Objective: how to make butter in a mason jar

Which of the following solutions is more sound in terms of naive physics reasoning?

Available options: I. Pour heavy cream into the mason jar, filling it half-way full. Screw the lid on. Shake mason jar for approximately 5-7 minutes. After the first 2 minutes you'll have whipped cream. Keep shaking until you hear that a lump has formed inside, and shake an additional 30-60 seconds after that.Remove the solids from the jar. The remaining liquid is buttermilk. You can save that for other recipes, or discard it. Place the solids into a small bowl. Pour cold water over the butter and use your hands to squish it into a ball. next heat in microwave. II. Pour heavy cream into the mason jar, filling it half-way full. Screw the lid on. Shake mason jar for approximately 5-7 minutes. After the first 2 minutes you'll have whipped cream. Keep shaking until you hear that a lump has formed inside, and shake an additional 30-60 seconds after that.Remove the solids from the jar. The remaining liquid is buttermilk. You can save that for other recipes, or discard it. Place the solids into a small bowl. Pour cold water over the butter and use your hands to squish it into a ball. you now have butter.
II.