Recoil of gun and motion of rocket are example of .....

Two forces $10 N$ and $15 N$ act on a body of unit mass in opposite direction. The minimum acceleration is:

A man of mass $60$ kg on the top floor of a skyscraper steps into an elevator. What is the man's weight as the elevator accelerates downward at a rate of $1.5$ m/s²? (g $=10$ m/s²)