Exploring Newton's Third Law of Motion: Evidence and Experiments

Newton's third law of motion, often referred to as the law of action and reaction, is a fundamental principle in physics. It states that for every action, there is an equal and opposite reaction. This law is not a simple statement to prove in the traditional sense of mathematical proof. However, it is extensively tested and found to match reality so closely that it is widely accepted as a fundamental truth in the science of physics.

Testing Newton's Third Law: The Bathroom Scale Example

To understand why Newton's third law is so fundamental, let's consider a simple experiment involving a bathroom scale. When you stand on a solid floor, you feel your weight on the scale. Despite Earth's gravitational force pulling you downward, there is an equal and opposite force from the floor pushing you upward. This is precisely what Newton's third law describes. The scale measures the force with which your body is being pressed on it, which is the reaction to the gravitational force exerted on you.

Scientific Proof: Bathroom Scales and Newton's Third Law

Imagine standing on a bathroom scale, which measures the force with which your body is pressed on it. Suppose you weigh 70 kg on Earth, meaning the Earth pulls on you with a force of 700 N (Newtons) downward. The floor (or any solid surface) exerts an equal and opposite force of 700 N upward. This is the force the scale measures, confirming Newton's third law in action.

More Evidence: Pendulum Collisions and Newton's Third Law

Another fascinating way to test Newton's third law is through pendulum interactions. Consider two pendulums, one weighing 1 kg and the other 2 kg, each equipped with a G-forcemeter to measure the forces during a collision.

The Pendulum Experiment and Newton's Third Law

When the 1 kg pendulum strikes the 2 kg pendulum and the 1 kg G-forcemeter registers a peak of 10 G (10 times the gravitational acceleration), the 2 kg pendulum's G-forcemeter will register 5 G. These readings confirm that the force involved in the collision is the same for both pendulums, just acting in opposite directions. The difference in the acceleration levels corresponds to the differing masses of the pendulums, as described by Newton's second law (F ma). The 1 kg pendulum has twice the acceleration of the 2 kg pendulum, consistent with the conservation of momentum and Newton's third law.

Natural Phenomena and Newton's Third Law

Newton's third law is not just confined to laboratory experiments. It is observable in countless natural and everyday phenomena. For instance:

Walking and Action/Reaction Pairs

When you walk, your feet exert a force backward on the ground, the ground exerts an equal and opposite reaction force forward. This allows you to move forward.

In car driving, the engine (action) pushes the car forward, and the ground exerts a backward (reaction) force on the tires, propelling the car.

Even when you play your favorite sports, the force you exert on a ball (action) is met with an equal and opposite force from the ball (reaction), affecting its trajectory.

Evidence from Extreme Conditions

Newton's third law remains unchanged even in extreme conditions, such as in space. For example, when a bullet is fired from a gun in zero gravity, the bullet moves forward, and the gun moves backward, both with equal forces but in opposite directions. Similarly, when a rocket launches, the thrust of its engine (action) pushes the rocket upward, while the exhaust gases are expelled downward (reaction).

Conclusion

From bathroom scales to pendulum collisions and everyday activities, the evidence supporting Newton's third law is overwhelming. There has never been a single instance where this law has been contradicted in the real world. Newton's third law of motion continues to be a cornerstone of our understanding of physics, underpinning the functioning of our universe. While it cannot be proven in the strictest sense, its veracity is uncontestable based on the wealth of experimental evidence and natural phenomena it explains.