Video Transcript
The following figure depicts four identical metallic balls thrown into four different
liquids at the same instant in time. Which fluid is the least viscous? (A) W, (B) X, (C) Y, or (D) Z.
In this question, we are shown figures of four identical metallic balls that were
thrown into four beakers of liquid at the same time. We are asked to work out which fluid is the least viscous. In order to figure this out, let’s first remind ourselves about what viscosity is and
how it affects the movements of objects in a fluid.
Viscosity is defined as the magnitude of internal friction in a fluid. The greater the viscosity of the fluid, the greater the internal friction. Because of this friction, any object that moves through a fluid will experience a
drag force that affects the object’s speed.
Drag forces always act to oppose the motion of an object. So, if an object is moving through a fluid, the drag force will oppose that motion
and slow the object down. The more viscous a fluid, the greater the drag force it will apply to objects that
move through it and so the slower the object will move. This is a fairly intuitive result. Imagine dropping a ball bearing into a tub of water and a tub of syrup. We would expect the ball to fall through the syrup much more slowly than the water
because the syrup is much more viscous. And hence, the ball experiences a greater drag force.
Now that we remember what viscosity is and how it affects objects moving through a
fluid, let’s take another look at the figures we were given and see where each ball
is in which liquid.
We can see that the ball in beaker W is near the bottom. The ball in beaker X is near the middle. The ball in beaker Y is right at the very bottom. And the ball in beaker Z is near the top, just below the surface of the liquid. We already know that the more viscous a fluid is, the greater the drag force it will
exert on an object moving through it, and the slower the object will move. This means that objects moving through less viscous fluids will move a greater
distance in a given time because the drag forces are smaller.
In this case, because the metallic balls are identical and they were all thrown into
each liquid at the same time, the beaker with the least viscous liquid will have the
ball that has moved the greatest distance. This is simply the ball that has gotten the closest to the bottom of the beaker. This means that the liquid in beaker Y must be the least viscous, since the ball in
beaker Y has already reached the bottom before any of the other balls. So, option (C), Y, is the correct answer.
