An objects’ centre of mass is the point where its mass is equally balanced. When an object rolls, it rolls about its centre of mass. 

Since the jar has a ping pong ball its density is inconsistent (non-uniform).  This means that the centre of mass isn’t actually in the middle of the jar, especially as it keeps moving! While the jar is rolling the ping pong ball being less dense than its surrounding attempts to float. This results in the jar moving in one direction while the ball attempts to move in the opposite direction. This produces the rocking motion that you see on the ramp.  

This ‘stir-thickening’ of the cornflour slime shows that the material is a Non-Newtonian fluid, which means that the material does not follow the properties described of fluids by Newton’s law of viscosity which defines the relationship between the sheer stress to sheer rate of a liquid at a given temperature and pressure (in the case of a Non-Newtonian fluid, the viscosity is not constant and is dependent on the sheer rate i.e. in this case, the amount of pressure applied).

Variables to test

More about variables here

• Can you recreate this effect using plain water? Why or why not?
• Does the slope of the ramp change how the jar rolls?
• Can the effect be recreated using denser objects like large marbles instead of ping pong balls?
• Why not try and see if multiple ping pong balls (you may need a larger jar for this one).

How can this science be used?

Non-Newtonian fluids can be handy! Plenty of research is being done about how to use this special material in modern technology. Local and international scientists as well as engineers have been developing liquid body armour using non-Newtonian fluids, since they are really good at stopping bullets!

In 2010 a number of articles were released about the development of a speed bump dynamically changed depending on how fast cars were travelling over it! If you travel over the speed bump at a slow speed the fluid inside the speed bump would move out of the way and you’d barely feel the speed bump. However, if you travel over the speed bump too fast the speed bump would react quickly to the increased force and become almost solid… making your car bounce and reminding you to slow down!