For opening (or closing) a tap or a bottle cap we use force on two opposite direction. Why?

When two equal and opposite forces are applied to an object, they tend to rotate the object. The pair of forces are known as couple. They produce no translational motion, only rotational motion because their resultant is zero.

When a hand is applying an equal and opposite force to the top of a tap, in a clockwise direction, a rotating effect is produced which opens the tap. The rotation is achieved around the central axis of the tap. When the tap is to be closed, a couple is applied again in an anti-clockwise direction.

We do the same action while opening a bottle cap. We twist the cap by applying a couple to produce rotation.

Try doing the same action using force on one side only, you will not succeed in opening either the tap or the bottle cap!

How does a garden sprinkler works?

Garden sprinklers are employed to water the garden; they seem to be able to rotate continuously and spray water all around automatically. If you knew some facts about rotational motion, you will not say this! There are many types of garden sprinklers in the market, but all of them work on employing torque and angular momentum to their advantage.

A sprinkler is placed vertically on a pipe. Water is sent up to the pipe with some force. The sprinkler has openings or nozzles, which are off the vertical axis. The water, when forced out of a nozzle, produces a force (or a torque) at an angle with respect to the vertical axis. This sets the sprinkler rotating about the axis. Once the sprinkler is rotating, the angular momentum it acquires keeps it rotating again and again around the axis. The sprinkler ofcourse has to sit on the pipe loosely. Sometimes when the sprinkler gets stuck, a little tangential push is sufficient for it to start rotating again.

In a circus, how does the death well trick is done without any accident?

In a death well a motor cyclist cycles in all directions inside an enclosed sphere. It is fascinating to watch the driver go at a fast speed inside the sphere. Sometimes to add some excitement and thrill more than one motor cyclist or a motor cyclist and a car is also included.

The motorcyclist is undergoing non - uniform circular motion. He does not fall down as he moves against the upper surface of the chamber because he moves at sufficiently high speed. At the highest point, his speed cannot be less than the minimum value (gr)^1/2.

The rider is aware of this and knows the radius of the circle. He therefore ensures that his speed is high enough to overcome the downward pull of gravity.