Week 23 - Work and Efficiency
Reading pg. 278-286
So far in our exploration of mechanical devices we have been calculating speed ratios and mechanical advantage based on the designs of the simple machines. These theoretical values showed that they had the same values. But in real life this never happens because there are physical factors that affect the operation of all machines. The mechanical advantage is reduced by friction. You may recall from science seven that friction is a force that resists motion. The rougher the surfaces of the simple machine the more the force of friction reduces the mechanical advantage of machine. The energy lost by the force of friction is usually converted to heat.
On cold days many people rub their bare hands together to warm them up. What force are they using? Try it for yourself. Rub your hands lightly together for about 15 seconds. What did you feel? Now do it again, but this time press your hands together more firmly. What was the result this time? Finally, put some hand lotion between your hands and rub them together a third time. Did the addition of the lotion make any difference?
Friction can be good
Although the heat produced by friction might be useful some times, it is generally a problem when it comes to machines. For example, the friction in your car or truck engine creates a lot of heat. If this heat was allowed to remain in the engine block it would soon expand the moving parts until the engine seizes up - not a good thing! If this happens the engine is ruined and you will have to buy a new one which is very, very expensive.
To avoid this problem, water (plus antifreeze in Canada) is circulated through the engine and radiator. As moving air passes through the radiator it re-cools the water which then heads back to the engine. Another way of reducing friction is to lubricate the moving parts of the engine. What do you think is used to lubricate gas engines?
Determining the Efficiency of a Mechanical Device
So far we know that friction affects all mechanical devices. But how do we go about measuring the affect of friction? By using a simple formula we can determine the overall efficiency of a machine. Efficiency is the measurement of how well a machine uses energy. The efficiency of a machine can be found by dividing the mechanical advantage by its speed ratio.
The efficiency of the Lego would be 67%. That means that our machine lost 33% of its mechanical advantage due to friction.
The gasoline combustion engine found in most cars and trucks is only about 25% efficient while a diesel engine is about 40% efficient. That means most of the gasoline or diesel fuel that we put into an internal combustion engine is given off as heat - not moving us down the road. Not only is this extremely wasteful but it is very hard on the environment. Carbon dioxide emissions from vehicles is thought to be a major contributor to global warming. Hopefully as your generation gets older you will be driving more efficient machines that use hydrogen fuel cells instead of gas engines.
Go to BrainPop at " http://www.brainpop.com ".
Click on "Log In" then use the user ID "argyllbp" and the password "brainpop".
Then do the search by typing in "hydrogen fuel cells" and select the one that fits the search.
Exercise 3.1A: Managing Friction
Exercise 3.1B: Draw a Machine
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