Aball is thrown horizontally from a cliff at a speed of 10 m/s. calculate its speed one second later (be sure to show your math steps)

The ball is thrown horizontally at 10 m/s.

Its horizontal speed doesn't change, but because of gravity,
it picks up vertical (downward) speed at the rate of 9.8 m/s
every second.

So at the end of one second, the ball is moving ...

                 horizontally at    10 m/s
and
                 downward at      9.8 m/s  .

All together, it's moving in kind of a slanted direction,
at a speed of

                    √ ( 10² + 9.8² )

                 =  √ (100 + 96.04)

                 =   √  (196.04)   =   14 m/s        (rounded)