Frames of Reference, Inertia, and Newton's First Law.
Inertia and Frames of Reference
Things to remember from tonight's class demonstrations involving the ballistics cart. A question was raised: If you are flying in an airplane at a constant velocity, and you throw a ball straight up, will it land back in your hands (or elsewhere)?
We were able to show that it lands back in the cart.
https://youtu.be/lbGCa5EPZhQ
https://youtu.be/ZzzFlzoGg68
Data processing part 2:
So, why does the ball land back in the car? INERTIA - the ball keeps doing what it is doing (moving forward at the same speed) because there is nothing acting on it (in the horizontal direction) to make it stop. However, there IS something acting on it in the vertical direction - gravity - which causes the ball to come back down once it is shot upward. The ball lands back in the car because its horizontal speed forward is never changed; only the vertical speed is affected (by the spring inside the car and by gravity).
Inertia is a vital concept - its origins trace back to the Scientific Revolution, and Isaac Newton is usually given credit for it. However, Galileo also had a version of this concept, as did several other scientists of this period.
We'll get to Newton's laws shortly. First, a bit of pre-history...
Scientific Revolution: roughly 1550 - 1700
- notable for the introduction of widespread experimental (evidence-dependent) mathematical science.
- also notable for the 150 years that it took for geocentrism to finally die after published in 1543 (see below)
- sometimes thought of as "kick-started" by the publication of Copernicus' De Revolutionibus Orbium Celestium, in 1543 (the year of his death). This was the first major work arguing for a heliocentric (sun-centered) universe. Not initially a success of a book - its influence took decades to be realized (and very slowly)
Galileo (1564 - 1642) and Newton (1642 - 1727) are often thought of as the central figures of the Sci Rev'n.
Worth remembering about Galileo:
- discoveries with his telescope (craters on the Moon, phases of Venus, moons of Jupiter, many stars in the Milky Way galaxy, sunspots, rings of Saturn)
- convincing mathematical/logical argument for a Sun-centered universe (which he published, and which was cause for his trial)
Isaac Newton, 1642-1727
Newton, Philosophiae naturalis principia mathematica (1687) Translated by Andrew Motte (1729)
Lex. I. Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity. A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air. The greater bodies of the planets and comets, meeting with less resistance in more free spaces, preserve their motions both progressive and circular for a much longer time.
And now, in more contemporary language:
1. Newton's First Law (inertia)
An object will keep doing what it is doing, unless there is reason for it to do otherwise.
The means, it will stay at rest OR it will keep moving at a constant velocity, unless acted on by an unbalanced force.
http://www.iflscience.com/physics/what-happens-when-you-fire-ball-out-cannon-travelling-opposite-way/
Great Penn and Teller clip worth a view:
https://www.youtube.com/watch?v=mwkmgqbYXdE
Great Penn and Teller clip worth a view:
https://www.youtube.com/watch?v=mwkmgqbYXdE
Things to remember from tonight's class demonstrations involving the ballistics cart. A question was raised: If you are flying in an airplane at a constant velocity, and you throw a ball straight up, will it land back in your hands (or elsewhere)?
We were able to show that it lands back in the cart.
https://youtu.be/lbGCa5EPZhQ
-
the ball shot from the cart lands in the cart, because it was moving at
the same speed as the cart. This is similar to Galileo's ship problem,
or the idea of apples falling near the base of the apple tree.
Now some data processing:
This graph is a little confusing, since it shows 2 different motions. The red dots are the horizontal position of the ball - continually moving forward at a constant speed (which is the same as the car). The blue dots show the ball rising and then falling, due to gravity. These 2 motions combine to make the mathematical curve called a parabola.
2nd demonstration:
Now some data processing:
The blue dots highlight the actual path of the ball - a parabola!
This graph is a little confusing, since it shows 2 different motions. The red dots are the horizontal position of the ball - continually moving forward at a constant speed (which is the same as the car). The blue dots show the ball rising and then falling, due to gravity. These 2 motions combine to make the mathematical curve called a parabola.
2nd demonstration:
-
the ball released from the top of the hook (on the cart) still lands in
the cart itself, for the same reason: ball is moving at the same speed
as the cart.
Data processing part 2:
Descriptions are the same as above.
So, why does the ball land back in the car? INERTIA - the ball keeps doing what it is doing (moving forward at the same speed) because there is nothing acting on it (in the horizontal direction) to make it stop. However, there IS something acting on it in the vertical direction - gravity - which causes the ball to come back down once it is shot upward. The ball lands back in the car because its horizontal speed forward is never changed; only the vertical speed is affected (by the spring inside the car and by gravity).
Inertia is a vital concept - its origins trace back to the Scientific Revolution, and Isaac Newton is usually given credit for it. However, Galileo also had a version of this concept, as did several other scientists of this period.
We'll get to Newton's laws shortly. First, a bit of pre-history...
Scientific Revolution: roughly 1550 - 1700
- notable for the introduction of widespread experimental (evidence-dependent) mathematical science.
- also notable for the 150 years that it took for geocentrism to finally die after published in 1543 (see below)
- sometimes thought of as "kick-started" by the publication of Copernicus' De Revolutionibus Orbium Celestium, in 1543 (the year of his death). This was the first major work arguing for a heliocentric (sun-centered) universe. Not initially a success of a book - its influence took decades to be realized (and very slowly)
Galileo (1564 - 1642) and Newton (1642 - 1727) are often thought of as the central figures of the Sci Rev'n.
Worth remembering about Galileo:
- discoveries with his telescope (craters on the Moon, phases of Venus, moons of Jupiter, many stars in the Milky Way galaxy, sunspots, rings of Saturn)
- convincing mathematical/logical argument for a Sun-centered universe (which he published, and which was cause for his trial)
Siderius Nuncius
Dialogue on Two World Systems
Isaac Newton, 1642-1727
Principia Mathematica, 1687
Newton, Philosophiae naturalis principia mathematica (1687) Translated by Andrew Motte (1729)
Lex. I. Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.
Every body perseveres in its state of rest, or of uniform motion in a right line, unless it is compelled to change that state by forces impressed thereon.
Projectiles persevere in their motions, so far as they are not retarded by the resistance of the air, or impelled downwards by the force of gravity. A top, whose parts by their cohesion are perpetually drawn aside from rectilinear motions, does not cease its rotation, otherwise than as it is retarded by the air. The greater bodies of the planets and comets, meeting with less resistance in more free spaces, preserve their motions both progressive and circular for a much longer time.
And now, in more contemporary language:
1. Newton's First Law (inertia)
An object will keep doing what it is doing, unless there is reason for it to do otherwise.
The means, it will stay at rest OR it will keep moving at a constant velocity, unless acted on by an unbalanced force.
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