# Newton's Third Law of Motion

Source: The Martha Stewart Show, May 2008

## Introduction

In his work, Isaac Newton described universal gravitation and the three laws of motion, laying the groundwork for classical mechanics, which dominated the scientific view of the physical universe for the next three centuries and is the basis for modern engineering. In mechanics, Newton enunciated the principles of conservation of momentum and angular momentum. In optics, he invented the reflecting telescope and developed a theory of color based on the observation that a prism decomposes white light into a visible spectrum. He also formulated an empirical law of cooling and studied the speed of sound.

In mathematics, Newton shares the credit with Gottfried Leibniz for the development of calculus. He also demonstrated the generalized binomial theorem, developed the so-called "Newton's method" for approximating the zeroes of a function, and contributed to the study of power series.

Newton's First Law of Motion
Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Newton's Second Law of Motion
The relationship between an object's mass, its acceleration, and the applied force is force = mass x acceleration. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

Rocket Propulsion
Newton stated in his third law of motion that "for every action there is an equal and opposite reaction." It is upon this principle that a rocket operates. Propellants are combined in a combustion chamber where they chemically react to form hot gases, which are then accelerated and ejected at high velocity through a nozzle, thereby imparting momentum to the engine. The thrust force of a rocket motor is the reaction experienced by the motor structure due to ejection of the high-velocity matter. This is the same phenomenon that pushes a garden hose backward as water flows from the nozzle, or makes a gun recoil when fired.

Balloon Experiment
Holding the balloon with the opening facing down, let the air flow out from the bottom. The balloon is pushing the air down, which proves Newton's third law. If the balloon pushes down on the air, then the air pushes up on the balloon. When you to let go of the balloon, you will see it go straight up.

Tricycle Experiment
This tricycle experiment is another example of Newton's third law. Instead of using air and a balloon, the tricycle experiment is powered by a carbon dioxide fire extinguisher. The fire extinguisher is pushing backward and the tricycle is going forward. This also happens with a rocket; hot gas is coming out of the rocket and the rocket goes in the opposite direction of the gas.

Special Thanks
Special thanks to MIT physics professor Walter Lewin for showing us these fantastic experiments. You can find his lectures on iTunes U, MIT Open Courseware, Google, and YouTube.