How does the toy rotate ? / about which point
toy rotating ? (Centre)
Answers
Answer:
Toy rotation is simple. You choose a small selection of toys for your child to play with and box up the rest. At regular intervals, you'll swap the current toys for and the ones you stored away.
Answer:
A spinning top is not just a child’s toy. It is also an adult toy, in the sense that it helps to have a PhD in physics to figure out how it works. Besides, some adults DO play with spinning tops (check the web), and some make very elegant ones (see www.puccimanuli.com under spinners). A spinning top is remarkable because it tends to defy gravity while it is spinning. If spun fast enough, a spinning top will rise to a vertical position and happily stay there, despite the fact that it will fall over if it stops spinning.
A spinning egg is just as amazing. If a hard-boiled egg is spun fast enough it will rise up and spin on one end. A tippe top does something similar, but is even more spectacular. It turns itself completely upside down and ends up spinning with its peg underneath the sphere that was originally spinning underneath the peg used to spin it.
The basic physics behind all these effects is that a torque is required to rotate an object. The torque is equal to the rate of change of angular momentum. There is nothing magic about that. It is the rotational equivalent of what happens when an object accelerates along a straight line. In that case, the force on the object is equal to the rate of change of its momentum. Angular momentum is similar to linear momentum, but it refers to motion in a circular rather than a straight line path.
Usually, the torque acting on a spinning top is just due to the weight of the top. If the top is perfectly upright there is no torque acting on it but if it leans sideways then it will tend to fall over due to the torque about the bottom end. It will indeed fall over if it is not spinning. If it is spinning then it does something else. Instead of falling down, it “falls” sideways. That’s the amazing part. The effect is described as precession, and is explained in simple terms below. A spinning top precesses slowly around a vertical axis through its point of support while it spins rapidly about its own axis. The usual “explanation” is that the change in angular momentum must be in the same direction as the torque on the top – that is, in the sideways direction. The spin axis must move sideways instead of down, but that is just stating the observed facts in fancy technical words.
The following slow motion video clips show what happens with different types of tops, including a spinning egg and two types of tippe top. The tops were filmed at 300 fps to measure their spin and rate of precession. You will see the tops spinning ten times slower than they actually did. The first two are a 100 gram aluminium disk with a pointy bottom end, viewed from the side and the top (just before it fell). The third and fourth is the same disk supported on a round, brass knob. The bottom end makes a big difference. The brass end top takes a while to stand up straight, as shown in the fourth video clip. Its centre of mass rises slowly since the brass ball rolls and the friction force at the bottom end is relatively small. It is the torque generated by friction at the bottom end that causes tops to rise upward and defy gravity. However, all tops eventually fall when the spin drops to a low value. Further details can be found in “The rise and fall of spinning tops”, Am J Phys 81(4), 280-289 (2013).
Explanation: