Newton's Second Law of Motion

Now another great thinker whom I'm sure you've heard of - the infamous Aristotle - once said that the force of an object is equal to its mass multiplied by its velocity. Now any one with common sense would believe this to be true. However, Aristotle did not take into account the amazing forces of friction and this blinded his insight. However of course as you know, another clever man, called Sir Isaac Newton came and proved this fellow wrong with his second law of motion:

"The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma"


In other words, he took upon Galileo's original idea, but tweaked it so that it was RIGHT. He found that when you took into account frictional forces, force was not equal to mass multiplied by velocity, but mass multiplied by acceleration. As he proved in his first law, an object tends to maintain a constant velocity and when a force is applied it does not keep this velocity, but accelerates in accordance to the force. Since Aristotle didn't think about friction, he saw an object slowed down if force was removed, and so thought that you needed to apply force to maintain a constant velocity, which we of course know is wrong.

This is one of Newton's most powerful laws as it gives us these useful equations:

Force = Mass x Acceleration

and from this...

Acceleration = Force / Mass

and...

Mass = Force / Acceleration


You can think of an example of this law very easily. Think of a huge ball of mass 10 stone (or 60 kg), then another ball of mass 20 stone (or 120 kg). If you imagine these two balls each being rolled towards you in turn at the same acceleration (ouch!) which would hurt more? The answer of the heavier ball is obvious. This is due to it having double the force due to its mass being doubled. Similarly you could think of two balls of the same mass each being rolled at you at different accelerations. The one speeding up more quickly would produce more force. You can see from the derivatives of the original equation that we can calculate mass and acceleration as well as force if we have the other values.

I hope you know understand this law, crucial to physics as we know it! If you don't, or just generally have something you'd like a post/topic on, just feel free to leave a comment below :).