What is the average speed, over the first 4.0 s of its motion, of a pebble released from rest off a bridge? what is the average speed, over the first 4.0 s of its motion, of a pebble released from rest off a bridge?

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! 

moving ball hits still ball. mover stops, hit ball moves. momentum transfer.

the answer is a. at the equator