Why an object reaches terminal velocity

Air resistance - this is a frictional force acting in the opposite direction to the movement of the object. Note that in space and other vacuums there is no air resistance. Three stages of falling When an object is dropped, there are three stages before it hits the ground: At the start, the object accelerates downwards because of its weight.

There is very little air resistance. There is a resultant force acting downwards. The acceleration is constant when the object is close to Earth. As it gains speed, the object's weight stays the same but the air resistance on it increases. Eventually, the object's weight is balanced by the air resistance.

The resultant force is zero because the frictional force acting against it is now the same as the weight of the object. The object does not stop falling once its resultant force is zero, unless it has hit the ground. Terminal velocity Near the surface of the Earth, any object falling freely will have an acceleration of about 9. Three stages of falling There are three stages as an object falls through a fluid: at the start, the object accelerates downwards due to the force of gravity as the object's speed increases, frictional forces such as air resistance or drag increase at terminal velocity, the weight of the object due to gravity is balanced by the frictional forces, and the resultant force is zero The weight of an object does not change as it falls, as long as it stays whole.

A skydiver The diagram shows what happens to the speed of a skydiver from when they leave the aircraft, to when they reach the ground after their parachute opens.

Before the parachute opens: Immediately on leaving the aircraft, the skydiver accelerates downwards due to the force of gravity. There is no air resistance acting in the upwards direction, and there is a resultant force acting downwards so the skydiver accelerates towards the ground.

As the skydiver gains speed, their weight stays the same but the air resistance increases. There is still a resultant force acting downwards, but this gradually decreases. Eventually, the skydiver's weight is balanced by the air resistance. There is no resultant force and the skydiver reaches terminal velocity. Velocity-time graphs for falling objects The diagram shows a velocity-time graph for an object falling through a fluid, eg air, water, oil.

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. Under such conditions, all objects will fall with the same rate of acceleration, regardless of their mass. But why? Consider the free-falling motion of a kg baby elephant and a 1-kg overgrown mouse.

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 kg baby elephant would experiences a greater force of gravity. 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 kg baby elephant would accelerate faster.

But acceleration depends upon two factors: force and mass. The kg baby elephant obviously has more mass or inertia. This increased mass has an inverse effect upon the elephant's acceleration. 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. All objects placed upon Earth's surface will experience this amount of force 9.

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. As such, all objects free fall at the same rate regardless of their mass. Because the 9. Gravitational forces will be discussed in greater detail in a later unit of The Physics Classroom tutorial. As an object falls through air, it usually encounters some degree of air resistance.

Air resistance is the result of collisions of the object's leading surface with air molecules. The actual amount of air resistance encountered by the object is dependent upon a variety of factors. 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. Increased speeds result in an increased amount of air resistance.

Increased cross-sectional areas result in an increased amount of air resistance.