Machines - all complex machines are made up of six simpler machines: ¾ Inclined plane - the gentler the slope, the greater the MA. ¾ Wedge - the longer and narrower the wedge, the greater the MA. ¾ Screw – an extremely long inclined plane. The finer the pitch, the greater he MA ¾ Lever – the greater the ratio between effort arm and load arm, the greater the MA. ¾ Wheel and axle – the greater the ratio between the wheel diameter and the axle diameter, the greater the MA. 9 ¾ Pulley – the greater the number of supporting strands, the greater the MA. In class and at home: Activity 19E Review 19.4 What are these? How would you get a mechanical advantage fro each one? 10 19.5 Efficiency of Machines Machines are NEVER 100% efficient because they always have to overcome friction. Some machines have a higher degree of friction so they are less efficient. There are two ways to calculate efficiency: Efficiency = ___work output___ X 100% work input Or Efficiency = ___load force X load distance___ effort force X effort distance X 100% Review 19.5 Chapter Review. 11