4) Describa una situación en la cual un cuerpo se encuentre en reposo para un observador, y en movimiento para otro.

Rest and motion are the relative terms.

Explanation:

When the position of the object does not change with respect to time, the object is said to be at rest.

When the position of the object changes with respect to time, the object said to be in motion.

let there is a tree, the tree is at rest with respect to the person which is standing near by the tree.

But when the person is in outer space and able to see the tree, the tree is in motion as the earth is in  motion.

A person sitting in the train which is moving is at rest when observed by the other person sitting in the train but he is in motion with respect to a person which is standing on the platform.  

b. lifting dumbbell weight at 20-degree angle from the body after a 45-degree angle