We’ve all heard Newton’s third law of motion: For every action, there is an equal and opposite reaction. But what does that really mean? And why does it matter? Although this law may seem vague and abstract, it actually affects every physical activity that we engage in, whether it’s walking, running, or even sitting. It also explains events and objects that we observe in nature all the time, besides serving as the founding principle for the functioning of things like airplanes and rockets.
The essential components of this law are simple. Any contact interaction between two objects will generate force or, more specifically, a pair of forces. Any time that these forces are equal in size but opposite in direction, it results in one force reacting to the other.
This reaction force is the reason that birds are able to fly. The bird is exerting downward force by moving its wings downward. In turn, the air exerts an opposite force, pushing upward, to keep the bird in the sky. This is an example of two objects (the bird and the air) exerting equal and opposite force on each other. A similar reaction happens to a fish when it swims through the water.
Our own movements have a lot to do with the force of the Earth’s gravitational pull. The force of the Earth pulling us downward reacts with the force of our pushing upward in order to move.
Let’s consider the movements of walking. When we walk, we are pushing backwards with our legs in order to move forwards. That’s because the surface we walk on (whether it’s the floor or the ground) exerts an opposite force to move us forward when we exert force by pushing backward.
Another clear example of this is the action of pushing off against the wall of a swimming pool. We push backwards on the wall with our hands or feet, and an opposite force from the wall reacts to push us forward through the water. Even the act of sitting in a chair illustrates Newton’s third law at work. The force of our body pushes downward, and the chair exerts an opposite force to hold us up.
The third law of motion not only helps us swim, walk, and sit but even allows us to fly in a plane. That’s because an airplane wing is constructed to generate lift, which results when the airfoil pushes air downward and the equal force of the air reacts to bear the plane up. Another great example of this is the movement of a rocket. Unlike a plane, a rocket does not exert the force on the air, but on the gas within its combustion chamber. The rocket pushes the gas out of the chamber with great force, and the gas reacts by exerting an equal upward force on the rocket. This interaction is called thrust.
To sum up, Newton’s third law of motion is very much present in our daily lives. We can easily find many other examples of this law at work as we observe interactions around us.
- “Newton’s Third Law.” The Physics Classroom, http://www.physicsclassroom.com/class/newtlaws/Lesson-4/Newton-s-Third-Law. Accessed 12 August 2016.
- Hall, Nancy. “Newton’s Third Law Applied to Aerodynamics.” NASA, 15 May 2015. https://www.grc.nasa.gov/www/k-12/airplane/newton3.html.
- “What is Newton’s Third Law?” Khan Academy, https://www.khanacademy.org/science/physics/forces-newtons-laws/newtons-laws-of-motion/a/what-is-newtons-third-law. Accessed 12 August 2016.