Answer:
a = 12.8 m/s^2
Step-by-step explanation:
To find the centripetal acceleration you use the following formula:
(1)
v: tangential speed of the rock = 17.90mph
r: radius of the orbit = 1000cm/2 = 500cm = 0.5m
You first change the units of the tangential velocity in order to replace the values of v and r in the equation (1):

Then, you use the equation (1):

hence, the centripetal acceleration is 12.8 m/s^2