physics
posted by SAM .
You stand at the top of a cliff while your friend stands on the ground below you. You drop a ball from rest and see that it takes 0.90 for the ball to hit the ground below. Your friend then picks up the ball and throws it up to you, such that it just comes to rest in your hand.
What is the speed with which your friend threw the ball?

The same speed with which it hit the ground.
v = g t
v = 9.8 m/s^2 (.9 seconds) = 8.8 m/s 
Learning Goal: To learn to read a graph of position versus time and to calculate average velocity.
In this problem you will determine the average velocity of a moving object from the graph of its position as a function of time . A traveling object might move at different speeds and in different directions during an interval of time, but if we ask at what constant velocity the object would have to travel to achieve the same displacement over the given time interval, that is what we call the object's average velocity. We will use the notation to indicate average velocity over the time interval from to . For instance, is the average velocity over the time interval from to .
Find the average velocity over the time interval from 1 to 3 seconds.
Express your answer in meters per second to the nearest integer.