Lana Lazarevich, of Mount Roskill Grammar School, asks :-

Why is it that when a half-full bottle is spun in a vertical circle the liquid doesn't fall out when the bottle is upside down?

John Campbell, a physicist at the University of Canterbury, responded.

I am old enough to remember when milk was delivered by horse and cart and decantered from a large milk-can into billy-cans. All children of my day, who carried the billy-can into the house, knew this trick which has now advanced to a party trick using a bottle or glass.

The trick is to do it fast enough so that when the billy-can is highest, the vertically-upwards force due to the motion exceeds the vertically-downwards force due to gravity.

A mechanical force is exerted when an object is changing its velocity. In the case of a body in rotary motion, its speed of rotation is constant but the direction of its motion is constantly changing. Hence its velocity is changing and therefore it experiences a force that is radially outwards.

This explains, for example, why the Moon orbits the Earth. The attractive gravitational force towards the Earth is balanced by the radially-outwards force due to the Moon's obital motion. You will also see toys that involve cars looping the loop. They need a minimum speed to stay on the track.

Someone taking about year 12 physics could work out what speed the rotation needs to be for the liquid to not fall out of the billy-can when it is upside-down.

But have a go yourself. It may be easiest to practice first with a cheap liquid (water) in a bucket with a good handle. You will soon learn how to swing it vertically without the water coming out at the top (or when starting or stopping). The trick is in stopping slowly over the last three-quarters of a cycle so the water doesn't slosh out. ie start slowing down smoothly as it goes through the highest position.