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# Newton's first law of motion introduction

Newton's first law states that objects move with constant velocity unless acted upon by an unbalanced force. If the net force on an object is zero, it will remain at rest (if already at rest) or continue moving with constant speed and direction. A force is not required to keep an object in motion unless an opposing force is present. Created by Sal Khan.

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• In science my teacher said that theres a lot of weight pushing us down because theres a lot of air above us and he said our body is pushing back but how can other things push back to like a blade of grass and how can they push back too
• There is pressure outside, which pushes us, but there is high pressure inside us which doesn't let us crush because of the pressure outside. And measure your height when your lying down or sleeping on the floor.
• what is the definition of newton's first law of motion
• The definition of Newton's first law is: The velocity of an object will not change unless the object is acted on by an outside force.

The definition of Newton's second law is: When an object is acted on by an outside force, the strength of that force is equal to the mass of the object times the resulting acceleration.

A Newton= kg.m/sec2.
Force=(mass).(acceleration).

Here's an example of Force=(mass).(acceleration):
F=(m).(a)
F=(300 kg).(0.12 meters/second2
F=36 Newtons.
• what other forces exist?
• The universe consists of 4 major forces: Gravity, Electromagnetism (Light), Weak Nuclear, and Strong Nuclear. Some scientists say there is a fifth, but these 4 are the proven forces that basically "run" our universe.
• This may be a really simple question but I want to know, what force is keeping earth moving or orbiting the sun. You might say that in Vacuum, with one force, the object will keep moving. Okay but where did the first force come from if that is true. Also, Why is it that the earth is moving in circles (orbiting) when when you push something, it goes straight, not in curves?
• It's not such a simple question, at all! In fact until Newton came along and figured it out, nobody had a really good explanation for your really important observation, that the Earth and other objects in space going around the Sun follow a precise and predictable path, or orbit. You know what he called the force that keeps planets (and anything else!) from flying off in a straight line once they start moving-- gravity. I think it makes more sense to think of gravity as a pull, not a push, because gravity is a force of attraction that exists between any objects with mass. And the force is actually a pretty small one... you need a lot of mass involved before we start to notice it. Andrew M explained pretty well to another question how the solar system got moving at its formation and its been going ever since. But why in elliptical orbits? Well most of the mass in our solar system is found in the Sun and so the Sun has the greatest gravitational pull in our neighborhood in space. When you get an object pulling you toward it, but you're already moving, you start moving in a curved path. That's what happened to the planets going around the sun. I've heard the Sun's gravity described like a rope or tether pulling on the Earth, with not so much force that we spiral in toward it (which would be bad), but enough force to keep us from spiraling away (which would be pretty bad, too.) There's just enough gravity to keep us going around and around and around...
• Why is it that when a car accelerates the person on the passenger seat feels like they are being pressed into their seat?
• All objects want to stay at rest is they are at rest. This is also called inertia. When the car accelerates, your body's inertia kept your body from moving until it was pushed by the car seat.
• So if Galileo and Descartes had the same general ideas before Newton why does Newton get ALL the credit
• Does inertia have any relation with gravity?
• Yes, very good question!
In fact there are two different concepts of "mass":
1.) the mass which is attracted by gravity (let's call it mass(g))
2.) the mass which resists any accelleration, i.e. the mass which is responsible for inertia (let's call it mass(i))
It is not obvious at all that mass(g)=mass(i) This was the discovery by Galileo and indeed it is one of the most important physical relationships: it is called the "equivalence principle"
• So, thus, if the block was "pushed" in space it could float forever, right? Well, there is no friction and putting aside any astronomical objects like stars, planets, black holes aside; then it would just go on forever, right?
• Yes, if nothing applied a force to the block after the initial push the block would continue to move with the same velocity forever.
• If the objects at rest and costant velocity have 0 net force,does it mean that to give an object speed you have to exert some force on it.And once you stop exerting force,an object will just infinitely move forth?