Question

1. A liquid at rest in a fixed container exerts a force perpendicular to the wall of the container. Two students make claims about the microscopic cause of this force. Student A says that the force exerted by individual molecules as they bounce off the wall is always perpendicular to the wall. Student B says that the molecules may strike the wall at angles that are not perpendicular. Which student is correct and why?

A. Student A . If the molecules hit at nonperpendicular angles, there would be a component of force parallel to the wall exerted on them. The net result would be macroscopic motion of the fluid.

B. Student A . The molecules in a liquid are in random motion at the microscopic scale, so they move in every direction. Near the wall the molecules collide with each other in such a way that the components of their motion not parallel to the wall cancel.

C. Student B . The molecules in a liquid are in random motion at the microscopic scale. For every atom that hits at an angle to one side of perpendicular, there is likely to be another atom hitting at the same speed at the same angle on the other side of perpendicular. On average the nonperpendicular components cancel.

D. Student B . The walls of the container cannot move, so near the walls the parallel component of the motion of the molecules is cancelled by the net force the wall exerts on the liquid.

Answer 1

C option is correct, student B is correct.

As shown in the figure below, let any two molecules travel with equal speed (V1 = V2) and strike at same angle to the wall . It can be clearly seen that non perpendicular components (Sine components) cancel each other whereas the perpendicular components (cosine components) add up and thus giving a net perpendicular impact (force ) on the walls . Since all the molecules are in continuous random motion, therefore for each and every molecule striking the wall , there must be another molecule which strike the wall at same angle on the other side of perpendicular.

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