Whey powder is a protein that is easier and quicker to absorb than protein found in most foods. While this is convenient, it doesn't necessarily mean it is better than foods. Protein is required to repair the tears in muscles from strenuous and intense exercise. When you put stress upon your muscles, you cause tears that actually break down the muscles. And yes this means that technically by working out you are causing your muscles to become weaker.
When you consume protein, whether it be whey, soy, etc, the tears get repaired. Not only do these tears get repaired to their original state but rather the tears become more adapted to exercise, thus leading to strength gains.
Before ending this little section, let's not forget that appropriate amounts of carbohydrates and fats are also required to build muscle and strength.
Watch this video for more info about whey protein danger