"As your jet plane speeds down the runway on take off, you measure its acceleration by suspending your yo-yo as a simple pendulum and noting that when the bob (mass 40g) is at rest relative to you, the string (length 70cm) makes an angle of 22 degrees with the vertical. Find the period T for small angle oscillations of this pendulum." I've tried to do this using total force on the bob = tension + mg + ma, where a is the acceleration of the plane. Resolving mg and ma into components I came out with: g sin 22 = a cos 22 which gives me a = 3.96m/s^2. Then using T = 2 pi (L/(g+a))^1/2 I found T = 1.42s, however the solution given in the book for this problem says T = 1.62s. Where have I gone wrong?