Mechanics is the study of the motion of objects. This is a great place to start your studies in physics since it’s such a BIG idea. We’ll be learning the language, laws, concepts, and principles that explain the motion of objects. We’re going to learn about kinematics, which is the words scientists use to explain the motion of objects. By learning about scalars, vectors, speed, velocity, acceleration, distance, and more, you’ll be able to not only accurately describe the motion of objects, but be able to predict their behavior. This is very important, whether you’re planning to land a spaceship on a moon, catapult a marshmallow in your mouth from across the room, or win a round of billiards.

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We can describe how something moves with words, numbers, graphs, charts, or equations. To do that, we need to measure things with rulers and stopwatches. If I asked you how fast your car goes on the freeway, you could say fast or you could also say 55 mph. That 55 mph is a quantitative number that describes the motion of your car. The car travels 55 miles every hour. It’s also a scalar quantity, since you only mentioned the magnitude (how fast the car is going) and not it’s direction. A vector quantity is when you’d say 55 mph southeast. Vectors include a number and a direction. Scalars deal only with numbers.

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Distance and displacement sound the same, don’t they? But they’re just a little different from each other, and here’s how: distance is a scalar quantity, like 5 miles. Displacement is a vector quantity that describes how far out of place an object is, like going up and down the same flight of 8 steps. Your distance is 16 steps, but your displacement is zero, since you physically traveled 8 steps up and 8 steps down, but your total is zero since we also take into account the direction of travel, and everything cancels out.

Have you noticed that scalar quantities ignore direction, and vector quantities take direction into account? Speed and velocity also sound the same, don’t they? But again, one is a vector and one is a scalar. Speed is the scalar quantity that describes how fast something is moving, like 100 mph. It’s the rate that something covers over a distance.

Rockets are fast, so they have high speeds, which means they cover large distances in a short amount of time. Compared to the speed of light, however, rockets are quite slow. (You always have to keep in mind what you are comparing to.) Velocity is a vector quantity that has a magnitude and a direction, like 100 mph north. It doesn’t matter if your speeding up or slowing down (we take that into account when we look at acceleration of an object). Velocity is the change in distance over a given time, or v = d / t. If a jet travels 600 miles in an hour, then it’s moving at 600 mph. A car going 25 miles in a half hour is moving at 50 mph. A snail crawling an inch every four minutes is moving at 0.25 inches per minute. You can mix up the units of distance and time to be whatever is most useful to you, whether it’s miles per hour, feet per minute, or meters per second. Most objects don’t just travel at one speed, however.

When you travel in a car, sometimes it’s on the freeway (65 mph), sometimes you’re at a stoplight (zero mph), sometimes you’re driving through the neighborhood (25 mph), and so forth. Your car has a lot of speed changes, so it’s useful to be able to calculate the average speed and average velocity of your car. It’s also useful to know the speed or velocity at a given instant in time, called your instantaneous speed or instantaneous velocity.

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Acceleration is defined as a change in velocity. In other words, it is a change in speed or a change in direction. It is how much time it takes something to go from one velocity to another. Remember that velocity is speed and direction. If you go straight ahead on your bike at a constant speed of 5 mph, you are not accelerating because neither your speed nor your direction is changing. Now, if you are stopped at a stop light and it turns green, you are accelerating as your speed increases from zero to 10 mph.

The word ‘acceleration’ is a little confusing, since sometimes people say someone is ‘accelerating’ when they really mean that they are ‘moving really fast’. Acceleration simply means changing speed or direction, not if they are going fast or not. Also, in physics we don’t use the word deceleration. We use positive and negative acceleration. So if you went from 10 mph to zero, you’d say that you have a negative acceleration, not deceleration.

Now what happens if you are in a car and it turns a corner at a constant speed of 15 mph? Is it accelerating or not? Well, the speed is not changing but its direction is, so it is indeed accelerating.Remember back when we talked about gravity? We learned that gravity accelerates things at 32 feet per second². Now this may make a little more sense. Gravity made something continue to increase in speed so that after one second of having the force of gravity pull on something, that something has reached a speed of 32 feet per second. When that thing started falling it was at 0 velocity, after a second it’s at 32 feet per second after 2 seconds it’s at 64 feet per second and so on.It’s the old formula v = gt or velocity equals the gravitational constant (32 ft/s²) times time.

If you need a refresher on how to convert units, here’s a video on how to do it:
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