Question

A particle moves along the y-axis so that its position at time $0\le t\le 20$ is given by $y\left(t\right)=5t-\frac{t^2}{3}$. At what time does the particle change direction?

• A. $5$ seconds
• B. $7.5$ seconds
• C. $10$ seconds
• D. $15$ seconds
• E. $18$ seconds

Answer 1

The correct option is B $7.5$ seconds

• The particle change direction when the derivative of $y\left(t\right)$ becomes zero
• So we get $5-2t/3=0$ , which implies $t=15/2=7.5$ seconds