Terminal velocity is the maximum velocity attainable by an object as it falls through a fluid, such as air or water. It occurs when the sum of the drag force and the buoyancy is equal to the downward force of gravity acting on the object. As the speed of an object increases, so does the drag force acting on it, which also depends on the substance it is passing through (for example air or water). At some speed, the drag or force of resistance will equal the gravitational pull on the object. At this point, the object stops accelerating and continues falling at a constant speed called the terminal velocity.

Some key points about terminal velocity include:

• Terminal velocity is achieved when the speed of a moving object is no longer increasing or decreasing; the object’s acceleration (or deceleration) is zero.
• An object with a large projected area relative to its mass, such as a parachute, has a lower terminal velocity than one with a small projected area relative to its mass, such as a dart.
• For the same shape and material, the terminal velocity of an object increases with size.
• The terminal velocity of an object changes due to the properties of the fluid, the mass of the object, and its projected cross-sectional surface area.

Terminal velocity can be calculated using the drag equation, which depends on the drag coefficient, air density, the square of the air velocity, and some reference area of the object. The equation tells us that an object with a large cross-sectional area or a high drag coefficient will fall slower than an object with a small area or low drag coefficient.

In summary, terminal velocity is the constant speed an object acquires after falling through a fluid, like air or water, and it occurs when the sum of the buoyant force and the drag force equals the force due to gravity.