Study Drag Forces and Motion in Different Media
✅ FREE Experiment • 📊 Real-Time Analysis • 🎓 NEB Class 11 Physics
When objects fall through a fluid (air, water, or oil), they experience a resistive force called drag force or air resistance. This force opposes motion and increases with velocity. Eventually, the drag force equals the gravitational force, and the object reaches terminal velocity - a constant maximum speed where acceleration becomes zero.
This experiment allows you to drop different objects (feather, ball, brick) through various media (vacuum, air, water, oil) and observe how air resistance affects their motion. You'll see velocity-time graphs, calculate terminal velocities, and understand why skydivers reach a constant falling speed.
Earth: 9.81 • Moon: 1.62 • Mars: 3.71
Note: In vacuum, objects fall with constant acceleration (9.81 m/s²). With air resistance, acceleration decreases as velocity increases, eventually reaching zero at terminal velocity where drag force equals weight.
Air resistance is a frictional force that opposes the motion of an object through a fluid. Unlike friction on surfaces, air resistance increases with velocity. The drag force can be approximated by:
Where:
When an object falls, two forces act on it: gravitational force (downward) and drag force (upward). Initially, gravity dominates and the object accelerates. As velocity increases, drag force increases until it equals the gravitational force. At this point, net force is zero and acceleration becomes zero. The object continues falling at constant velocity called terminal velocity.
At terminal velocity, forces balance:
Solving for terminal velocity v_t:
Drag force is proportional to v² (at high speeds). This means if you double the speed, drag force quadruples. This quadratic relationship causes terminal velocity behavior.
Larger objects experience more air resistance. A parachute has a large area, creating high drag and low terminal velocity (safe landing). Streamlined objects minimize area in the direction of motion.
The drag coefficient C depends on object shape:
Denser fluids produce more drag. Water is about 800 times denser than air, so objects reach terminal velocity much faster in water. In vacuum (ρ = 0), there's no air resistance.
Applying Newton's Second Law:
Initially (v = 0), F_drag = 0, so a = g. As velocity increases, F_drag increases, causing acceleration to decrease. When F_drag = mg, acceleration becomes zero.
Unlike free fall (straight line), velocity with air resistance follows an exponential curve, asymptotically approaching terminal velocity. The steeper initial slope gradually flattens.
Acceleration starts at g and exponentially decreases to zero as the object approaches terminal velocity. This is opposite to free fall where acceleration remains constant at g.
| Aspect | Free Fall (Vacuum) | With Air Resistance |
|---|---|---|
| Acceleration | Constant (9.81 m/s²) | Decreases with time |
| Velocity | Increases indefinitely | Reaches terminal velocity |
| All objects | Fall at same rate | Different rates (depends on mass/area) |
| Distance equation | s = ½gt² | Complex (exponential) |
Air resistance (or drag) is a frictional force that opposes the motion of an object through air or any fluid. It acts in the direction opposite to the velocity and increases with speed. Unlike surface friction, air resistance depends on velocity squared at high speeds.
Terminal velocity is the maximum constant velocity reached by a falling object when the drag force equals the gravitational force. At this point, net force is zero, acceleration is zero, and the object continues falling at constant speed. Formula: v_t = √(2mg / ρCA).
In air, feathers have a much larger surface area relative to their mass compared to stones. This gives them a much lower terminal velocity. However, in a vacuum where there's no air resistance, both fall at the same rate (demonstrated by Apollo 15 astronaut on the Moon).
Drag force is proportional to velocity squared (F_drag ∝ v²). This means if you double the speed, drag force becomes four times larger. This quadratic relationship is why terminal velocity occurs - as speed increases, drag increases rapidly until it balances weight.
Four main factors: (1) Velocity - higher speed means more drag, (2) Cross-sectional area - larger area experiences more drag, (3) Shape (drag coefficient) - streamlined shapes have less drag, (4) Fluid density - denser fluids create more resistance.
The drag coefficient (C) is a dimensionless number that quantifies how streamlined an object is. Lower values mean less drag. Sphere: C ≈ 0.47, Flat plate: C ≈ 1.28, Streamlined body: C ≈ 0.04. It depends on shape but not size or velocity.
Initially, only gravity acts (a = g). As the object speeds up, air resistance increases, creating an upward force. Net force = mg - F_drag, so acceleration a = g - (F_drag/m). As F_drag increases, acceleration decreases until F_drag = mg and a = 0 (terminal velocity).
Not always. Terminal velocity is reached if the object falls for sufficient time/distance. For short drops (like from a table), objects hit the ground before reaching terminal velocity. In vacuum, there's no terminal velocity - objects accelerate continuously.
Parachutes greatly increase the cross-sectional area (A in F_drag = ½ρv²CA), dramatically increasing drag force. This reduces terminal velocity to safe landing speeds (around 5-7 m/s). Without a parachute, a skydiver's terminal velocity is about 53 m/s (190 km/h).
The Moon has no atmosphere, so there's no air resistance. All objects fall at the same rate regardless of mass or shape (a = 1.62 m/s², the Moon's gravity). This was famously demonstrated by Apollo 15 astronaut David Scott with a hammer and feather.
At high velocities (Reynolds number > 1000), drag is proportional to v² due to turbulent flow. The object must push aside air molecules at rate proportional to v, and the momentum change of these molecules is also proportional to v, giving F ∝ v². At very low speeds, drag can be proportional to v (viscous drag).
Air density decreases with altitude. At higher altitudes, air is thinner, so drag force is smaller and terminal velocity is higher. This is why high-altitude skydivers reach higher speeds. At 40 km altitude, air density is only 0.3% of sea level.
Surface friction is independent of velocity and depends on normal force (F = μN). Drag force increases with velocity (F ∝ v²), depends on fluid density, and doesn't need physical contact (just motion through fluid). Both oppose motion but have different mathematical forms.
Surface tension causes water to minimize surface area, forming spheres. Small raindrops are nearly perfect spheres. Larger drops (>4mm) deform to oblate spheroids due to air pressure from below. Raindrops reach terminal velocity of 2-9 m/s depending on size (not 200+ m/s as they would in vacuum).
Birds spread wings to increase drag for landing (like parachutes), and fold wings to reduce drag for diving. They adjust wing angle to balance lift and drag. Tail feathers act as air brakes. This precise drag control allows for agile maneuvering and controlled landing.