Aball’s mass is 1.5 kilograms, and it leaves the racket with a horizontal speed of, say 90 mph. find the average force on the tennis ball as it is served

The answer is 1.5 * 10  = 15 Newtons. This is calculated by multiplying Mass and acceleration. In this case, the acceleration is the gravitational pull of 10 meters per second squared.  Even though the ball is played horizontally, gravity still applies on it and this is why its elevation from the ground decreases with distance until it hits the ground.

b. the carbon cycle ensures living things have a source of hydrogen