Q.A resultant force of $15\ \text{N}$ acts on a block of mass $3\ \text{kg}$ that is initially at rest on a frictionless surface. Find

Given: force $F = 15\ \text{N}$, mass $m = 3\ \text{kg}$, initial velocity $u = 0$ (at rest), time $t = 4\ \text{s}$.

(a) Acceleration. By Newton's second law, $a = \frac{F}{m} = \frac{15\ \text{N}}{3\ \text{kg}} = 5\ \text{m/s}^2$

(b) Velocity after $4\ \text{s}$. Using the equation of motion $v = u + at$, $v = 0 + (5\ \text{m/s}^2)(4\ \text{s}) = 20\ \text{m/s}$

Find acceleration from $a = F/m$, then substitute into $v = u + at$ with $u = 0$.

• Forgetting to divide by mass and simply treating $15\ \text{N}$ as the acceleration.
• Using an equation of motion that requires distance or time-squared when the simpler $v = u + at$ is all that's needed here.