Freefall

Freefall is the motion of an object when gravity is the only force acting on it. Near the Earth’s surface, gravity pulls objects downward with almost constant acceleration.

Key facts

• Acceleration of free fall is approximately constant: $g \approx 9.8\,\text{m s}^{-2}$ (often rounded to $10\,\text{m s}^{-2}$). It acts downward.
• Weight and mass: weight is the force of gravity. $W = mg$. Mass $m$ measures how much matter is in an object and does not change with location; weight changes if $g$ changes.
• If an object is dropped (starts from rest): $u=0$, so $v = gt$ and $s = \tfrac{1}{2}gt^{2}$.
• If an object is thrown upward, it slows down at $g$, reaches momentary zero speed at the top, but acceleration is still downward ($-g$).
• On a speed–time graph, freefall without air resistance is a straight line with slope $g$. The area under the graph is the distance fallen.

With and without air resistance

• No air (vacuum): all objects accelerate equally at $g$, whatever their mass.
• With air: air resistance (drag) increases with speed and opposes motion. Net force decreases, so acceleration becomes less than $g$.
• Terminal velocity: the steady speed reached when drag equals weight. Then acceleration is zero and speed is constant. A parachute increases air resistance, giving a much lower terminal velocity and a large (upward) deceleration when it opens.

Common misconceptions

• “Heavier objects fall faster.” In a vacuum, they do not. With air, heavier or more streamlined objects can have a higher terminal velocity, but $g$ is the same for all.
• “Acceleration is zero at the top of a throw.” Speed is zero there, but acceleration is still $g$ downward.