Daily Archives: April 18, 2017
It’s Rocket Science
Newton’s Third Law of Motion states, “For every action, there is an equal and opposite reaction.”
Anyone else experiencing this? When it comes to things I don’t want to do, I find that the harder I try to push myself, the stronger my resistance is. When I’m feeling pushed by life… or people… the same thing happens. My inner mule comes out, and I push back. I thought this was me being stubborn, now I find out that its simple rocket science!
Here’s a simplified explanation from Live Science:
Forces always occur in pairs; when one body pushes against another, the second body pushes back just as hard. For example, when you push a cart, the cart pushes back against you; when you pull on a rope, the rope pulls back against you; and when gravity pulls you down against the ground, the ground pushes up against your feet. The simplified version of this phenomenon has been expressed as, “You cannot touch without being touched.”
One might ask, “If the two forces are equal and opposite, why do they not cancel each other out?” Actually, in some cases they do. Consider a book resting on a table. The weight of the book pushes down on the table, while the table pushes up on the book with an equal and opposite force. In this case, the forces cancel each other because the book does not accelerate. The reason for this is that both forces are acting on the same body, while Newton’s Third Law describes two different bodies acting on each other.
If one object is much, much more massive than the other, particularly in the case of the first object being anchored to the Earth, virtually all of the acceleration is imparted to the second object, and the acceleration of the first object can be safely ignored. For instance, if you were to plant your feet and throw a baseball to the west, you would not have to consider that you actually caused the rotation of the Earth to speed up slightly while the ball was in the air. However, if you were standing on roller skates, and you threw a bowling ball forward, you would start moving backward at a noticeable speed.
Consider a horse and a cart. The horse pulls on the cart, and the cart pulls back on the horse. The two forces are equal and opposite, so why does the cart move at all? The reason is that the horse is also exerting a force on the ground, which is external to the horse–cart system, and the ground exerts a force back on the horse–cart system causing it to accelerate.
How, then, can a rocket move through space if there is nothing for it to push against? When the fuel is ignited in the rocket nozzle, the gas expands rapidly in all directions. Some of it goes backwards and has no effect on the rocket; however, some if it goes forward and crashes into the back of the rocket exerting a force that causes the rocket to accelerate in the forward direction. This is why Newton’s Third Law is considered to be the fundamental principle of rocket science.
Another explanation:
What did Newton mean when he said action? He meant — at any instant — force, or, over time, a change in momentum. So if you punch your enemy in the face … your fist exerts a force on his face, and his face exerts an equal and opposite force on your fist.
And, if you use a small gunpowder explosion to change the momentum of a bullet, the equal and opposite reaction is the equal and opposite change in momentum of you holding the gun, known commonly as recoil.
It pays to be aware of this.
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