![]() ![]() Let’s look at another simple example, in which you are trying to push a piano across the stage for your performance at your high school graduation. Similarly, if the car were to turn to the left or the right, your head, due to inertia, would feel like it was being moved to the right or left. This is because the part of your body that is attached to the car would speed up too, but your head wants to obey the law of inertia and continue moving at a constant velocity, that is, with the same speed and direction. If we look at the head as an object moving in the inertial frame of reference of the car, if the car sped up, your head would feel like it’s being thrown backward. If the car changed speed or direction, you would feel it because your body wants to keep moving in the straight line with constant speed. In that frame of reference, you and I are not moving with respect to each other we are both moving in the same inertial frame. Imagine if you and I were sitting in that car and there were no other objects around. However, what if we made the car the inertial frame of reference and discussed the motion of the passengers inside it? What we’ve just described is the constant motion of an object that is in a frame of reference with zero velocity. ![]() By doing this, we can state Newton’s first law, mathematically, as the sum of the external forces is equal to the upward force of the ground on the car minus the downward force of gravity on the car, which is equal to zero. We define force by the symbol \(F\), noting the force is a vector, which means it has both magnitude and direction. Since you have two equal forces acting in opposite directions, the forces cancel out, meaning that the car does not move in the vertical direction. But what about gravity? Isn’t that an external force? While it’s true that gravity is pulling the car downward toward Earth, the car is on the ground and the ground is providing a force in opposition to gravity. It would seem that there are no net external forces acting on the car since the car isn’t moving. ![]() Let’s take a look at an object in an inertial reference frame that has a velocity of zero. What we do instead is use Earth as a point of reference, or an inertial reference frame that is, we discuss the motion of objects in reference to Earth. However, it’s generally pointless to include the motion of things like the solar system and Earth when we talk about the motion of things on Earth, like cars or people. So objects that we consider to be “stationary” on Earth are technically moving at a speed of almost 1 million miles per hour! Remember, we’re on a planet that’s rotating at about 1,000 miles per hour, our solar system is moving at about 140 miles per second, and our galaxy is rotating at 130 miles per second. When you kick a soccer ball, and it soars through the air, the soccer ball isn’t the only thing that’s moving. An inertial reference frame is a frame of reference that is moving at a constant velocity. One of the key things to remember, though, is that this law is only valid for objects in an inertial reference frame. This statement is much more intuitive than it sounds. Newton’s first law of motion states that an object at rest will stay at rest, and an object in motion will stay in motion unless acted on by a net external force. In this video, we’ll be discussing his first law of motion, also known as the law of inertia. The basis of our current understanding of motion comes from Sir Isaac Newton’s three laws of motion, which he established in 1687. Over time, calculations and experiments helped pave the way for a deeper understanding of motion, with Galileo Galilei defining the laws of gravity in the 1580s and Johannes Kepler penning his laws of planetary motion in 1618. Understanding how and why things move has been the goal of scientists for thousands of years. Hi, and welcome to this review of Newton’s first law of motion! In this video, we’ll see how it forever changed people’s perceptions of how and why things move. ![]()
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