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Newton's Three Laws in Words and Formula. Inertia at Rest, and More.

Updated on July 23, 2012

Yes, you do need to memorize these.

You probably had them memorized when you were in middle school, but by now you've forgotten. Here's a quick refresher.

Isaac Newton's Three Laws
Isaac Newton's Three Laws

1. Law of Inertia

An object at rest remains at rest. An object in motion remains in motion unless acted upon by an outside force.

If F=0, then a=0

(no Force, no acceleration)

When you throw a ball on earth, gravity acts upon it as an outside force, which eventually stops its motion. If you throw a ball in space, it will never stop. It will go on forever until it encounters something that makes it stop, such as the gravitational pull of a star.

2. Law of Acceleration

The acceleration of an object is directly proportional to the force on it and inversely proportional to its mass.

f/m=a (force divided by mass equals acceleration)


f=ma (force equals mass times acceleration)

Basically, a smart car will accelerate much faster than a dump truck. The smart car has smaller mass, so the force applied to it results in higher acceleration. Apply the same amount of force to a dump truck, which has higher mass, and acceleration will be lower.

3. Law of mutual action

For every action, there is an equal and opposite reaction. Forces occur in pairs and the force on one object is equal and opposite of the first force.

A > B + -B > A (Equal and opposite)

Take a look at the skaters below. Their force on each other is equal, but in opposite directions.

Newton's Third Law
Newton's Third Law


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    • stephaniedas profile image

      Stephanie Das 5 years ago from Miami, US

      Hi KenWu- My philosophy is, who really wants to read a long academic hub? When I'm looking for information on things like this, I want it short and sweet. I know what you mean though...E=mc2...I don't even know what it means but I'll always remember it, thanks to the Wizard of Oz. Thanks for commenting!

    • KenWu profile image

      KenWu 5 years ago from Malaysia

      Short but very academic :)! I had never really interested in Science but those Newton's theory really made sense. BTW, the only science equation that never leaves my mind is Einstein's relativity theory, E=mc2

    • stephaniedas profile image

      Stephanie Das 6 years ago from Miami, US

      I'm glad you think so too! I think its just basic knowledge...if Isaac Newton did all this work for us, we might as well learn his theories.

    • CWanamaker profile image

      CWanamaker 6 years ago from Arizona

      Newton was a pretty smart guy. Not only is he credited with these three laws, he created calculus as well. His laws are the basis for the principles that engineer's use to design a lot of the things that we see every day. These concepts are very important for everyone to understand, so I am very excited to have read this hub.

    • stephaniedas profile image

      Stephanie Das 6 years ago from Miami, US

      Thanks! I'm glad I was able to explain them. Sometimes a lot of words get confusing, so I hope the formulas can help people who are more math-minded.

    • ktrapp profile image

      Kristin Trapp 6 years ago from Illinois

      I like the examples you give for each of the three laws. It puts a picture in my mind so I can comprehend the formula better. This is a very nice, concise summary of these 3 laws.