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Newton's Laws of Force & Motion

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  • 0:04 Newton's Laws of Motion
  • 0:32 Newton's First Law
  • 1:41 Newton's Second Law
  • 2:29 Newton's Third Law
  • 3:50 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 of motion explain why we observe the motion we do in the world around us - they explain how forces work. In this lesson, we take a look at each of Newton's laws one by one.

Newton's Laws of Motion

The study of kinematics (1D and 2D motion) was about describing how things move, from displacements to velocity to acceleration. However, when Newton began to do his work, he was concerned with explaining why things move the way they do. He achieved that by defining his three laws of motion. These use the concept of force (a push or pull) to explain the motion we observe in the world around us. Let's take a look at each of the laws of motion one by one.

Newton's First Law

Newton's first law says that an object in motion will remain in motion, at a constant velocity and in a straight line, unless acted upon by an unbalanced (or net) force.

What does this mean? Well, we tend to think of forces as being things that keep objects moving. To keep a shopping cart moving, it seems like you have to keep pushing it and applying a force. However, according to Newton, that's not true. When you let go of a shopping cart, there is still a force acting on it: the force of friction. If you push a shopping cart in space and then let go, it will keep going in a straight line, at a constant velocity, forever. It's the unbalanced force of friction that makes it stop when you push it along the ground on Earth.

Thanks to Newton's first law, we know that if we compare the forces acting on an object in each direction, we can figure out whether the motion will stay the same or change. If the forces are balanced, a stationary object will stay stationary, and a moving object will keep moving at the same velocity. If the forces are unbalanced, the object will accelerate, which means it will either slow down or speed up.

For example, if there is more force to the left than there is to the right, the object will have an acceleration pointing to the left.

Newton's Second Law

Newton's second law is related to the first law, but tells us exactly how much acceleration we get because of those forces being unbalanced. It says that the acceleration of an object is proportional to the net force on that object and inversely proportional to the mass of the object. This can be written as an equation which looks like this:

F = ma

Here, F is the net force on the object measured in newtons (N), m is the mass of the object measured in kilograms (kg), and a is the acceleration of that object measured in meters per second squared (m/s.

This means that an object with a bigger mass (heavier) takes a greater force to accelerate it. It also means the larger the unbalanced (or net) force, the more acceleration will occur. It's an equation we can use to solve a lot of problems.

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