School of Physics & Astronomy

# Units and Concepts

## Density

Density is mass per unit volume, but what is the range of densities? How dense can we go, or similar how un-dense? Start by considering empty space (which isn't actually empty), then compare that to water. Now let's go to super dense things like white dwarfs and beyond to neutron stars, the densest objects we know. In passing we consider pulsars, filling a cup with paper clips and how long it would take to make a neutron star.

Duration: 00:07:45

## Mass

What do we really mean by mass? There are two different definitions: inertial mass (which leads to the concept of inertia), and gravitational mass. Are they actually the same thing? [Note: this was filmed back before the Higgs field and the Higgs boson was discovered.]

Duration: 00:08:38
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## Energy

Energy comes in many forms, and of course appears in the most famous equation in all physics E=mc2. The awkward truth is that no one really knows what energy is. You can't watch it, or touch it -- you can just work out how much different things have of it. It turns out to be a very useful property, as it is always preserved across changes, as long as you measure it carefully. It can be used to work out if Einstein can pole vault over a bar and how to drive a swing, both of which are explored in this video.

Duration: 00:07:02
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## Kelvin

K is for potassium - or is it Kelvin? Chemists and physicists will have to disagree. Named after the scientist Lord Kelvin, in this case it is being used to measure temperature on the absolute scale. The coldest temperature on this scale is 0K, although in practice you can't reach this temperature. Strange things happen to some materials (in particular liquid helium) when they get VERY cold at close to 0K.

Duration: 00:04:03

## Measuring Error Bars with Aero Bars

In physics, you should always have three parts to your answer: the value, the units, and to what precision you believe the value is correct (the possible error). Here we look at this concept -- and set the fire alarm off in the process!

Presenters: Philip Moriarty
Duration: 00:13:15

## Pressure

Pressure is a force that tends to push things apart: we can feel this when blowing up a balloon, or pumping up a tyre. But what is actually happening to cause this pressure if we look very closely? Here we consider the molecular theory of pressure, using a demonstrations with ball bearings and a piston.

Presenters: Roger Bowley
Duration: 00:03:46
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## Gigayear

When we measure time, we usually use seconds in the SI system. Other quantities such as people's age or historical events are usually measured in years. Geologists often measure rocks in millions of years. Astronomers however, beat them all. Many phenomena only occur over time scales of BILLIONS of years. So enter the gigayear! We can perhaps get an idea of just how big this term is if we consider a snail moving around the Earth.

Duration: 00:04:25
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## Parsec

In astronomy, distances are vast. They are mind bogglingly, hugely, mind-numbingly vast. So metres and kilometres just won't cut it. When measuring distances around the solar system, the Astronomical Unit is typically used...but going out to the stars even this huge unit is not suitable. In astronomy measurements are typically done in parsecs (or kiloparsecs, megaparsecs or even gigaparsecs). Where does the name come from and how is it defined? Find out here.

Duration: 00:05:16
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## Mass and Weight are (sort of) the SAME

Weight, force, mass and Newtons. What is the difference, and does it really matter? Mass and inertia are the same thing (at least we think so). These are concepts that Einstein tackled when he was formulating the special of relativity.

Presenters: Michael Merrifield
Duration: 00:09:45
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## Work

Work is closely related to force and energy. Here we look at the work done on a rubber band. Does heating (putting energy into) a rubber band make it contract or expand? What do you think will happen?

Presenters: Roger Bowley
Duration: 00:04:51
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## School of Physics and Astronomy

The University of Nottingham
University Park
Nottingham NG7 2RD