圓周運動 (位移&速度)

(1) angular velocity = 2兀/3 s^-1
angular displacement = (2兀/3) x 0.5 radians = 兀/3 radians

(2) since angular displacement = 兀/3 radians = (兀/3).(180/兀) degrees = 60 degrees
Displacement = 2 x [2.sin(60/2)] m = 2 m

Q: 平均的加速度是怎樣得出?
The average acceleration is given by v^2/R (where v is the speed and R is the radius of the circular motion) in a radial direction towards the centre.

Q: 那 速度是怎樣計?..
If you refer to the instantaneous velocity, it is v with direction tangent to circular path at the point where the object is.
But if you refer to the average velocity, it is the linear displacement over the time taken.
For example, if an object moves with constant speed of 10 m/s around a circle of raduse 2 m. Then the time to complete half a circle is 2.pi.2/10 s = 2.pi/5 s.
The average velocity of the object in moving over a time interval of, say pi/5 s (i.e. half a circle) = diameter/(pi/5) m/s = 4/(pi/5) m/s = 20/pi m/s