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.

humans, along with other animals and plants, have linear chromosomes that are arranged in pairs within the nucleus of the cell. the only human cells that do not contain pairs of chromosomes are reproductive cells, or gametes, which carry just one copy of each chromosome.