When an object is at rest in water, only the downward force of gravity and upward buoyancy forces act on the object. If these two forces are not balanced, the object will begin to accelerate in the vertical (up/down) direction.
Once an object in a fluid such as water starts moving vertically, it also "feels" a force of resistance due to its movement through the water caused by the water molecules. This "drag force" — similar to air/wind resistance — pushes against the object in the direction opposite of its motion. As the object speeds up, this drag force also increases. Before long, the drag force will be large enough so that the sum of all forces acting on the object in the vertical direction is zero. At this point, the object will reach its maximum speed and continue moving at this same constant speed.
The same thing happens to objects falling through the air: they will reach a maximum speed when the drag force (due to air resistance) is equal to the force of gravity. This maximum speed is called the "terminal velocity."
The shape of the object affects the drag force from the fluid on the object. For example, a tear-shaped object will experience less drag force than a spherical object. But the reasons why different shaped objects experience different amounts of drag force are pretty complicated. You may learn about this later in a fluid dynamics class, most likely in college (especially if you're an engineering or science major).
***Note for Teachers: You can access more information about this website and find other resources for science inquiry on the ISP Tutor website.
©2022 Klahr Lab. All Rights Reserved
The TED project was funded in part by the Institute of Education Science (IES), Grant R305H060034, and in part by the National Science Foundation, Grant SBE035442. The ISP Tutor project was funded by IES, Grant R305A170176.
The research reported here was supported by the Institute of Education Sciences, U.S. Department of Education, through Grant R305A170176 to Carnegie Mellon University (Klahr Lab). The opinions expressed are those of the authors and do not represent views of the Institute or the U.S. Department of Education.