Equations of motion Common pitfalls

Using the equations when acceleration is not constant

SUVAT assumes constant acceleration. If acceleration changes, go back to Kinematic definitions and use calculus or graph areas.

Mixing distance and displacement

$s$ means displacement. If motion reverses direction, total distance travelled may be larger than $s$.

Forgetting signs

Choose a positive direction before substituting numbers. For vertical motion, either upward or downward can be positive; just stay consistent. If upward is positive, gravity is $a=-9.81\mathrm{m}{\mathrm{s}}^{-2}$.

Treating speed and velocity as identical

Speed is a magnitude. Velocity has direction/sign. The equation $v=-5\mathrm{m}{\mathrm{s}}^{-1}$ can describe motion just as validly as $v=+5\mathrm{m}{\mathrm{s}}^{-1}$.

Solving for the wrong interval

Initial and final values belong to a chosen time interval. If the problem has stages, split it into stages and reset $u$, $v$, $s$, $a$, and $t$ for each one.

Related: Equations of motion Examples, Equations of motion Equations and definitions, Mechanics MOC.

Unit drift

Keep units consistent before substitution. A speed in $\mathrm{km}{\mathrm{h}}^{-1}$ must usually be converted to $\mathrm{m}{\mathrm{s}}^{-1}$, and centimetres must be converted to metres if acceleration is in $\mathrm{m}{\mathrm{s}}^{-2}$. Unit mistakes often look like plausible numbers, so check dimensions at the end.