Find the speed of the pebble after every second starting with t=0.
For this we can uses the newtons equation, v = u + a*t
where v is the final velocity, u is the initial velocity,  a is the acceleration which is 9.8 m/s²
When t = 0, Vf = 0 m/s
When t = 1, Vf = 9.8 m/s
Using v = 0 + 9.8*1
When t = 2, Vf = 19.6 m/s
Using v = 0 + 9.8*2
When t = 3, Vf = 29.4 m/s
Using v = 0 + 9.8*3
When t = 4, Vf = 39.2 m/s
Using v = 0 + 9.8*4
Then, you should just be able to average out the velocities and divide by five to get an average speed of 19.6 m/s!Â
