About the discovery of momentum and force...

[albertlee]

We all know that momentum and force is related particularly

 

but when it comes to reality, how can we set up an experiment that shows force, momentum, and acceleration, that is practical enough to be calculated and experimented???

 

I mean, there is friction, air resistance all over on Earth... It comes to me that those ideas become so theoretical rather than practical

 

Any help?/

 

Albert

[swansont]

Typically you do these experiments with an air track, which minimizes friction. With decent springs on the ends of the carts the collisions can be very close to elastic.

[albertlee]

Ok, so how does Newton discover force and momentum and their proper definition??? through discovery and experimenting?? back then there was no spaceship, how did he discover such things that would not or rarely happen on Earth??

 

Albert

[Johnny5]

Ok' date=' so how does Newton discover force and momentum and their proper definition??? through discovery and experimenting?? back then there was no spaceship, how did he discover such things that would not or rarely happen on Earth??

 

Albert[/quote']

 

Albert, I asked myself the same question long ago, and I can tell you what I think.

 

First of all, he used different words than we do today. He did not use the word 'momentum', he used the word 'motion. He would refer to the "quantity of motion" in a thing. And he also referred to the "vis insita" which usually gets interpreted as inertia, and he also says somewhere in Principia, that the exact formula for gravitation remains for the mathematicians to figure out, so I am not sure even if he got that right. Feynman couldn't understand Newton's proof for gravity being inverse square, if that tells you anything, and he tried very hard to.

 

But really, if you pay attention to history, you will see that all Newton did, was try to restate what Galileo had already figured out. In other words, Newton really didn't figure out Newton's laws, and this is my stance.

 

Instead, I hold that Galileo figured out "Newton's laws," only Galileo stated them in Latin and Italian, Newton re-stated them in English.

 

The reason I say this is obvious. Of the two men, Galileo was the one who performed experiments. It is obvious to me, that Galileo was trying very hard to figure something out, and wasn't going to stop until he did. He performed experiments with pendulums trying to figure something out. He performed experiments in gravity, trying to figure something out, and he performed experiments with 'friction' trying to figure something out. His greatest experiment of all, is the one which helped him figure out his Law of Inertia, which is exactly what Newton's first law is.

 

And more to the point, in Galileo's free fall experiments, it was absolutely essential that he figure out how do define speed, and acceleration. And in one of his books (i think it was Dialogue concerning two new sciences) he meticulously explains the modern definition of speed and acceleration.

 

It is right there in black and white, Galileo invented the differential calculus. You can see galileo focusing on tiny changes in time, in order to figure out how to define acceleration, a concept he initially struggled with. But of course once he figured it out, he knew how to explain free fall.

 

I might add, that Leonardo Da Vinci also tried to explain free fall mathematically, although his explanation was incorrect. Galileo realized that in order to figure out the correct mathematical formula, he had to perform experiments, so that is exactly what he did, and he was successful.

 

Since free fall happens so fast, Galileo needed a way to slow things down, so he decided to use gently sloping inclined planes. This way, the time it takes something to reach the ground from a height of three feet, is about a minute or so. Then, Galileo needed a way to measure time, accurate to the second. At first he used his pulse, but eventually figured out a way to supposedly get an accuracy to the hundredth of a second, by developing a water clock. You make a small hole in a container, fill it up to a certain point with water, then when you take your finger off the hole the clock is ticking, and then the draining water fills a bucket. Later you weigh the bucket, and deduce the time.

 

But my whole point is, the only way to figure out "Netwons laws" is to do the experiments that Galileo did (most importantly the free fall ones, since you need them in order to figure out the meaning of acceleration, in fact you need them to even have a reason to figure out the meaning of acceleration).

 

Also note that galileo was at some point concerned with the fact that different objects fall at the same rate of speed regardless of their weights. He performed many many experiments, designed to figure out whether or not this was true. Eventually he was able to convince himself that it is indeed true that "all objects fall to earth with the same acceleration regardless of their weights, when dropped from the same height"

 

And lastly, Galileo thought quite a bit about the motion of projectiles. He figured out that the path is a parabola, and realized that horizontal and vertical motion could be treated separately. It is Galileo, not Newton, who figured out that the parabolic motion is caused by the horizontal part following Galileo's law of inertia, and the vertical part having the acceleration caused by earths gravity.

 

But not to detract from Newton, I think Newton's third law was due purely to Newton, not Galileo, but then I could be wrong about this.

 

Read Galileo's book and decide for yourself whether or not I am right, and think about the experiments Galileo was doing, and ask yourself this, "What was this man (Galileo) trying to figure out?"

 

Regards