[Physics] Why does the water bottle not rotate when it is half full

rotational-dynamics

Consider this water bottle: enter image description here

When it is full and thrown up in the air, it rotates at a constant velocity.

When it is less than 1/8th full, the water bottle rotates even faster than when it was full.

When it is half full, however, the water bottle rotates for one half-spin, and then it stops rotating.

Why is this?

By the way, I tested it with a 16.9oz bottle, but the bottles are mathematically similar.

Best Answer

When it is half full, however, the water bottle rotates for one half-spin, and then it stops rotating.
Why is this?

This is why you don't want to ship oil across the ocean in a half-full oil tanker. If you do, you had better equip that tanker with some very good anti-slosh mechanisms. The same goes for trucks, trains, and spacecraft carrying fluid. One quarter to three quarters full is when fluid sloshing (linear fluid dynamics), slamming (non-linear fluid dynamics), and whipping (highly non-linear fluid dynamics) are at their worst. Above three quarters full, most of the fluid cannot participate in the sloshing, slamming, or whipping. Below a quarter full, most of the fluid is free to participate in the sloshing, slamming, or whipping, but there's not much fluid in the container.

In the case of the bottle full of water, there's no room for the fluid to slosh or splash. The angular momentum you impart to the bottle is quickly transferred to the fluid. The bottle and fluid rotate as one.

In the case of the half-full bottle of water, you aren't immediately transferring angular momentum to the water. Instead, your initial flip initiates a slosh wave. That slosh wave is large in amplitude and doesn't have to travel far before it hits the other side of the bottle. This is non-linear dynamics. That wave smashing into the other side of the bottle marks when a good deal of angular momentum is transferred to the water. The bottle's rotation rate slows down markedly at this point (but it does not come to a stop).

In the case of the nearly empty bottle of water, the transfer of angular momentum to the water once again isn't immediate. Once the transfer has been complete, the bottle rotates about a point well below the center of the bottle. Given the reduced mass of the water and the lowered center of rotation, a good share of the angular momentum remains with the plastic bottle rather than being transferred to the water. The bottle rotates faster than is the case with the half empty bottle.

I can't fully replicate your results. I can give a full bottle of water a very hefty rotation rate by imparting some backspin while I toss the bottle. I couldn't make a partially filled bottle rotate anywhere near that fast. The half-full bottle wants to come to a near stop mid-flight unless I crank my arm around a few times before letting go. (This lets the slosh wave hit the other side prior to release.) The near empty bottle does rotate faster than the half-full bottle, but not as fast as the plumb full bottle.

Related Question