Kayaking is not only a good way to have fun but is also a great example of how objects move using Newton’s third law. This law states that if a force is being exerted on an object, than that objects exerts an equal and opposite force on whatever is exerting a force on it. This idea can be used to show how pushing a paddle through the water can move a kayak. When a paddle is pulled back in the water, the water exerts an equal and opposite force on the paddle. This force on the paddle by the water is then transferred to the kayak, since you are holding the paddle and sitting in the kayak. This allows the kayak to move in the direction at which the force is applied. Depending on the position at which the paddle exerts a force on the water, the direction of the force can be forward, “upward” (lifting the kayak up), or “downward” (pushing the kayaking down into the water). When the paddle is pushing the water at a shallow angle the force component, which is perpendicular to the paddle blade, is pointing towards the sky. This causes the force exerted on the kayak by the water to lift the kayak upward and “out” of the water. This force lifting the kayak up is balanced by the force of gravity, preventing the kayak from flying out of the water. Alternatively, if you leave the paddle in the water too long, causing it to have a deep angle, than the force component will be pointing down into the water, pushing the kayak down and “into” the water. This force, causing the kayak to be pushed down, is balanced by the buoyant force of the water on the kayak.
Another force that must be considered while kayaking is torque. The equation for Torque is T = F * r * sin(θ), where r is the length of the lever arm, F is the force exerted on the lever arm and sinθ is the angle between the lever arm and the axis of rotation. When kayaking, the axis of rotation is the center of the kayak, the paddle is the lever arm, and the force exerted by the water on the paddle is the force acting on the lever arm. To turn the kayak, the paddle must be extended far enough away from the kayak and swung in a wide arc to produce enough torque to turn the kayak. The farther the paddle is from the kayak the greater the torque, and the more the kayak rotates. Understanding how torque and Newton’s third law allows us to kayak is just another example of how physics affects us in our daily lives.