![]() For each case, use the diagrams to determine the net force and acceleration of the skydiver at each instant in time. In the diagrams below, free-body diagrams showing the forces acting upon an 85-kg skydiver (equipment included) are shown. Why does an object that encounters air resistance eventually reach a terminal velocity? To answer this questions, Newton's second law will be applied to the motion of a falling skydiver. Increased cross-sectional areas result in an increased amount of air resistance. Increased speeds result in an increased amount of air resistance. To keep the topic simple, it can be said that the two most common factors that have a direct effect upon the amount of air resistance are the speed of the object and the cross-sectional area of the object. The actual amount of air resistance encountered by the object is dependent upon a variety of factors. Air resistance is the result of collisions of the object's leading surface with air molecules. (Gravitational forces will be discussed in greater detail in a later unit of The Physics Classroom tutorial.)Īs an object falls through air, it usually encounters some degree of air resistance. Because the 9.8 N/kg gravitational field at Earth's surface causes a 9.8 m/s/s acceleration of any object placed there, we often call this ratio the acceleration of gravity. As such, all objects free fall at the same rate regardless of their mass. Being a property of the location within Earth's gravitational field and not a property of the free falling object itself, all objects on Earth's surface will experience this amount of force per mass. All objects placed upon Earth's surface will experience this amount of force (9.8 N) upon every 1 kilogram of mass within the object. The gravitational field strength is a property of the location within Earth's gravitational field and not a property of the baby elephant nor the mouse. This ratio (F net/m) is sometimes called the gravitational field strength and is expressed as 9.8 N/kg (for a location upon Earth's surface). The ratio of force to mass (F net/m) is the same for the elephant and the mouse under situations involving free fall. And thus, the direct effect of greater force on the 1000-kg elephant is offset by the inverse effect of the greater mass of the 1000-kg elephant and so each object accelerates at the same rate - approximately 10 m/s/s. This increased mass has an inverse effect upon the elephant's acceleration. The 1000-kg baby elephant obviously has more mass (or inertia). But acceleration depends upon two factors: force and mass. This greater force of gravity would have a direct effect upon the elephant's acceleration thus, based on force alone, it might be thought that the 1000-kg baby elephant would accelerate faster. If Newton's second law were applied to their falling motion, and if a free-body diagram were constructed, then it would be seen that the 1000-kg baby elephant would experiences a greater force of gravity. ![]() But why? Consider the free-falling motion of a 1000-kg baby elephant and a 1-kg overgrown mouse. Under such conditions, all objects will fall with the same rate of acceleration, regardless of their mass. ![]() Objects that are said to be undergoing free fall, are not encountering a significant force of air resistance they are falling under the sole influence of gravity.
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