Question

The tube as shown in the figure is of uniform cross section. The liquid flows through it at a fixed speed in the direction shown by the arrows. Then the liquid exerts on the tube will be given as,

A. a net force to the right
B. a net force to the left
C. a clockwise torque

Answer 1

Hint: Draw the diagram of the tube through which the liquid is flowing and mark the points at which the change in momentum is taking place. The change in momentum can sometimes cause a torque to act on the body. This information will help you in solving this question.

Complete step-by-step answer:

The force exerted by the liquid at the regions of bends are depicted in the figure. The liquid will undergo a variation in momentum at these particular points. According to law of the conservation of angular momentum, if there is no external resultant torque acting on a system, the angular momentum of the system is conserved. A resultant torque will be creating a variation in angular momentum which will be equivalent to the product of the torque and the time interval over which the torque is applied. Therefore as here the change in momentum at the bends are experiencing, there will be torque acting also. These two forces can form a couple which will exert a clockwise torque on the tube. Hence the liquid will exert a torque on the tube which will make the tube rotate in a clockwise direction. The correct answer is given as option C.

So, the correct answer is “Option C”.

Note: The torque can be expressed as the rotational equivalent for the force acting in classical mechanics. The angular momentum can be the expression of the rotational equivalent of the linear momentum also. In short we can say that the torque will be the rate of variation of angular momentum.