Tag Archives: physics

Solvent Drunk Ideas on a Common Afternoon

A couple of things that are quite fun to try on a dragging afternoon in the lab:

  • Explain how a laboratory centrifuge works without using the term “centrifugal force”.
  • Give a proper explanation why and how a cooling mixture of dry ice (CO2(s)) in isopropyl alcohol works pretty good while dry ice in water gives a rather poor coolant.
  • Determine if (or under which circumstances) it is a good idea to externally warm a fractionating column including a Vigreux condenser with a fan.
  • Think of what is going on when the temperature of an oil bath of a distilling apparatus is increased while it is already above the boiling point of the liquid inside.
  • Give proper explanation of the processes linked to a rotary evaporator. (Pressure is of special interest.)
  • Pour ether on your desk and set it on fire.

The Magic Tool

Just quoting a recent Physics exercise:

Question: A man with a mass of 80 kg jumps horizontally from a wagon (on friction free horizontal rails) with a mass of 800 kg. At the liftoff, his speed relative to the wagon is 3 m/s. How much energy did the jump cost the man?
Answer: E = 327.3 J

The other day, I broke my window pane and had to purchase a new glass. Hence I had to know the dimensions of my window. Unfortunately, I didn’t have an appropriate measuring tape and one has to know it very exactly when ordering glass. No problem: I expected the window to be almost quadratic and I guessed that the area would be about one and a half square meter. Now I pulled out my good old pocket calculator, hit 1.5 + sqrt + enter and – surprise – got exactly 1.2247448713915890491 m as the side length. I think most scientific laboratories would – even using sophisticated equipment – have had serious troubles to meet that precision.

P.S. The inconvenient fact that the new pane did not fit, I can only explain by incredible malpractice of the manufacturer.

Physics Lecture at ALDI

Elastic collosion of bodys of equal mass.

Elastic collosion of bodys of equal mass.

Another illustration of elastic collision between bodys of equal mass.

Another illustration of elastic collision between bodys of equal mass.

Instead of visiting today’s Physics lecture, I went to ALDI. (Which is a supermarket.) While waiting in the queue, I had the opportunity to watch two cashiers practicing.

As most supermarkets ALDI provides triangular bars to be used as stop signs at the checkout’s conveyor belt. (Almost like “Toblerone” except you can’t eat ’em.) This bars are supported at a runner parallel to the conveyor. The problem is that people always take them from the back end of the runner while the cashier puts them on at the opposite end. If there are only a few bars left, the cashier might wish to push them back in order to make things easier for his customers. It was quite exciting for me to watch two ways of doing this.

Cashier A knocked his bar firmly against the ones already lying at the runner. The bars are made from aluminum sheets. Neglecting the (rather thin) terminal plastic capes, they show almost ideal elastic behavior. Hence the bar hitting the first obstacle of equal mass preforms a complete transfer of its momentum. (As illustrated in this animation.) This is repeated with every bar. In the end only one (namely the last) bar is pushed to the end of the conveyor while all others stay in place.

Next time he knocked the bar harder — making things even worse. Now the last bar collided with the stopper at the end of the runner, was reflected and the reaction occurred a second time the other way round, leading to an unchanged arrangement of bars except he got back the one he just propelled forward.

Cashier B, however, was more concerned about the physics behind his working place. He took the bar and accelerated the other bars gently by pushing it against them while still holding it in his hand. When he could no longer stretch his arm, he let the bar go and was satisfied by the intended movement of all the bars to the runner’s end.

So you see, it’s quite crucial to pay attention to your physics lectures. What was the first sentence again?

God spoke: Shame on Physics

Is god really great or isn’t he?

It’s Wednesday, 21th October, a shiny autumn day. 11:30 AM in the Gerthsen audience hall on the site of a renominate German university. Physics, lecture #1. For the sake of clarity I should maybe add, that it’s the year 2009 AC.

The common practice of starting a lecture with a definition and an overview of the subject including its potentials and limitations resulted in unexpected consequences. When mentioning the importance of the natural constants a (30 minutes) word about the amazing fact that Continue reading