Motion Through Fluids - Drag and Terminal Velocity

At the moment of jumping, the skydiver's velocity is zero. Drag depends on speed — if speed is zero, drag force is also zero.

Using $F_{net}=W-D$, and substituting $D=0$: $F_{net}=W-0=W$ The entire weight acts as net force downward.

Since the net force equals the full weight of the skydiver acting downward, they experience maximum acceleration at the instant of jumping. This acceleration is approximately $g=9.8{\text{m/s}}^2$ downward.

The correct answer is: Only weight acts on the skydiver, so they accelerate downward at maximum rate.

We need to determine the forces on the skydiver at the instant their velocity is zero (just after jumping).

'Weight acts downward and drag acts upward, and they are equal' describes terminal velocity, not the starting instant. This is incorrect because drag is zero when speed is zero.

'Drag is greater than weight, so the skydiver decelerates' is impossible at the start — there is no drag when the skydiver is stationary, so drag cannot exceed weight.

'Weight and drag both act downward' is incorrect because drag always opposes motion — it acts upward when falling. Also, drag is zero here anyway.

The remaining option — Only weight acts on the skydiver — is correct. At zero velocity, drag = 0, so only weight acts, producing maximum downward acceleration.

A fluid is any substance whose particles can flow past one another freely. Both liquids (like water and mercury) and gases (like air and steam) are fluids.

For the choice “Water, air, mercury, steam”: Water (liquid ✓), Air (gas ✓), Mercury (liquid ✓), Steam (gas ✓). All four are fluids. The other options include solids like ice, steel, or sand, which are not fluids.

Water, air, mercury, steam is the only list containing exclusively fluids.

We need a list where every substance is a fluid (liquid or gas). Any list with a solid is automatically eliminated.

'Water, ice, air, oil' — Ice is a solid; its particles are locked in a rigid structure and cannot flow. This list is eliminated.

'Steel, air, water, olive oil' — Steel is a solid metal. This list is eliminated.

'Sand, water, air, nitrogen gas' — Sand consists of solid particles. Although sand can pour, individual grains are solids. This list is eliminated.

Water, air, mercury, steam — all four are liquids or gases, confirming they are all fluids.