HomePhysicsP1: Forces and MotionP1.5 Gravity, Weight, and Drag

P1: Forces and Motion

P1.1 Distance and SpeedP1.2 AccelerationP1.3 Forces and Newton's LawsP1.4 Balanced and Unbalanced ForcesP1.5 Gravity, Weight, and DragP1.6 Hooke's Law – Springs and Elasticity
P1: Forces and Motion

Gravity, Weight, and Drag

Understand gravitational force, weight calculations, and terminal velocity

Skydiver demonstrating terminal velocity

Falling Through Air

Weight, drag, and terminal velocity

Gravity and Weight
The force that pulls everything down

Gravity is a fundamental force of attraction between all objects with mass. Near planets, it pulls objects toward the center.

Weight is the force of gravity on an object:

Weight = mass × gravitational field strength

W = m × g

On Earth, g ≈ 10 N/kg (or 10 m/s²). This means every kilogram of mass experiences 10 N of gravitational force.

Mass vs Weight

  • Mass (kg): Amount of matter - constant everywhere
  • Weight (N): Force of gravity - varies with location

A 60 kg person weighs 600 N on Earth but only 96 N on the Moon (g = 1.6 N/kg).

Air Resistance and Drag
Forces that oppose motion through air

Drag (air resistance) is a friction force that opposes motion through air. Key properties:

  • Speed dependence: Drag increases with speed (proportional to v² at high speeds)
  • Surface area: Larger area = more drag
  • Shape: Streamlined shapes reduce drag
  • Direction: Always opposes motion
Terminal Velocity
When falling objects stop accelerating

When an object falls, it initially accelerates because weight is greater than drag. As speed increases, drag increases until it equals weight. At this point, forces are balanced and the object reaches terminal velocity—constant speed with no further acceleration.

Skydiver example:

  1. 1. Initially: Weight > Drag → Accelerates downward
  2. 2. Speed increases → Drag increases
  3. 3. Eventually: Weight = Drag → Terminal velocity (~55 m/s)
  4. 4. Parachute opens: Drag > Weight → Decelerates
  5. 5. New terminal velocity reached (~5 m/s)
Gravity, Weight, and Drag Simulator
Explore weight on different planets and terminal velocity
70kg

v = 0.0 m/s

t = 0.0 s

Settings

Calculations

Weight: W = mg = 70 × 10 = 700 N

Current Drag: 0.0 N

Net Force: 700.0 N

Terminal Velocity: ~118 m/s

Key Concepts

  • • Weight = mass × gravitational field strength
  • • Drag increases with speed squared
  • • Terminal velocity: when drag = weight
Key Terms Flashcards
Click the card to reveal the definition

Gravity

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Worked Example
Calculating weight on different planets

Question:

An astronaut has a mass of 80 kg. Calculate their weight on: (a) Earth (g = 10 N/kg), (b) Moon (g = 1.6 N/kg). (c) Explain why a skydiver reaches terminal velocity.

Answer:

(a) Weight on Earth:
W = mg = 80 × 10 = 800 N

(b) Weight on Moon:
W = mg = 80 × 1.6 = 128 N

(c) Terminal velocity explanation:
As the skydiver falls, their speed increases. Air resistance (drag) increases with speed. Eventually, drag becomes equal to weight. At this point, forces are balanced and acceleration is zero. The skydiver continues at constant velocity (terminal velocity).

Test Your Knowledge
Question 1 of 6

What is the weight of a 50 kg person on Earth? (g = 10 N/kg)