مقاله انگلیسی در مورد حرکت جسم

 

D Motion

Physics has a lot of motion in it. Who wants to do calculations about stuff that just sits there? One of the fundamental rules of physics is that an object at rest stays at rest. This means that if an object is just sitting there, nobody wants to study the physics of it. -Pause for groans- Actually it means that for any object to move, a net force needs to act upon it. If you have some equal and opposite forces pushing on your car, it's not going to go anywhere. Once your car starts moving, however, you need a force to stop it, such as friction. If you hit a patch of ice in the road, there isn't any friction and your car isn't going to stop before it hits a few mail boxes. Before I get to stuff like momentum and energy, you need to learn the basics of motion. One-dimensional motion is any movement that is in a straight line. This is useful for stuff that's dropped or moving along a ramp or other surface. First, let's discuss falling objects. Everybody has heard the story about Isaac Newton getting hit in the head with a falling apple and then figuring out gravity. Actually, he merely observed a falling apple. But how fast was that apple falling? Well, if it was falling from a 5 meter tree, it would have actually reached a speed of 22 mph, (disregarding the small effects of air resistance). How exactly did I find this out? Well, with 1-D physics. First, I found the height of the tree (well, estimated it to be 5 meters tall). Then I just used a formula for how long something should take to fall, or: Most physics is just a matter of thinking about what you already know, what you need to know, finding the right equations, and just plugging and chugging. This may sound boring, but there are some cool physics experiments out there. Ever wanted to drop something off of a really high up place? Maybe it's a guy thing. Here's an experiment to find the speed of an apple (or any object) dropped off various landmarks. (Note: This experiment requires Macromedia Flash 5 or higher. If you don't have this plugin, you can try the JavaScript Version)

All I did was take the height of the landmark and plug that distance into the time formula. It wasn't that hard. If we were to do the car problem, the setup would be pretty much the same. Just figure out what you know, what you need to know, and then find the correct equation(s). Lets say you are driving along in your car going 10 (25 mph), and suddenly, a wild llama jumps on to the road 20 meters in front of you! Should have paid attention to the signs.

Whether it's a llama or a deer, you don't want to hit it. If your reflexes are good, it will take you 1 second to hit the brakes. How fast do your brakes need to de-accelerate the car in order to not hit the llama? First, lets look at what we know. There's a one second reaction time, which, if you're traveling 10 meters-per-second, means that you just used up 10 of the 20 meters. So now you know that you have 10 meters to change your velocity from 10 m/s to 0 m/s.

What handy equation should we use that has the four variables of V final, V initial, acceleration, and distance? How about Vf = Vi + 2*a*d. Plugging in the known variables gives 0 = 10 + 2*a*10, or 0 = 100 + 20a. Rearranging the equation gives -100 = 20a, so a = -5 . That's not that bad, considering how gravity is always pulling at you with 9.8 . You probably want to know where I'm getting all these equations from. Well, I have a note card that I keep in my calculator case with all the worthwhile physics equations. It's very useful, and I'll let you have a copy for free. (And actually, I can derive almost any equation I need to know with calculus, but I'm not going to go into that yet...)