Third law.Originally Posted by babygirl
Because as the force of the air is flying in one direction an equal force pushes the ballon forward.
You can also say Newton's Second law.
Before the blowing up of balloon, both are at rest. When air begins to escape out of the opening, the balloon moves in the other direction so that the total momentum of the system remains zero.
Actually, if you think over newton's laws, Newton's Second Law (Law of conservation of Momentum) is the basic law. The first and third law are just the special conditions which we observe frequently. With the statement of second law, you can arrive at the other two.
Ummm...Just to be precise here, the Law of Conservation of Momentum is a special case of Newton's Second Law when the net external force on the system is zero.Originally Posted by malaygoel
I usually get myself in trouble while teaching Newton's laws. I typically segregate the Law of Inertia (Objects in motion will stay in motion) and Newton's laws and state that the first law is a definition of what a force is. (unless acted upon by an external force) Then Newton's 2nd simply defines (inertial) mass. So the way I teach it there are really only two laws: The law of inertia and Newton's 3rd, and we get 2 extra definitions out of it.
Needless to say, no textbook I've found agrees with this approach. However, I've often found it a convenient method to approach the subject.
In my view, conservation laws are important tools for analyzing systems.And since forces always occur in pairs, we can always treat forces as internal forces.Originally Posted by topsquark
What are those two extra definitions?So the way I teach it there are really only two laws: The law of inertia and Newton's 3rd, and we get 2 extra definitions out of it.
For your first comment, it depends on what system we are referring to. Consider a book sitting motionless on a table. Yes, there are both a weight and a normal force acting on the book, and the sum of those forces is in fact zero. However the weight and normal forces are not "pairs" in the sense you are referring to; ie. involving Newton's 3rd law. (There is a force on the book by the Earth, thus there is an equal but oppositely directed force on the Earth by the book. This second force is has nothing to do with the normal force on the book.) In many situations in Physics we are dealing with only one of the pair of 3rd law forces.Originally Posted by malaygoel
Also note that neither the normal force on the book nor the weight can be considered to be "internal" to the book since they both originate from external sources.
The defintions we get from my teaching system are a definition of force (that which causes or changes the acceleration of an object) and the definition of mass (which is simply a measurement of inertia): . (We can measure the acceleration directly and the first law gives a method to measure the net force.)
By the way, I was not trying to discredit the importance of the Law of Conservation of Momentum. I was simply trying to put a bit of distance between it and Newton's 2nd. They are, in fact, somewhat different.
What you say is true except for two problems, one practical, one theoretical.Originally Posted by malaygoel
The practical reason is that if you give these (my) definitions on an exam you won't be quite correct according to the Professor. Your Professor will be looking for you to spit out your book's version of Newton's Laws. My interpretation of them are subtly different. In application the two conceptualizations give the same numbers so in working with them there's no big deal.
The theoretical reason is that I've wound up switching the base units for the system of Physics. Normally one would use something like the MKS system where mass in kg is one of the base units. By switching the Newton's 1st definition what I've done is essentially made the unit of force (N) to be the base unit. (The unit of mass now has an explicit and well-defined meaning given by Newton's 2nd.) This sets up an array of interesting problems to solve. For instance, in the usual setup for Newton's laws we have the question of whether inertial mass and gravitational mass (mass calculated using Newton's Law of Gravitation) are the same. In my system there is no longer a question of this...they must be the same. However, we have the (roughly equivalent) new problem: Do forces that arise from different sources have the same units?
Needless to say I don't bring the theoretical problem up in my Intro Physics class. I use my system primarily because I feel it gives me a much better way to make the distinction between the mass and weight of an object and stresses the Law of Inertia. Then I leave well enough alone! (Except in my personal research, of which this is a part.)