Why is it important to have a control group in an experimental investigation?

How would doubling the made of an object change the objects potential energy

Asmall object with mass 0.200 kg swings back and forth on the lower end of a light rope that is 3.00 m long. the upper end of the rope is attached to the ceiling. as the object swings through its lowest position, where the rope is vertical, the speed of the object is 5.80 m/s. at this point in the motion, what is the tension in the rope? (use g = 9.80 m/s2.)

What is the weight of a feather (mass = 0.0001 kg) that floats through earth's and the moon's atmospheres?