Moments

Have a look at this, then think what it might have been used for. It’s a balance used in Roman times

Which hook did the merchant use for hanging the object up in his shop for the customer to see?

The weight W was probably a Roman ‘Libra” where one libra = 12 unciales or ounces. An ounce is about 30g. So, W would have been about 360g.

It’s a movable weight. It can move from left to right and there would have been markings on the horizontal bar.

The merchandise being weighed would have been hung on hook H”. Why on this side? This should tell you where the pivot is.

Additional known weights could be hung on hook or loop H’. Why?

Finally, what kind of merchandise do you think this balance might have been used to weigh? Look at the design and think about the range of weights that might have been hung on H””.

Here’s a question on  The Roman Balance plus the physics of moments for you to download and try.

The Khan Academy video link on Introduction to Moments  is here…

This is ‘The Decorator Problem.

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A decorator is standing on a horizontal plank supported by two ropes equidistant from the ends.

Use the Law of Moments to calculate the tensions in each plank. How would the calculation change if we had to consider the weight of the plank as well?

Here’s a little bit of fun. Go online and google “Vasa”. Find out “why the King of Sweden’s flagship ‘Vasa’ sank moments after leaving port.”

Khan Academy

Click on the image. This site is making hot news worldwide. What do you want to know? It’s all here as short video tutorials. Maths, Physics, Chemistry – almost anything to any level is either online or being rolled out. All you need to do is type a search word, sit down and watch.

Click here as an example, showing refraction of light in water – or why a drinking straw looks bent.

 

Redox Reactions

Redox (reduction-oxidation) reactions are a family of reactions that are concerned with the transfer of electrons between reactants. Redox reactions are a matched pair – you don’t have an oxidation reaction without a reduction reaction happening at the same time. Oxidation means loss of electrons while reduction means gain of electrons. Each reaction by itself is called a “half-reaction”, simply because we need two half-reactions to form a whole reaction.  We write them out like this.

Mixing  magnesium powder and copper oxide together – a displacement reaction.

The copper ion has gained 2 electrons, and is reduced, the Mg atom has lost 2 electrons and is oxidised

The OILRIG rule: Oxidation is loss, reduction is gain of electrons.