Understanding Newton’s Third Law in Fluids: Action and Reaction Forces Explained

When we think of Newton’s laws, particularly his third law concerning action and reaction forces, it often involves solid objects. However, the application of these principles extends beyond solids, especially in the realm of fluid motion. Let’s delve into how the laws apply in various scenarios and explore the fascinating physics behind swimming and other fluid interactions.

Newton’s Third Law: Forces in Pairs

Newton’s third law of motion states that for every action, there is an equal and opposite reaction. This law applies universally, whether dealing with fluids or solids. The key point is that forces come in pairs acting on different objects. An action force and its corresponding reaction force are equal in magnitude but opposite in direction. This principle explains how we move through water and air, seemingly defying the expectations set by solid interactions.

Floating and Pushing: A Fluid Example

Imagine you are in a canoe, pushing against the water to move forward. The fascinating part is that the water pushes back with equal force, just like if you were pushing against a solid object. However, water, being a fluid, behaves differently than a solid. When you push on water, the water might move out of the way, but the force is still present. This force is what allows swimmers and canoers to move through the water.

Efficiency in Force Application

It's true that pushing water or air might not be as efficient as pushing a solid object, but the same principle applies. When you push on a solid object, the object pushes back, and if you push too hard, it will eventually break, stopping the force. In contrast, when you push on a fluid, the fluid moves out of the way, but the force still acts. This is why swimmers use specific techniques to minimize water displacement while maximizing force application.

Newtons Third Law and Fluids: Specific Examples

Consider the scenario of a diving into water. When someone does a belly flop and lands horizontally in the water, they need to move a large volume of water quickly. This requires both a large action force and a large reaction force from the water. The water applies a large force to the diver, causing them to slow down rapidly and not go deep into the water. This is a perfect example of Newton’s third law in action.

Another example is pushing your hand through the air. When you push quickly through the air, you move a small amount of air, requiring a small force. The air pushes back with an equal force, but it is so small that you barely notice. If you want to apply a large force to the air, you need to move your arm incredibly fast, which is not practical with human muscle power.

Real-World Application in Vehicles

However, vehicles provide a more practical way to experience the force pairs defined in Newton’s third law. When you are in a car traveling at a high speed, like 70mph, and you extend your hand out the window, your hand is stopping or moving a large amount of fast-moving air. This requires a significant force, and you can feel this force on your hand. The air pushes back with an equal and opposite force, demonstrating Newton’s third law in a tangible way.

Conclusion

Newton’s third law of motion holds true for fluids as well as solids. The principles of action and reaction forces are consistent, whether you are pushing a solid wall, displacing water, or driving a car. Understanding these principles helps us grasp the physics behind movement, whether it’s in the air or underwater.

Keywords: Action and Reaction Forces, Newton’s Third Law, Fluid Motion, Swimming Physics