Year 12 / 13 Mechanics Inertia demos:


  • These demos can be used with Y12 when looking at newton's laws, or with Y13 when introducing rotational inertia.

String break
Table cloth pull
Toilet Paper Tug
Bed of nails:
Hoop/Ring and Pen Cap
Eggs into cups
Rotational inertia helps man on high ropes:

Demo 1:

Breaking string:

  • 2 identical strings
  • a large mass
A large mass (e.g. brick) is suspended by a single strand of breakable string. A strand of this string is also attached to the bottom of the mass. Students are asked which string will break first: the one above the weights, or the one below, when the strings below the weights are pulled. Which one actually breaks depends on how the string is pulled. If the string is pulled very fast, the string below the mass breaks first every time, while if the string is pulled slowly, it breaks above the mass every time. This is because of the inertia of the mass. A quick pull does not give the mass a chance to move, i.e. the mass resists the change. In order for the string above the weight to get tighter, the weight must move down a bit. If the pull is fast enough, it gets much tighter below the weight than above, and that is where it will break. If the string is pulled slowly, it gets tighter above the weight, for the weight does have a chance to move, and the string above has to support the weight and resist the tug.For more info click here:

Demo 2:

Table cloth pull

  • Table cloth (smoother the better - having seams can catch the crockery and destroy this demo!)
  • A smooth bench
  • Some old crockery (Note: the lower the COM the better - as the movement of the table cloth does create a torque about the COM which if large enough does cause the objects on top of the cloth to fall over
  • NOTE: plastic crockery does not work, as the objects need to have significant inertia.


Demo 3:

Boiled and Raw Eggs?

  • 2 boiled eggs (boil slowly so not to crack the shell)
  • 2 raw eggs (leave in cold water for the same amount of time as the boiled so they get the same discoloring)

Ask students to tell you which one(s) are raw and which one(s) are boiled.

Spin the eggs, one at a time, on the side on a smooth hard surface, stop them fast and let them go immediately. The boiled egg will spin easily to begin with but will not spin any more after stopped suddenly and released. However, the raw egg will spin again after quick stopping and releasing it because the liquid inside the raw egg is still in motion due to inertia. Please wash your hands when handing raw eggs. This technique can be used to find out if the egg is raw or boiled.
More info click here

Demo 4:

Toilet Paper Tug

  • two rolls of toilet paper (one new the other almost empty) - the bigger the difference in radius the better.
  • a broom handle or similar
Ask students which one is more easier to snap off a piece of paper by a single tug?

Demo 5:

Bed of nails:

Demo 6:

Hoop/Ring and Pen Cap

Demo 7:

Eggs into cups

Demo 8:

Rotational inertia helps man on high ropes:

Demo 9:

Inertia Rods:

  • These rods weigh the same, but due to the distribution of the mass, one is harder to rotate than the other.
Click to watch video demo:

There are heaps more inertia demos. Feel free to add more, or improve the descriptions above :)

References / other comments:

These rods weigh the same, but due to the distribution of the mass, one is harder to rotate than the other.