Forces only have a “speed limit” in the presence of friction. An object falling in Earth’s atmosphere for instance has a terminal velocity, but this terminal velocity is dependent on the friction, not the force. Two objects with the same mass, but different shapes, would experience the exact same force, but different air resistance, and so would have different terminal velocities. Remove the air though, and you have no terminal velocity.
This comes directly from one of the most basic laws of physics, Newton’s second law, F=m*a. Where F is the sum of forces on a mass. Only when that sum is zero can you have constant speed. If you have thrust larger than friction you have acceleration.
If you haven’t examined this law and understood its consequences you are extremely unlikely to produce meaningful interpretations of the physics of force, acceleration and speed.
You are suffering from a complete misunderstanding of how the laws of physics work. Forces don’t have a speed limit, they don’t care how fast you are already going. If you apply a force to an object, that object will accelerate regardless of how fast it is already going, unless that force is completely opposed by other forces such as friction and air resistance. In a frictionless scenario, a constant force will result in constant acceleration and a constantly increasing speed with no limit.
This is basic high school level Newtonian physics here. If you don’t understand this, you don’t have a hope of understanding how jet engine reversers work.
Yep , you’re wrong. If an object is “out of gravity” (presumably, you mean out of the significant reach of the earth’s gravitational field) and you exert a force on it, it will accelerate, i.e. go faster. If you continue to exert force on it, it will continue to accelerate, i.e. go faster. In theory, if you could continue to exert force, you could continue to accelerate the object. Yes, in theory, you could get it close to the speed of light. At some point, its mass would increase and things get complicated, but as long as a force is imparted to an object, it will accelerate (in the absence of any contrary forces, of course). Deep space probes from Earth are moving as a result of a force that was imparted many years ago. The force is no longer with them, as it were, so they don’t accelerate, except as they are affected by other celestial bodies. If they could get a force applied to them now, they’d go even faster.
This is more or less the concept behind the ion engines being used on some space probes now. They produce a tiny amount of thrust, but they can produce it for months on end. Even though the thrust is tiny, with no friction or anything else to oppose it a constant thrust still results in a constant acceleration that adds up to significant speed eventually.
I agree totally that a fair foundation is needed to really follow the answers.
You need to understand what F=ma means. You need to understand exactly what position, velocity, and acceleration mean, and how they are related. You need to understand what vectors are, and that force, velocity, and acceleration are vectors. You need to be able to construct and understand free-body diagrams. This would be covered in the first few weeks of an introductory physics course. Done right, at least one semester, and better two, of calculus would be a prerequisite. People I know who took non-calculus physics don’t seem much better off than the OP. Issac Newton pretty much had to invent calculus to explain the physics after all.
Absent such a foundation, the question can only be answered as “trust us, it works.”
It seems you are saying that if force x is applied to an object and force 2x was applied to a similar object in space, that eventually all speeds attained by force 2x will be attained by force x. as time goes on. i.e. force x will always be half the speed of force 2x at any given time during the constant acceleration which I assume you mean speeding up.. until of course close to light speed is attained.
I already I told you that I know how the reversers work, but you don’t accept it, something has to act on the plane, and it’s the exhaust. I think one of the posters were satisfied with my conclusion that I had it right.
What we don’t agree on is what is going on inside the engine. someone said they don’t care what is going on in the engine but this is the crux of the matter.. look at the multi directional disco light that spins. the one with lots of square glass. lets say the inside was empty, open one of the squares and throw a molotov cocktail in there and close the square, imagine the explosion in the middle of the light.. what would happen? every glass square would blow out at the same speed/mph (acceleration aside). equal FORCE on each little square. That proves that the force of combustion that is expansion is equal in all directions. every direction that a glass was facing has glass going towards it.. I don’t think you can deny this deny .
2) fortify all other squares and leave two opposite squares as the example before and start the combustion again, the two opposite sides would fly off at a certain MPH ( miles/feet per sec per sec of course. however technical you want to get). Why? because equal force on both sides.The ball would not move, why ? because the force going north and south have no part of the light to PUSH on. remember the glass flies out because force is PUSHING on it. I don’t think you can deny this.(unless you want to apply an analogy of some gravity theories that we are being pushed into the earth rather the pulled). The other sides have expansion force applied to them yes but because they are fortified the expansion is only free to PUSH out the two non fortified sides. so there is a PUSH going on. I don’t think you can deny this.
3)Fortify all sides except the south side and start the combustion, what happens? remember the forces are acting on all sides, the south side blows out and the north side not having the south side closed to counteract the force is PUSHED in the north direction.. Once again, I proved that there is a north force vector when the north glass was not fortified. so the south force you feel is not the north force so to speak.
Imagine a ten mile long tube north to south orientation; with one end open, in the vacuum of space. so no air is in the tube. have a fuel air mixture and light it. I guarantee you that before that expansion of gasses reaches that opening ten miles away that canister is on the move. why? because the expansion of gases expanding to the north hits the closed end of the tube.
What folks are saying in here is that we have to wait until the expansion goes ten miles and exits the tube before you have movement, which is absurd!! That tube is long on its way from the north force. Its the south force vector you feel coming out the tube.. i don’t think you can deny this, Hope I am clear this time. And also I want to give heartfelt thanks for everyone discussing with me. I appreciate the time and effort. I think it is a very informative discussion. maybe the same views with different ways of saying it maybe not..
vyVY
If this is what it means, then I believe I am right. a force pushing something 20 mph could not push an object twice the mass 20mph. if that is what this is saying that is.
Q:What does “F=ma” mean in Newton’s Second Law?
A:Newton’s second law of motion describes the relationship between an object’s mass and the amount of force needed to accelerate it. Newton’s second law is often stated as F=ma, which means the force (F) acting on an object is equal to the mass (m) of an object times its acceleration (a). This means the more mass an object has, the more force you need to accelerate it. And the greater the force, the greater the object’s acceleration.
vyVY
according this they produce what adds up to huge amounts of thrust over time by sending a narrow field of the thrust:
Ion thrusters create very small levels of thrust compared to conventional chemical rockets but achieve very high specific impulse, or propellant mass efficiencies, by accelerating their exhausts to very high speed.
The ion engine has advantages and disadvantages, nicely summarized here: “Yet the probe’s thruster is 10 times more efficient than a chemical thruster. A typical chemical spacecraft could accelerate to 1 kps in about 20 minutes, but would require 300 kg of propellant, according to Rayman. The Dawn spacecraft’s ion propulsion system could attain the same speed with only 25 kg of xenon, but would take nearly 100 days of continuous thrust to reach that velocity.”
From the other articles I read it seems to say that the ion engine takes longer to get UP to speed that a normal rocket would take , using a lot less fuel. I see where the say it broke a thrust record, don’t know if that means overall or if just for ion engines. It seem to me it is saying that it creates extremely high power but in small streams, so the power is there but takes a while to get the mass going.. Laymans terms would be at the carnival when you shoot the water pistol to blow up the balloon, which is the ion propulsion analogy. and the rocket would be using a water cannon. and blow the balloon in seconds.. even though the stream from the water gun is smaller, it’s velocity will one day win out over the rockets speed. I could be wrong. You can only go as fast as the thrust is pushing you, when you reach the speed the thrust is pushing you you will balance out.
vyVY
F is the force applied, m is the mass, and a is the acceleration.
Note that speed is not mentioned anywhere in that equation. Acceleration, which is the rate at which speed changes, is.
If you double m while keeping F the same, then a will be halved. The object will accelerate half as fast if you double the mass while keeping the force the same. This means it will take twice as long for the object to reach a given speed.
Top speed is not mentioned in the F=ma equation, because an object’s top speed is going to depend on a large number of other factors such as wheel friction and air resistance. In a frictionless environment, there is no top speed, and as long as you keep applying a constant force you will get constant acceleration.
An ion engine works by using electric and magnetic fields to accelerate a small amount of noble gas at high speed. They take a lot of power to run, have extremely high fuel efficiency, and produce tiny amounts of thrust. They can run for months, and have set records for total amount of thrust provided over their total run time - but the amount of thrust generated at any one time is tiny.
If I have a force that moves an object through space at 100 mph and a force that moves a clone of the object 20mph, you are saying if I put the force that makes the object move 20 mph with the force that moves the object 100 mph I would get 120 mph? My thinking is that to get the 100 mph force to move 120 mph I need to put on it a force the moves the object 120 mph. because the force that moves the object 20 mph is a fifth of the force used to move it 100 mph. It would just match the the fifth of the 100mph force, not do anything to increase the speed.. think of batteries in series and parallel. in series more power, in parallel same but longer power. could be wrong. but that’s what i see
vyVY
No, you are wrong. Acceleration is not the same thing as speed. A given force will simply take twice as long to accelerate an object with twice the mass.
There is really no point in proceeding further with this thread. You have apparently no knowledge of physics whatsoever. We cannot teach you physics from the ground up in this thread, especially because you choose to argue with everything anyone tries to teach you that is in conflict with your totally uninformed misunderstanding. We cannot explain the principles behind the operation of a jet engine at anything other than the most basic level to someone who does not know the difference between acceleration and speed. These are layman’s terms!
I think that’s basically what I said. the key is the velocity of the noble gas, I understand it to be very high speeds? if that is the action the object will attain very high speeds over time
vyVY
Ok, stop right there. You are still laboring under your same impression, that a force moves an object at a specific speed. It doesn’t. A force applied to an object will accelerate that object. An acceleration is the rate at which an object changes speed over time. Saying that a force moves an object at a given speed is meaningless. At best, you can say that a force will accelerate an object to a given speed in a certain time.
For example, you might have an object X, and apply a force F to it that is sufficient to accelerate that object at 10 mph per second. If you apply that force for ten seconds, the object will reach a speed of 100mph. Now say you have a second object Y which weighs five times as much as X. If you apply the same force F to Y, it will accelerate at a rate one-fifth as much, or 2 mph per second. After ten seconds of having force F applied to Y, it will be traveling at 20mph. After 50 seconds, it will have reached 100mph.
You need to understand the basic concept of force and acceleration as a rate of change of speed over time to start with before you can understand this.
Try this: gravity is a force. Let’s say you have an object, a, that you place on a bathroom scale, which essentially measures the gravitational force between an object and the earth. It reads 100 lbs (of force). Let’s say that you have a different object, b, that you place on a different scale, and it reads 20 lbs (of force). Now, take object b and place it on top of object a. Now, the force of one combines with the force of the other, and the total gravitational force of the objects is 120 lbs. There’s no “matching” of forces or anything else. Forces can add up and do. And, in the case of these objects, the combined work they do in pushing down the springs in the scale is more than the work they do independently.
If you read all my posts you would see where in one I indicated That acceleration could be change in direction not velocity I will see if I can find it., I know this from my studies in mechanics. I was just trying to be clear on which was being used. read my post about the 10 mile long pipe is space and tell me where I have the wrong idea of what goes on in a jet engine
vyVY
While I’m not about to read through all your previous posts, you indict your study of mechanics with this one. Velocity has both speed AND direction. If you change direction, you change velocity.
Here’s where I said the part on acceleration. And if I had knowledge of physics as you put it, I would need The straight dope. I come in here for the straight dope, If i want overly scientific answers that I couldn’t grasp I would go to a physics forum, get the answers and probably leave just as confused, and end up in here asking for explanations of the explanation. So expect lots of questions, and questions on questions. If not go to the physics forum and answer physics students who understand when you drop an equation, or am I in the wrong place?
Quote:
Originally Posted by Richard Pearse View Post
The reverser buckets. In the sense that they are the last part of the aircraft interacting with the exhaust, and it is the buckets that are accelerating the exhaust from going backwards to going forwards.
virtually yours:As far as I remember, that’s all I ever said.. We are in COMPLETE agreement, if I assume by acceleration you mean change of direction, or increase in speed but not increase in power. I.e the same lbs of force coming towards the buckets cannot increase after it leaves.
YES!! a true layman explanation! But the issue was not only the object but the force itself i.e what about if force 2F comes into play on the same object?
Ok. tell me in this scenario where the force is, if it is applicable or not.
I am walking pushing a car along at five mph. me personally am only going 5 mph. so it’s no way that car can go faster than 5 mph.
If you put only the car in space and take away gravity from it and I am pushing it (not holding it) walking on earth I.e. just touching the bumper of the car, the car would still be going at 5mph even though it is in space and no gravity. it would not move away from me.
now say I go into space with the car, sit on the bumper and face backwards and blow a steady stream of air out of my mouth. let’s say a wind meter measures the speed of the air at 5 mph and I maintain this constant 5mph flow, Are you saying me and the car would speed up to almost light speed?
vyVY