what is the best radius for a fillet in objects

According to wiki, the applications of fillets are:

1. to eliminate the stress concentration
2. to reduce aerodynamic drag
3. for manufacturing cost reduction(in milling)
4. it prevents the edges from damages
5. to eliminate the injuries from sharp edges
6. also it is given as an aesthetic feature

now, what is the best radius for a fillet in objects to eliminate the injuries from a sharp edge?
but before that another question should answer.

does a sharp edge really cause injuries?

A sharp edged object cannot make an injury as a knife does. because the included angle in the knife is really small to make the serious cut.

consider the sharp edge on a rectangular block.the included angle is 90 degrees. does it can make a cut on your body. if "burrs" are present on the surface obviously it will cause injuries. otherwise if the sharp edged block is having a surface finish like "i phone" it is difficult to make a cut.

is the electric car really green?

This is "Love Bird". India's First electric car made by Eddy current controls Ltd, Kerala. they manufactured this car long back in 1993.

"Lovebird" is a cute two-seater Electric Car of Eddy Electric series. Unlike the conventional cars that use petroleum fuels to power internal combustion engines, the electric car is run by a direct current electric motor powered by a rechargable lead acid battery pack. The electronic chopper controller gives smooth speed control. Four-speed with reverse gearbox will make your ride comfortable on all road conditions.

With the range of 60 Km per charge, lovebird suits most of your daily city drives which are normally within this range. After the day's trip it can be fully charged in about 6 to 8 hours and be ready for drive the next morning. The gradeability limit of 15 degrees can negotiate any gradient. Lovebird is built on a light weight high tensile steel chassis and fibre glass reinforced polyster body. The ride is noiseless and pollution free.
courtesy:Eddy current controls Ltd, Kerala

now many companies are making electric vehicles which are really powerful.

but is it really a greener vehicle?????

of course, it is not producing any direct emission. batteries are recharged with electricity.
but this electricity is produced in a power plant  which is emitting carbon dioxide.

now what is happening?

all gas cars are polluting atmosphere wherever they go.
all electric cars are not polluting wherever they go but more pollution near the electric power plants.

is it true?

waste bin

 


this a waste basket with a lid. The lid can be opened by pressing on the lid. is it a good design?
Identify the problems of this waste basket.

Actually it seems to be a good design. because the lid can be open and close without touching on it. but it fails when form starts to follow the function.
here when we press on the pedal, the lid supposed to be open 90 degree to horizontal. but because of this poor design, pedal will touch on the ground so lid will not open completely(approx. 45 degrees).

so, while standing it will be difficult to put the waste into bin. another problem of this waste bin is, the inner volume of bin is actually less. since the pedal mechanism is consuming lot of space at the bottom of the waste basket.