The Hu-Washizu principle is a fancy way of saying that when you apply a force to something, it will push back with an equal force in the opposite direction. It's like playing a game of catch with a friend - when you throw the ball to them, they catch it and then throw it back to you with the same amount of force you used.
This principle is really important when we're talking about things that are made of a material that can bend or stretch, like a rubber band or a spring. When you stretch a rubber band, it gets longer - but it also starts pushing back against you with a force that's equal to the force you're using to stretch it.
The same thing happens when you're dealing with more solid materials. For example, if you push on a wooden desk with your hand, the desk will push back with an equal force. This is because the atoms and molecules that make up the wood are all connected to each other, and they don't like to be pulled or pushed out of position.
The Hu-Washizu principle helps engineers and scientists design things that can withstand forces without breaking or bending too much. They can use math and physics to calculate how much force something can handle before it breaks, and then design it to be strong enough to handle that force.
So, if you ever hear someone talking about the Hu-Washizu principle, just remember that it's all about things pushing back with an equal force when you apply a force to them.