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Newton's Second Law of Motion concerns the mass of objects and their acceleration. What the law states is "the change of momentum of a body is proportional to the impulse impressed on the body and happens along the straight line on which that impulse is impressed." Now in regular English what that means is a force applied to an object will accelerate it in a straight line and the equation for that is F=ma or Force = Mass times Acceleration.
And with that equation we can derive two additional points. One is more force accelerates the same mass faster and more mass requires more force for the same acceleration. So, if something is bigger it's going to require more force to move it. Or if you use more force on something, you are going to accelerate it a little bit faster. So, let's take a look at this. Here we have a simple ball and the red arrow indicates a force.
So, as the force is applied to the object, the object will accelerate, which means it will move a further distance with each unit of time. So, the first unit of time it will move this far, the second unit of time it will move this far, then this far, then this far. So basically, it's going faster and faster and faster, the longer the force is applied. So, as the force is applied, the object will continue to accelerate. So, let's see how this looks.
Now, the other part of this is that the amount of acceleration depends upon the mass of the object. So, let's go ahead and bring in a bigger, heavier object. And let's apply the same force to that object. When we do this you will see that this object doesn't move nearly as fast and that's just because it's heavy. Now this may be common sense, but what this does is it indicates the weight of an object. How fast that object can move with any given force will tell the audience how heavy that object is.
And this is very important for character animation. If you want a big character, you are going to have to move him a little bit differently than a light character. Now, let's take a look at another thing here. If we increase the force on the mass, so in other words, this large red arrow means a bigger force, you can get that same acceleration. Now another thing with force and acceleration is that acceleration can also become deceleration.
So, for example in this case, we have a force that is applied to this object and half way through we take away that force and add in the opposite force. Now, what this will do is it will basically reverse the effects of the original force and decelerate it until it's at a stop. Now, if we continue with that force, it will then continue to accelerate it in the opposite direction until another force slows it down. So, let's go ahead and play this.
So again, forces can accelerate or decelerate. Now, when you have an object that has no force applied to it, again you are going to get steady motion. You are just going to get equal distance per unit of time. The only time an object will accelerate or decelerate is when a force is applied and that force will accelerate or decelerate that mass according to how heavy it is.
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