when do you say an object is at rest
Rest (physics) Rest is a state in physics that refers to an object being stationary relative to a particular frame of reference or another object; when the position of a body with respect to its surroundings does not change with time it is said to be "at rest". According to the theory of relativity, it is said that an object is "at rest relative to" another. Two or more than two objects are said to be at rest, if its position with respect to each other is not changing or moving with uniform velocity with respect to each other; rest and motion are relative terms.
Serve (tennis) A serve (or, more formally, a service) in tennis is a shot to start a point. A player will hit the ball with a racquet so it will fall into the diagonally opposite service box without being stopped by the net. Normally players begin a serve by tossing the ball into the air and hitting it (usually near the highest point of the toss). The ball can only touch the net on a return and will be considered good if it falls on the opposite side. If the ball contacts the net on the serve but then proceeds to the proper service box, it is called a let; this is not a legal serve in the major tours (but see below) although it is also not a fault. Players typically serve overhead, but serving underhand, although rare, is allowed. The serve is the only shot a player can take their time to set up instead of having to react to an opponent's shot. But as of 2012, there is a 25-second limit to be allowed between points.[1]