Behaviour (Fogg B=MAP)
Physical Simplicity
How physically demanding is it?
What it is
Physical Capacity is the Ability lever of physical effort required to complete the action — taps, scrolls, typing, swipes, movement.
Why it works
Physical effort is one of Fogg''s six simplicity factors, and on mobile it is frequently the one that binds. Every tap, scroll, swipe and especially every character typed is a cost, and small costs compound across a flow in a way that is invisible in design review and obvious in analytics. Fitts''s Law formalises part of it: the time to reach a target depends on its distance and its size, which is why small controls near screen edges are measurably slower to hit. Typing is the most expensive interaction of all on a phone, which is why autofill, one-tap authentication, camera capture and stored payment credentials produce conversion gains that no amount of copy optimisation can match. The lever is removal, not persuasion.
How to apply it in design
Reduce taps, scrolls, and typing wherever possible. Use one-tap auth, autofill, voice input, photo capture instead of forms. Every saved finger movement is ability gained.
Where you'll see it
- One-tap authentication instead of typed credentials
- Autofill and correctly typed input fields on every form
- Camera or file capture in place of manual entry
- Stored payment credentials and platform wallets at checkout
- Touch targets sized and positioned for thumbs, not cursors
Example
Apple Pay's one-tap checkout vs. typing a 16-digit card number on a tiny mobile keyboard is a Physical Simplicity victory worth billions in conversion.
Failure mode
Long forms on mobile without autofill are the most common physical-ability sin in product design. Every required field on mobile is an ability tax.
When not to use it
Deliberate friction is correct for destructive and irreversible actions — deleting an account, wiring money, publishing to a large audience. Making those effortless is a design failure, and a confirmation step is effort well spent. Physical simplicity also has limits where accuracy matters more than speed: autofilling a legal name or a dosage without confirmation trades a small time saving for a large error risk. And accessibility sometimes requires more steps rather than fewer, since the shortest path for a thumb is not always the most navigable one for assistive technology.
The ethical line
One-tap flows are exactly as good as their reversibility. A purchase completed in a single tap with no confirmation and a difficult refund path is not convenience, it is an accident engine — and the accidents are not evenly distributed, since they fall hardest on users who are rushed, distracted or unfamiliar with the pattern. The rule is proportionality: the easier an action is to take, the easier it must be to undo. Where effort is removed from committing, equivalent effort must be removed from reversing.
What to measure
Count taps and required keystrokes for the core flow and track the number down over time; it is one of the few UX metrics that improves monotonically. Segment funnel drop-off by device, since physical cost is mostly a mobile problem and desktop averages hide it.
Frequently asked questions
What is physical effort in the Fogg Behavior Model?
Physical effort is one of the six simplicity factors determining Ability in B=MAP: the bodily work an action requires, measured in taps, scrolls, swipes and keystrokes. On mobile it is often the scarcest resource, and typing is the most expensive interaction of all.
What is Fitts's Law in UX?
Fitts's Law states that the time to acquire a target depends on the distance to it and its size. In interfaces it explains why small controls placed near screen edges are slower and more error-prone to hit, and why primary actions should be large and within comfortable thumb reach.
Why does one-tap checkout convert so much better?
Because it removes the single most expensive interaction on a phone — typing a long number accurately on a small keyboard. Payment wallets replace dozens of error-prone keystrokes with one authenticated gesture, which is a larger ability gain than any copy change can deliver.
Should you ever add friction on purpose?
Yes, for destructive or irreversible actions. Account deletion, large transfers and public publishing all deserve a confirmation step, because the cost of an accidental action far exceeds the cost of one extra tap. Ease of undoing should always match ease of doing.
Related skills
Source: Growth.Design — B=MAP (Ability)
Practise Physical Simplicity on a real brief
Quest gives you a client brief that targets this skill and 60 minutes to ship an iteration.