# Feel free to smack me accross the head for asking this, but...

I don’t quite get what a newton of force does.

The definition is such: 1 N is the force that, when applied to a mass of 1 kg, produces an acceleration of 1 m/s^2.

My problem is lies with the acceleration bit. To me, it seems like this definition is telling me that if I apply 1 N of net force to a 1 kg object, I will set it in motion and it will continue to accelerate forever at 1 m/s^2. That means, it will continue to travel faster and faster as long as I keep that 1 N of net force pushing the object.

But the thing is, if I push this 1 kg book that’s in front of me right now with a force of 1 N, what happens is (as far as I can tell) that the book will accelerate until it reaches a certain velocity and then it will continue to travel at that constant velocity as long as I keep pushing with 1 N of force. Constant velocity = zero acceleration… so what gives?
An explanation I’ve been toying with is that the acceleration of the object will slowly decrease as the object travels faster. Initially, the acceleration is 1 m/s^2, but because the object travels faster, more force is required to move the object, and thus acceleration begins to decrease. Is this a correct assumption? Or am I way off base? If this assumption is correct, could someone explain it further, because I don’t really trust my own explanation.

Thanks.

The reason that your book doesn’t keep accelerating is because there is a force that opposes your applied force. That is friction. If there was no friction, then it would continue to accelerate if you continued to apply the force and it would maintain a constant velocity when you stopped pushing.

Ignoring relativistic effects and friction, as long as you are pushing an object you are increasing its speed, ie accelerating it. In the example you gave, the reason your book is maintaining a constant speed is because you’ve balanced the force of your pushing with the friction from the table. If you had a frictionless table, the book would maintain a constant speed without being pushed and any additional pushing would increase its speed. In an ideal state, the energy required to change from 0 to 10 mph is equal to that required to change from 100 to 110 mph.

[quote]
The reason that your book doesn’t keep accelerating is because there is a force that opposes your applied force. That is friction. If there was no friction, then it would continue to accelerate if you continued to apply the force and it would maintain a constant velocity when you stopped pushing.[/quote}

Well, I can’t see friction makes any difference at all. First of all, I was speaking in terms of NET force, meaning, after all the vectorial quantities have been added up, the resultant is 1 N of force in a certain direction. Besides, if I push my book accross the icy walk outside and then accross the thick carpet in my living room, the final velocity is the same. If it’s the same, I would think friction does not play a factor (considering the fact I’m speaking of NET force, and friction has already been accounted for… has it not?)

Oops. The paragraph break in the above post, is where the quote should have ended.

The final velocity of a book on ice or on carpet may end up the same, but it takes more force to achieve it on the carpet. You may not consciously realize it, but you’re pushing the book harder.

You’re not pushing it with a constant force. Your arm has a certain maximum velocity (you can’t move your arm with infinite speed), and once the book reaches that speed, you’re not exerting any force other than that needed to counteract friction. Get a spring and push the spring against the object. You’ll see that the spring is compressed the most when the object first starts moving.

Another problem, Arthur, is that as the velocity of the book increases, the effect of friction will increase. Unfortunately for your average home experiment, this effect is insignificant until well beyond the speed you can push the book.

As for pushing a book on a table, try buying a little device that, using a spring, can measure force. Attach it to a block and pull with a net force of 1 N. You’ll see that the book is accelerating.

If you had no friction or relativistic effects, and you could, in fact, pull the block faster and faster (that is, if a theoretical force could be applied continuously forever), why would it not continue to accelerate forever?

I think the key would be buying the doohickey (can’t remember what it’s called) and pull at what you can be SURE is a constant force.

Is “scale” the word you’re looking for, there?

[QUOTE]
*Originally posted by Arthur *
**

The net force, when you add it all up, is zero. When you are exerting a force to maintain a constant velocity the force of friction and on your arm exactly cancel. Try pushing the book really hard and try to maintain that high degree of force. The book will accelerate because it will have a net force acting on it.

This is usually demonstrated in physics labs with an air track so as to minimize friction. If you apply a constant force to car on the track it accelerate. I’ve seen and done it myself. It is a very real effect.

You are right about this part, but it has nothing to do with friction. According to relativity, the closer an object gets to lightspeed, the more massive it becomes and therefore the more force is required to get the same acceleration. This effect is only noticable at something like >70% lightspeed, which I don’t think you’ll be able to acheive by throwing your book around.* It has been observed in particle accelerators though. I don’t know the equations, but the mass of, say, an electron increases exponentially as it approaches lightspeed. The best they’ve been able to do is about 99.999%, and the mass is measureable in grams instead of the usual kilo-electron volts.