Does the question say anything about when the particle begins accelerating?
A particle in Switzerland travels in a circle of radius 10 km. initially the time taken to complete one circuit is 1 second. After 10 seconds, the particle is travelling so fast that it can complete the same circuit in 0.5 seconds. What is the average tangential acceleration?
(Hint: a particle travelling on a circle of radius R has tangential speed of Rdx/dt, where x is the polar angle in radian. Initially dx/dt= 2π, in this question. And the tangential acceleration is R(d^2 x)/(dt^2 )