CSC 395 (Fall 2021)

Reading: Physical Realities

Physical Realities

Because our user interfaces are usually tangible artifacts—by design, they are meant to be interacted with by users—we must consider the limits of physical interaction. In this set of readings, we’ll look at two aspects of physical interaction and see how they shape the design of user interfaces.

Fitts’ Law

Fitts’ law models the time \(T\) required to perform a “pointing task”, abstractly defined as a function of the distance \(D\) from the target and the width \(W\) of the target.

\[ T = a + b \log_2 (1 + \frac{D}{W}) \]

Fitts’ law gives us a basis for such design heuristics as “make the buttons big and clickable areas large” as well as novel interface widgets such as radial menus and corner hotspots.


In addition to locomotion, we must also consider our ability to perceive time. Nielsen observes a so-called “powers of 10” rule concerning human perceptions of time. This rule has implications regarding how we design systems to appear responsive to users.

The Illusion of Multitasking

Finally, consider the action of multitasking—performing multiple tasks simultaneously. While many people have the belief that they are “excellent multitaskers”, it is important to realize that limitations on our working memory make multiasking a suboptimal mode to work under.

Finally, something fun to try to reflect on what we do when we multitask is the old-school Multitask flash game: