A moving mass carries energy, and the amount is $E_k = \tfrac{1}{2} m v^{2}$ — read aloud, E-k equals one-half m v squared. The subscript k is for kinetic. $E_k$ is the kinetic energy in joules, $m$ is the mass in kilograms, and $v$ is the speed in metres per second. Solve it for whichever letter the question withholds.

Two features, both non-negotiable. The ½ is exact — it falls out of adding up $ma$ over the distance, and a force that grows from zero does, on average, half its maximum. The square is where the danger lives: double the speed and the energy goes up four times. That is not a curiosity, it is the reason a car at 100 km/h needs roughly four times the stopping distance of the same car at 50 — the brakes have four times as many joules to remove, and they remove them at much the same rate per metre.

Keep it separate from its lookalike. Momentum $p = mv$ uses the same two ingredients and comes out in kg·m/s; kinetic energy squares the speed, halves the product, and comes out in joules. Same pantry, different dish — and the units will tell you which one you cooked.