Who the heck is Newton?
So, Sir Isaac Newton was this scientist from England that studied calculus (yucky, complicated math) and physics (the science of how stuff moves) at the same time. Some people also think that he came up with the idea of gravity. You may have heard the old folk tale of an apple falling onto his head and him yelling "Eureka!" We have already talked about forces and that they are measured in Newtons. That unit takes its name from Sir Isaac. But those are discussions for another time. What we really need to focus on is Newton's work. During his studies, he came up with three basic ideas that can be applied to motion. These ideas were tested and verified by him during his research and they still hold up today. We call these ideas "Newton's Laws of Motion" and there are three of them.
Newton's 1st Law - Inertia
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Newton's 1st Law states that "an object at rest tends to stay at rest and an object in motion tends to stay in motion, with the same direction and speed, unless acted upon by an unbalanced force." Wow, that's a mouthful.
Basically, the first law means that objects want to keep doing whatever they were doing in the first place. If an object is sitting still, chances are it's going to stay sitting still unless someone or something does something to make it move. If the object was already moving, it's probably going to stay moving unless someone or something stops it or does something to change its speed or direction. This tendency of objects to keep doing whatever they were originally doing is called inertia.
An unbalanced force is just a push or a pull that causes some change in the object's motion. It could be a push that starts a ball rolling or the brakes that stop the car.
Sometimes on Earth it is hard to see this law in action because there are so many unbalanced forces on Earth that interfere. Gravity is always pulling things down and friction slows things down or eventually brings them to a stop. Friction, by the way, is the force that exists between two objects when they move past each other. Try rubbing your hands together. Feel the heat that builds up and the pressure of rubbing them against each other? That is friction.
Anyway, the easiest way to observe Newton's 1st law is to look at home objects behave in space. Watch the video below and then scroll down for an explanation of how this relates to Newton's 1st Law.
Basically, the first law means that objects want to keep doing whatever they were doing in the first place. If an object is sitting still, chances are it's going to stay sitting still unless someone or something does something to make it move. If the object was already moving, it's probably going to stay moving unless someone or something stops it or does something to change its speed or direction. This tendency of objects to keep doing whatever they were originally doing is called inertia.
An unbalanced force is just a push or a pull that causes some change in the object's motion. It could be a push that starts a ball rolling or the brakes that stop the car.
Sometimes on Earth it is hard to see this law in action because there are so many unbalanced forces on Earth that interfere. Gravity is always pulling things down and friction slows things down or eventually brings them to a stop. Friction, by the way, is the force that exists between two objects when they move past each other. Try rubbing your hands together. Feel the heat that builds up and the pressure of rubbing them against each other? That is friction.
Anyway, the easiest way to observe Newton's 1st law is to look at home objects behave in space. Watch the video below and then scroll down for an explanation of how this relates to Newton's 1st Law.
When the astronauts let go of their food, the food just stays where they left it. Sometimes if wobbles around a bit if they nudged it before they let go (a slight unbalanced force), but for the most part, it stays right where they left it. It was left at rest, so it stayed at rest. If they were to push off of something, even accidentally, like the astronaut in blue does when closing the box, he would immediately begin moving and not stop until another force stopped him. He has to grab the railing and put some force on himself until he is still again. Because there is not gravity and no friction, stuff in space behaves exactly as Newton said it should.
Newton's 2nd Law - Force, Mass and Acceleration
Most people know this law, even if they don't know the proper way to describe it. Newton's 2nd law describes how much force is takes to move objects, based on their mass and how fast we want them to start moving. Take the following example: Which one is easier to move from a resting position, a bicycle or a dump truck? No it isn't a trick question. The bicycle really is easier to start moving. Now, which one is easier to stop moving if it is rolling toward you at 20 miles per hour? Again, not a trick question. It seems like a simple question because it IS a simple question. The bicycle is simply easier to move around. Why? Because it has much less mass. You would have to use much more force to move the dump truck than the bicycle because the dump truck has more mass.
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Here we see Newton's 2nd law written out as a formula. You've seen it before when we talked about forces. This formula is a mathematical way to figure out just how much force would be necessary to move a specific amount of mass at a specific rate of acceleration.
The same idea we used with mass can be extended to acceleration. If you want an object to accelerate at 10 meters per second squared, you would need to use a lot more force than if you just wanted that object to accelerate at 2 meters per second squared.
The same idea we used with mass can be extended to acceleration. If you want an object to accelerate at 10 meters per second squared, you would need to use a lot more force than if you just wanted that object to accelerate at 2 meters per second squared.
Newton's 3rd Law - Action and Reaction
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Newton's third law is all about forces that exist in pairs. Actually, all forces exist in pairs. Whenever a force pushes in one direction, there is always a force of equal value pushing in the opposite direction. For instance, when a cannonball is fired into the air, the cannon it was fired from jumps backward. The cannonball and the cannon were both acted upon by equal forces moving in opposite directions. The cannonball flies much farther because its mass is less (see Newton's second law). The cannon, acted upon by the same amount of force in the opposite direction, moves less because it has more mass (see Newton's second law). Forces will always act in pairs, but not always on the same object.
Another example is when you sit down in a chair. If Newton's first law can be believed, some force would have to stop you in order to keep you sinking all the way to the floor. The chair stops you by putting the same amount of force upward as you are putting downward. The chair holds you up, and you get a nice comfy seat. Aren't you glad Newton figured all this out?
Another example is when you sit down in a chair. If Newton's first law can be believed, some force would have to stop you in order to keep you sinking all the way to the floor. The chair stops you by putting the same amount of force upward as you are putting downward. The chair holds you up, and you get a nice comfy seat. Aren't you glad Newton figured all this out?