Ivan astikov, I am not saying this to be snarky or insulting, but a basic physics class might do wonders to help you understand what we are saying. Seriously, Dude. If you are interested enough to continue this discussion, do you have access to a local community college for such a class? I assume you are past highschool age; if not, how about putting elementary physics on your class schedule?
You can have this same argument with a class instructor at any level, but the personal touch and possibly some lab experiments might open your eyes, at least I hope so.
Some of the assumptions you are making were the same as society made and Aristotle postulated. They lasted for a very long time. Galileo, Newton and others challenged them, and were proved right. The well-established knowledge of the past 300 years or so is readly available at any college or university.
It’s hard to talk to someone about vectors, acceleration and inertia if that someone isn’t clear on what those terms mean.
Velocity = How fast an object is moving.
Acceleration = Any change in velocity.
Force = Pushing or pulling on an object.
Applying a force to an object will accelerate it, i.e. change its velocity.
Say I’m an astronaut floating in space. There’s no air and no gravity. I throw a baseball away from me. What happens?
The baseball initially has a velocity of zero. When I push on it with my hand I’m applying a force. That force accelerates it to a new velocity and the baseball moves away from me. But the only way to change the velocity of an object is to apply a force. Once the baseball leaves my hand it the force goes away. It stops accelerating. So the baseball will keep flying away forever at exactly the same velocity it had the moment it left my hand! This seems very counterintuitive because in the real world when you throw a baseball it doesn’t keep going forever. Instead it slows down and drops to the earth. But that’s because in the real world there’s air resistance and gravity to complicate things. In the vacuum of space the baseball will just sail away and keep going forever.
Gravity is a force. If an object is near a planet gravity will pull on it continuously. This means the object will accelerate *and it will keep accelerating *until it smacks into something. That’s what happens when you drop the bullet out of your hand. It starts off with zero velocity and moves faster and faster until it reaches the ground.
When you fire a gun the exploding gunpowder applies a force to the bullet. This accelerates the bullet. But when the bullet leaves the barrel the force ends. It doesn’t keep accelerating. At that point he bullet keeps moving at a constant velocity away from the gun, just like the baseball thrown by the astronaut.
If the gun barrel is perfectly horizontal then all of that velocity is in the sideways direction. It’s not moving up or down at all. But then the force of gravity starts to pull on the bullet. This accelerates the bullet, increasing its downward velocity.
Here’s the important part: Since gravity is pulling straight down it has no effect on the bullet’s sideways velocity and vice versa. This makes intuitive sense. Could you stop a rolling car by pressing sideways on the door? No! If you want to stop a rolling car you need to push against the front bumper. If you want to affect the velocity of something you need to apply the force in the same direction (or the opposite direction) of its movement.
So … the bullet flies sideways at the same speed regards of whether there is gravity or not. And the bullet accelerates downward just the same regardless of whether it is moving sideways or not. If we add up the sideways movement and the downward movement we see that the bullet traces a downward arc until it hits the ground. Note that at the moment it hits the ground it’s still moving just as fast sideways as when it left the barrel. It doesn’t slow down and then drop. It keeps moving at a fixed speed sideways as it drops faster and faster.
Since the moving bullet’s sideways velocity has no effect on the acceleration caused by gravity it hits the ground at exactly the same time as the stationary bullet.
Actually, no. Exponentiation binds tighter than division, in regular math as much as in computer programming languages, so “9.8 m/s[sup]2[/sup]” was absolutely unambiguous as you gave it.
Not only that, but it’s only average speed. The car wasn’t going 50 mph the whole time. You had to accelerate up to a certain speed, and then decelerate at the end (unless you drove past the mark, of course.) In real life, you probably would have had to slow down and speed up multiple times during the journey.
I know that (and I suppose I should have added an asterisk), but ivan appears completely at sea on the concept of vectors, so I thought that commenting on the difference between velocity and speed would be useless and likely more confusing to ivan than it was worth.
Don’t feel bad Ivan, this one is hard to visualize and took me a while to believe and grasp when explained to me in physics.
It’s common to think that a bullet coming out of a horizontal muzzle is moving so fast that it must travel in a straight line horizontally for at least some distance before falling. Unfortunately it’s untrue. That bullet will start to fall to the ground the moment it comes out of that barrel.
It just seems to travel in a straight line because it has traveled horizontally much further than it has downward.
If you had a wall a mile long to graph the bullets flight path in full scale you’d see a line go from the end of that barrel all the way to the ground a mile away with a very slight arc to it. Look at that line close up and the first 10 meters will appear to be a horizontal straight line (it’s not). Look closer still and even at that 10 meter mark the line is lower than where it started.
If you took a laser pointer and mounted it perfectly level with the barrel of a gun and pointed it at a wall 20-30-50 meters away not a single bullet will hit that laser mark on the wall. They will all fall under it.
It is only unambiguous if one is familiar with the rules of math notation and also notices the subscript. ivan askitov appears unfamiliar with the rules and apparently missed the subscript, thus confusing acceleration for speed.
My phrasing was precise, but the level of presentation was not simple enough. Thus I was not clear to the audience.
As counter-intuitive as I find the evidence to be( a reflection of my ignorance, I accept.), I appreciate your efforts and patience to explain things, and I think I get it now.
I just want to say: Don’t feel stupid because this wasn’t obvious to you. In fact, it’s really not very obvious or common sense, for even physics majors. But that’s why it gets talked about a lot.
The vast majority of projectile motion ends up agreeing with common sense: if you throw something up hard, it takes longer to come back down than if you throw something up softly; things dropped from a big height land with more impact than things dropped from a short height; and so on.
But some things – like the two bullets – aren’t so common sense. So they get talked about a lot, because understanding these situations helps us understand how the laws of motion really work.