Limitations of Newton's Laws

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  • 0:00 Definition of Newton's Laws
  • 2:34 Relativity and High Speeds
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Newton's Laws are one of the first things people learn in physics classes across the world. But did you know that they don't always work? Learn about the limitations of Newton's laws, and then test your knowledge with a quiz.

Definition of Newton's Laws

Science, and especially physics, has come a long way over the last 300 years. One of the first people who made science the data-driven, precise process that we're used to today was Sir Isaac Newton. That's because he not only introduced his ideas about gravity and motion to the world in 1687, but he explained them all through the power of mathematics.

Newton's laws of motion are three laws that describe how the movement of an object relates to the forces (the pushes and pulls) that act on it. It's the basic foundation on which all the physics was built. However, a lot of the ideas and uses Newton formulated in his laws of motion are not necessarily that obvious.

For example, he realized that an object will keep moving at a constant speed even if the forces on it are completely balanced, or even if there are no forces on it at all. From our everyday experience, it seems like things have a natural tendency to stop. But he realized they actually have a natural tendency to keep going at a constant speed. This is Newton's first law.

In Newton's second law, he explained how to calculate the effect balanced forces would have on an object's motion. It says that the unbalanced force is equal to the mass of the object times the acceleration of that object. Or, F=MA.

Newton's third law is the famous idea that actions have equal and opposite reactions. Or, in other words, if you push on a wall, the wall pushes back on you with the same force. This again seems kind of strange, because one might wonder why anything moves if there are always two forces to cancel each other out. But if you push a shopping cart, the force the cart applies back to you has nothing to do with how the cart moves.

So, those are Newton's laws, and they work really well. In our everyday lives and experiences, Newton's laws are perfect. But it turns out that Newton's laws don't work quite right. Over 300 years later, Einstein realized the Newton's laws were really just a simplification. They work fine at slow speeds on Earth, but fail miserably when you take an object closer to the speed of light. There were limitations of Newton's laws.

Relativity and High Speeds

Einstein's special theory of relativity is a set of laws and ideas about how space and time work that he introduced to the world in 1905. The basic ideas behind it are quite simple: that the laws of physics work the same way no matter where you are in the universe, and that the speed of light is the same no matter where you're measuring it from. Those might seem so obvious that they're not worth saying. But it turns out that the consequences of these ideas being true are super weird and groundbreaking.

One of these consequences changed our understanding of Newton's laws forever. Einstein explained that mass and energy are really the same thing. What this means is that if you increase the amount of energy that an object has, you are also increasing its mass. For example, if an object gets faster, its mass increases. This effect is small enough that we don't see it when we're driving in a car. A car weighs the basically the same whether it's going at 10 miles an hour or 100 miles an hour.

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