Lesson 6 of 6 · Friction and drag
Explain resistance and a steady descent
8 minute lesson · 3 checks along the way
Your goal for this lessonIdentify relative motion at contacts or through air, and use a force balance to interpret a parachute descent.
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Words with a dotted underline open a short definition right here in the lesson.
Look at the contact, not just the journey
Friction is a contact force that resists relative slipping or the tendency to slip between surfaces. A box sliding across a floor experiences friction opposite its sliding direction. But friction does not always point opposite an object’s overall travel. A powered rover wheel pushing backward against firm ground can receive a forward frictional force from the ground, providing traction for .
acts when contacting surfaces do not slip relative to each other; applies during sliding. Static friction adjusts to the needed value up to a limit, rather than always having its maximum value. Real rover traction also depends on loose soil and wheel sinkage, so a simple firm-surface model is a starting assumption, not a complete prediction for every planetary terrain.
Use motion relative to the surrounding fluid
is the part of a fluid’s force opposing an object’s motion relative to that fluid. Air and water are both fluids. A parked object can experience aerodynamic drag in wind even when its ground speed is zero. For a descending parachute in still air, drag acts upward. Its magnitude depends on flow conditions, shape, area, and speed through the air.
A common model writes drag as D = ½ρCdAv², where ρ is fluid density, Cd is a drag coefficient, A is the chosen reference area, and v is relative speed. If density, coefficient, and area stay fixed, doubling speed quadruples drag. Those conditions matter: an opening canopy changes shape and area, and the coefficient can change when the flow changes.
NASA photographer Bill Ingalls recorded Soyuz TMA-18M descending beneath its main parachute on March 2, 2016. The complete archived color frame shows the deployed canopy and capsule above clouds. The lesson’s capsule mass and force values are explicitly fictional teaching inputs.
Look closely
Does the broad deployed canopy prove the capsule had reached terminal speed at the instant of this exposure?
See what to notice
No. It shows a structure that interacts with the air. Establishing terminal descent requires velocity measurements and a force-balance model, including the state of the canopy and surrounding air.
Check 1 of 3
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Work through a decision
Diagnose a descent from measured forces
Consider an invented vertical drop in still air. Treat capsule and parachute together as a constant- 200 kg system, use g = 10 m/s², and neglect buoyancy. At one instant its weight is 2,000 N downward and measured drag is 1,400 N upward. The is 600 N downward, giving acceleration 600/200 = 3 m/s² downward. A downward-moving system then gains speed.
Later, suppose drag reaches 2,000 N and the two forces remain balanced in unchanged conditions. Acceleration is zero, but the system continues descending at a nonzero . If drag temporarily exceeds weight while it is still moving downward, upward acceleration reduces its downward speed. An upward acceleration arrow alone does not mean the capsule is already rising.
Check 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Distinguish a visible canopy from a force measurement
The Soyuz TMA-18M return photograph shows a small capsule beneath a broad orange-and-white parachute above clouds. It makes the drag-producing structure visible. It does not establish terminal speed, air density, or instantaneous acceleration from appearance alone. The exercise’s 200 kg mass and force values are teaching inputs, not reported measurements of this crewed vehicle or this exposure.
To decide whether a descent is steady, compare measurements across time and account for changing air and canopy conditions. To decide why a rover accelerates, examine its contact with the ground. Both tasks use the same method as a : choose the system and frame, identify interactions, then add forces with directions. Resistance can slow a vehicle, provide traction, or balance gravity during continued motion.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
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Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: OpenStax: frictionStatic and kinetic friction, contact motion, and the limits of a simple model.
- Read: NASA Glenn: dragDrag depends on motion relative to a fluid.
- Read: NASA Glenn: the drag equationThe roles of speed, density, area, and drag coefficient.
- Read: OpenStax: drag and terminal speedForce balance at terminal descent and the assumptions of quadratic drag.