Why did Goliath fall facedown when he was hit from the front side? By physics laws he was supposed to fall backward.
The familiar story of David and Goliath is found in 1 Samuel 17. We won’t be able to read the whole account here, but I encourage you to spend time in the Word and read about this very interesting and important time. The Jews and the Philistines were at war with one another. We read in 1 Samuel 17:4, “And there went out a champion out of the camp of the Philistines, named Goliath, of Gath, whose height was six cubits and a span.” This means Goliath was approximately 9 feet, 9 inches tall, and he was a champion warrior of the Philistines. Verses 4-7 give a frightening description of this giant. Then, we read in 1 Samuel 17:8-10, “And he stood and cried unto the armies of Israel, and said unto them, Why are ye come out to set your battle in array? am not I a Philistine, and ye servants to Saul? choose you a man for you, and let him come down to me. If he be able to fight with me, and to kill me, then will we be your servants: but if I prevail against him, and kill him, then shall ye be our servants, and serve us. And the Philistine said, I defy the armies of Israel this day; give me a man, that we may fight together.” The deal Goliath offered was very clear: he would fight Israel’s mightiest warrior. If Goliath won, the Israelites would be their servants. If the Jewish warrior should win, then the Philistines would be the servants of Israel.
Goliath taunted the Israelites for 40 days with his challenge. For the sake of time, we won’t go into all the details of how David went to Saul and asked permission to fight Goliath. He refused Saul’s armor and sword and only took a sling, gathering five smooth stones from a nearby brook. To answer your specific question, let’s read 1 Samuel 17:48-51, “And it came to pass, when the Philistine arose, and came and drew nigh to meet David, that David hasted, and ran toward the army to meet the Philistine. And David put his hand in his bag, and took thence a stone, and slang it, and smote the Philistine in his forehead, that the stone sunk into his forehead; and he fell upon his face to the earth. So David prevailed over the Philistine with a sling and with a stone, and smote the Philistine, and slew him; but there was no sword in the hand of David. Therefore David ran, and stood upon the Philistine, and took his sword, and drew it out of the sheath thereof, and slew him, and cut off his head therewith. And when the Philistines saw their champion was dead, they fled.” Others, like yourself, have questioned how Goliath could have fallen face first after being struck in the forehead with this stone. Let’s be very careful with the exact wording, because we are not told that Goliath fell ‘face first.’ Rather, we are told that he ‘fell upon his face to the earth’. What is the difference between falling face first and falling upon his face? To fall face first is to fall forward, and as you stated that would be against the law of physics. That would be true if the blow to his head was powerful enough to drop him quickly, with his head being jerked backward. However, if the rock hit Goliath and he staggered for a moment, he could have easily fallen on his face. Another thought is that falling upon his face could mean that when the rock struck Goliath that it spun him around and then he fell upon his face, although not face first.
We don’t want to spend too much time in speculation about how this happened. We can trust that it did happen just as the Word said. Before we close, though, let’s consider the words of David to Goliath in 1 Samuel 17:45, “I come to thee in the name of the LORD of hosts…” When we go forward in the name of the Lord against those who defy and deny our blessed Lord, we are guaranteed victory. We have the words of the Lord in Jeremiah 1:19, “And they shall fight against thee; but they shall not prevail against thee; for I am with thee, saith the LORD, to deliver thee.” Even as the Lord was with David, so is He with every one of His children that He sends forth to battle in His name. “They shall not prevail against thee.” What a precious and reassuring promise from the Lord for us. (177.7)