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Imagine Steven, a typical nerd, walking down the school's main hallway. He is minding his own business, moving in a straight line at a constant speed. Now imagine Eric, the excessively muscular, sexually frustrated football player. He is very pissed off. Yet another girl had thwarted his flirtatious advances. Seeing in Steven an opportunity to vent his rage, Eric body slams the frail figured Steven into the lockers. All that remains of the nerd is a messy stain for the janitors to clean. |
In essence, that was the first law in action. If Eric had never been walking down that hallway, Steven would have continued along his merry way.
More technically stated the first law says: If an object has no forces acting on it (i.e. friction, wind, large football players, etc), it is going to keep moving in the same direction at the same speed until a force does act on it.
What's a force? Simply stated, a force is a push or a pull. If you hit your little brother in the face with a text book, you have exerted a force on him.
The idea of things moving in straight lines forever seems foreign to most people. The reason for this is we live on Earth. On our planet, or any planet for that matter, forces are inescapable. The air around us is constantly pushing on everything and slowing stuff down, gravity forever traps us on the gound. However if we could float in space, we would immediately see how true the first law is. Objects would always move around at constant velocities until something pushed them.
We call an object's resistance to changing its motion inertia. Inertia is measured in Kilograms (1000 grams). The more Kilograms an object has (the more massive it is) the more it's going to resist any change in motion. Keep in mind that change in motion doesn't just mean taking an object that wasn't moving and making it move. It also applies to things that are moving. The massive oil tanker moving through the water is going to want to keep moving at that speed, and a force, like an iceberg, it needed to make it change its velocity.
Keep in mind weight and mass are NOT the same thing. Weight is a force that comes from the pull of gravity. Mass is your resistance to motion, you can also think of it as the amount of 'stuff' in you. In outerspace your wieght would be zero, but your mass would stay the same.
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This is the law to know. This is the one your teachers are going to test you on. In words it says: If you add up all the forces pushing on an abject, (The funky looking letter 'E' is the Greek letter sigma, it just means add up) that force will equal the object's mass times it's acceleration.
For example if we knew Steven's mass was 60kg and when Eric hit him he accelerated away at 10m/s2 we can find out how hard he was hit. Just plug in the numbers
So we know that Eric slammed Steven with a 600N force
N? What is that N? Well, N stands for Newton. Since he was such a bright guy, he got a unit named after him. A newton is defined as the force required to accelerate one kilogram, one meter per second.
Most of the time in physics Sigma F=ma is just written as F=ma
You've heard it before and you'll hear it again. For every action there is an equal but opposite reaction.
Newton's 3rd law applies to pushes and pulls. Every force ever exerted has a counter force that goes along with it. Your fist exerts a force on your brother's kidneys and your brother's kidneys exert an equal force on your fist. Everything is like that. The earth pulls you toward it with gravity, and, believe it or not, you pull the entire earth toward you with the exact same force.
The reason you are able to stand on the top floor of a skyscraper is thanks to law number 3. Your weight pushes down on the floor, and the floor pushes back with a force equal in strength but opposite in direction (get used to that phrase, you'll hear it a lot in physics) If the floor did not push back, you would go crashing through it. Don't believe me? Read on.
| First lets start with the normal situation. If you weigh 600 Newtons, then you are pushing on the floor with a 600 Newton force. The floor has to push back on you with a 600 Newton force, so you stay in place. |  |
Now lets pretend the floor is slacking off and only pushes back with a 500 Newton force. This is where law #2, Sigma F=ma, comes into play.
Remember that Sigma? You have to add all the forces, yes it's a bitch, but you gotta do it. Your weight is -600 Newtons. Why negative? Because if you remember, weight is a vector quantity. The negative sign indicates that the force is being applied downward. The lazy floor is pushing upward on your feet with 500 Newtons. Add them together and you have a resultant force of -100 Newtons. If we know your mass to be 50 kg we can find your acceleration by plugging the numbers into the formula.
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This result means that in this situation, you would have to be accelerating downward through the floor at 2m/s2. Obviously this doesn't happen. The only way to avoid crashing through the floor or flying off of it, is if the floor pushes back with a force equal to your weight, but opposite in direction. |
Yes the above is true, but not in a good way. Newton, after many a long evening by himself, came up with the theory of gravitation. It simply says that every thing in the universe attracts everything else. He also had a formula to go along with that.
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In English, that says the force of attraction between two objects is equal to the gravitational constant (6.67 x 10-11) times the mass of the first object times the mass of the second object divided by the distance between there centers squared.
Because the gravitational constant is such a small number, we don't feel the effects of gravity from every day objects. It takes the mass on the entire earth to hold you in place. But none the less if you eat more, people will be more attracted to you.
If any of the above was unclear, or if you have any comments or suggestions, please E-mail me!
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