Newton's 3rd law

Sir Isaac Newton came up with three laws that revolutionized our understanding of motion.

While all three are profound and show great insights into how things work, the third law is, in some ways, the most radical and, to a certain extent, the hardest to see in action – although it happens all around us every day.

Fair is fair

In the old testament's justice system, there is a list of different ways justice was to be served for a whole list of wrongs. In the end, the main idea was as follows:

Breach for breach, eye for eye, tooth for tooth: as he hath caused a blemish in a man, so shall it be done to him again.” (Leviticus 24:20)

People have often used this phrase as an excuse for revenge, but the idea is, in fact, one of justice – if someone is wronged by someone else, the wronging person must give back a fair amount. No more or less.

So, what does all this have to do with Newton and his laws?

Well, it turns out that nature is the same way with forces. You apply a force to me (for example, you give me a push), and you will find that, at the same time, I am returning a force of my own back at you – same amount, opposite direction.

The law

All this gives us Newton's 3rd law:

"To every action (force applied), there is an equal but opposite reaction (equal force applied in the opposite direction)."

Forces (pushing, pulling, and so on) between objects come in pairs.

Take the medieval catapult or trebuchet; used back in the day to throw huge rocks at castles walls.

The catapult throws the rock forward with a large pull (a force). However, at the same time, there is a backward force from the rock to the catapult; the rock pushes with the same amount of force, just towards the back. This can be seen on a wheeled catapult when it gets pushed back as the rock leaves.

What does it mean?

Newton's 3rd law reminds us that interactions are, in fact, a two-way deal. Suppose I'm lifting a box. My lifting up on the box isn't the only thing going on; at the same time, the box is also pulling down on me.

So next time you've just polished off a big meal, such as thanksgiving, and you feel full and big.

Remember, the Earth may be pulling you with an increased weight due to your meal, but you have your revenge. You pull up on the Earth with a greater pull as well.

A heavy unit

A final word on units for measuring force. The most common unit of force is the pound, making it an excellent unit of weight – weight being simply the force of gravity on an object. By extension, the ton (2000 pounds) is also a valid unit of force.

However, the "proper" unit of force in science is the Newton (1 pound is about 4 ½ Newtons), and this brings me to my unit-bone to pick. Europe and the medical profession have adopted the gram or kilogram as a unit of weight. But grams measure mass, not force; it is the wrong unit. But since weight and mass increase together, this does not cause any grievous errors.

But whether one is dealing with newtons, tons, or whatever, the same basic message remains. In the end, there is an equality in everything; Thomas Jefferson wrote that "all men are created equal." When it comes to forces, this democratic statement still rings true.

On the web

Newton's Third Law of Motion

This page is part of the Physics Classroom. It explains Newton's 3rd law and has sample questions.

Newton’s Third Law of Motion Demonstrated in Space

Astronauts in orbit demonstrate Newton's law. Fun for anyone interested in seeing people float in space.

Newton's Third Law of Motion (Real life example)

Fun examples of the 3rd law with animations. The examples come as we follow a young couple in love that meet, fight, and then make up again. A lot of fun!

Neil deGrasse Tyson: My Man, Sir Isaac Newton | Big Think

This is a video tribute by Neil deGrasse to Sir Isaac Newton and his incredible contributions to science and math.

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