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It is hard to believe that many of the most fundamental concepts in physics are derived from Newton's three laws. The first law shows us that if no force acts on an object, it will stay at rest or travel with constant velocity. The second law tells us that the force acting on an object depends on both its mass and its rate of acceleration (or deceleration). And finally, the third law explicitly states the conservation of momentum. Newton's three laws are the basic tools physicists use to describe how things move. How can they be used to derive all the other concepts? That's what this section is all about. So, let's start with an easy one, acceleration. If you were asked to write Newton's first law for an object that has a constant velocity, what would you do? Of course! You could change it very little by replacing "F=ma" with "F=0=ma". But if you did that, the original statement would no longer be true. In this case, here , you would have written "Newton's first law is not true". Now the question is, what would happen if you displayed this statement to your physics class? What would they think? They might think that Newton's first law must be wrong! And so they would start looking for other laws. But how can they find other laws if they do not understand Newton's first law? They cannot. This is where Newton's second law comes into play. If forces are added to an object that has constant velocity, the object will accelerate (or decelerate). This can be understood very easily by drawing a diagram. Here it is. In this diagram, a mass moves at a constant velocity v. It moves the same distance, but has a force F acting on it. The object accelerates by a small amount α. Here's another diagram to demonstrate the idea of acceleration. In this diagram, a mass moves with a constant velocity v and experiences mechanical forces F from other forces. The force from the other force is "F". It equals "F+mv". So, if you have an object that moves with constant velocity, then it will move the same distance but accelerate by some amount that depends on both its mass and its acceleration (or deceleration). What about deceleration? If an object has a constant velocity, then to stop the object, you must apply a force in the opposite direction. Newton's second law says that when this force is acting in the direction of the acceleration (or deceleration), there is no net force on the object. That's because you are using up energy when you apply the force in the opposite direction to stop motion. But when your motion stops without any need for force, Newton's second law says that there is no change in velocity, so there is no need for mass or acceleration. Look at this diagram, where a mass moves with a constant velocity of v and experiences mechanical forces F from other forces. cfa1e77820
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