Observing a squirrel survive a long fall from a tree or building often sparks questions about physics and biology. These animals possess a remarkable resilience unavailable to larger creatures. To understand how far a squirrel would need to fall to die, one must first explore the physical law that dictates the maximum speed an object can reach during a descent. This principle explains why size and mass are the primary factors governing survival during a drop from a substantial height.
Terminal Velocity
When an object falls, gravity accelerates it downward, increasing its speed. As speed increases, the air resistance, or drag force, pushing back against the object grows stronger. Terminal velocity (\(V_t\)) is the maximum speed an object can achieve during a freefall when the upward force of air resistance exactly equals the downward force of gravity. At this point, the net force is zero, acceleration stops, and the object continues to fall at a constant speed.
Terminal velocity is determined by an object’s mass, cross-sectional area, and drag coefficient. A dense, compact object like a rock has a low surface area relative to its mass, resulting in a high terminal velocity. Conversely, a light object with a large, spread-out shape, such as a feather, reaches a much lower terminal velocity because air resistance quickly balances its low gravitational pull. This difference in the mass-to-surface-area relationship governs the outcome of a fall for all organisms.
Applying Terminal Velocity to Squirrels
The squirrel possesses a high surface area-to-mass ratio. A typical grey squirrel weighs about 0.5 kilograms, resulting in a low downward force from gravity. When falling, a squirrel instinctively spreads its limbs and flattens its body, creating a parachute that maximizes air resistance. This action ensures the drag force increases rapidly, balancing the low gravitational force at a low speed.
The terminal velocity for a squirrel is estimated to be in the range of 20 to 35 miles per hour. This speed is drastically lower than the terminal velocity of a human, which is approximately 120 miles per hour. Because the squirrel reaches this maximum speed quickly, falling from 50 feet results in the same impact velocity as falling from 500 feet. Therefore, the height required to reach a fatal speed is irrelevant, as the maximum speed attained remains constant regardless of the drop distance.
Anatomical Reasons for Survival
Survival at low terminal velocity is also due to the squirrel’s specialized anatomy. Their skeletal structure is light and flexible, helping to dissipate the force of impact across the body rather than concentrating it. The spine is able to bend and twist to absorb the shock of landing, preventing damage.
The squirrel’s bushy tail functions as an aerodynamic rudder, allowing the animal to control its descent and steer toward a landing spot. By using their tail and limbs, they can orient their body to land feet-first, similar to the righting reflex seen in cats. This combination of low-impact speed and a shock-absorbing body structure allows them to walk away from falls unscathed.
Conditions for a Fatal Fall
Fatal falls for a squirrel are rare and are not a consequence of the distance fallen. Since the maximum impact speed is non-lethal, death occurs only when a secondary factor introduces a force greater than the body can withstand. The most common cause of fatal injury is landing on a hard surface like concrete or asphalt. These surfaces do not allow the body to decelerate gradually, concentrating the impact force dangerously.
A lethal outcome can also be caused by striking a small, sharp, or fixed object during the descent or upon landing. Hitting a narrow branch or a spike at the wrong angle delivers a focused, shearing force that bypasses the squirrel’s natural shock absorbers. Another element is that a squirrel with wet fur would have a higher mass and less effective drag, potentially increasing its terminal velocity. A fatal fall is determined not by the height, but by the angle and nature of the landing surface.