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.

material selection

A grocery shop owner approached you and asked to design a carry bag .
which material, you will suggest for carry bag? and why?

• polyethylene
• non woven fabric
• paper
• textile
• jute

Science behind a product

There will be one scientific principle behind the working of every product. Analysing a product is a great way of learning.

When the scrap paper vendor who collected old news papers from my house, he used his hand to measure the weight of the paper. But i told, we have an instrument to measure the weight and i showed him this instrument.

And it showed 3 Kg more than what he told. He was resistive to that but finally he settled 2 kg more. We also didn’t bargain much.

Now how it is working and can we trust this instrument?

This instrument is designed and made with a rod of varying diameter. A piece of rope tied around the rod, is used to balance the rod. When the rope is at the centre of mass, the mass of the rod on both side of the rope will balance each other and become horizontal. This point will be marked as reference or zero weight. The rope should move towards the hook side and balance horizontally in order to weigh the objects.

When we add a weight centre of mass will change. Then if the rope is in the new centre of mass again rod will be balanced horizontally. The rod should be pre calibrated for getting the actual measurement.

This is clever and simple instrument for measuring the weight.

The main disadvantage of this instrument is we cannot add more weight as it will become too heavy to lift and balance.

soap dispenser

 

two different soap dispensers are shown here.
can you identify some advantages and disadvantages of each soap dispenser?
which design is better?