Describing Motion: what is it and how do we make sense of it? The words we are about to hear are not new words. Many use these words everyday and guess what? They carry the exact same meaning even in a scientific context. Those words are speed, velocity, and acceleration. Think about a car moving at 55 mph. The speed at which the car moves is 55mph, right? Now, if you say the car is moving at a speed of 55mph in the North West direction, you’ve just defined its velocity! Velocity is the speed in a given direction. The third is acceleration which defines how the speed changes in time. An example is when you have to decelerate at a stoplight, this is a form of acceleration because the speed is changing with time as you come to a stop.
Force and momentum: Forces directly impact the speed, velocity, and acceleration of bodies. Forces are what change the motion of any object and are usually from the interaction of more than one object. And if that force changes with time, we call that momentum. The momentum of a body is measured by how the forces driving it to move are changing. Humans experience angular momentum on the daily, too, did you know that? For instance, when you slow down to make a turn in your car, you generate angular momentum!
Gravity: Believe it or not, gravity is a force field! It manifests from the presence of two objects of any given mass (or even energy). It is quite literally a driving force to the creation of the Universe!
Free Fall: Free fall is actually just a synonymous phrase to gravitational acceleration. Free fall is the motion of an object when it is under the force of gravity. We specifically use free fall when describing an object that is responding to the Earth’s gravity. A popular example would be astronauts in SPACE! A common misconception is that there is no gravity in space. The astronauts that are literally orbiting around the Earth are certainly still experiencing Earth’s gravity. That’s how they are even in orbit to begin with! It’s just that they are in a constant state of free fall around the Earth such that the astronauts feel weightless.
Mass vs. Weight: The term ‘weightless’ brings a good segue into describing “mass” and “weight”. Mass is the amount of matter in your body. Weight is the measure of how gravity acts on mass. This is why people say you would weigh less on the Moon than on the Earth. You have the same exact mass on both planets, but the Moon’s gravity is much less than the Earth’s gravity, making the force of weight much less dramatic than the force of weight on Earth!
Want to know more?
Check out Chapter 4: Making Sense of the Universe in the textbook The Cosmic Perspective by Bennet, Donahue, Voit, and Schneider, Seventh Edition, 2014. Pgs 118–123
(On) Planet Nine 