newton's law

[coffeechocolat]

I don't understand about newton's laws( first law , second and third law). And i can not apply them to make Free body diagram.
I don't get what is the force act on an object. please help me

[Chris-T]

I don't remember which one is which about these 3 very "important"
Newton's Law of the traditional/classical Physics. Could be some
missing or mixed-up below (please reply w/ correct Law if you can,
I need to refresh my memory also).
1st Law: An object is stationary (not moving), it will remain
"stationary" forever (if No force applied to it)
2nd Law: An object is moving at "constant" velocity, it will remain
moving at constant velocity forever (if No force applied to it, and of
course only in free space).
3rd Law: If there is a Force "F" acting (applying) to an object, that
object will be moving at an acceleration of "a", and eq. F=ma hold
where m is the mass of the object.

Your second question asked about the application in solving problem
with Newtonian's Laws. In most of these problems, they ussually
involve 1 or more Forces "act on an object" & you need to draw
the free body diagram such that you can visualize & solve the problem
correctly. I agreed that is somewhat pretty "abstract" to visuallize
first then draw (once you can draw them out, it become easier for
you to solve using Vector sumations along with F=ma). Something to
note/remember are:
(1) Forces are Vector (must include direction, arrow)
(2) Accelerations a (and velocity) are vector.
When applying theories & eq into drawing ==> (since it's abstract)
try to make believe that "you" yourself are the "object". It would
help you visualize How the Force act on you (object). Ex: assume you
(object) is stand still, and something w/ Force F act on you from the
Left ==> you would draw an object (a box? as you) and an vector
Force F pointing to you/box from the Left. More vectors/Forces can be
applied at the same time (in this 1 Force case, you/object would move
to the Right at a=F/m).... Hope you have some picture. If there's any
specific problems, post it up someone here/VCB would help you out.
Good Luck :)

[CAptain DaLaZBoi]

YAH.. i hate this physic thing just failed my first midterm :(

[yeudoi]

-----------
coffeechocolat wrote:
...
I don't get what is the force act on an object. please help me.
------------

Force acting on the object mean thats. To applied a force to an object. For example:

1. I kick a soccer ball, this means I that applied a kicking force to the soccer ball to make it move.

2. Another example to consider here. Assume my car cannot started therefore i will push it to make it start. What I did there was applied a pushing force to my car to make it start.

OK. What is an object here. From example 1 an object was a soccer ball and in 2 an object was the car.

Physic is not hard if you consider it like math.

Practice makes perfect.

[Huyenthoai99]

Forces: Units; either Newton "N = kg*m/s^2) or lb and 1lb = 4.45N


Frictional force: This force always restraints the motion of the object
example: if you push a box on the floor due East.
Free Body Diagram "FDB"
F ( applied by you ) ------>[ Box ]<-------- F ( frictional force between the box and the floor F= e*m*g; where e= coefficient fo friction, g= gravitational acceleration, m = mass of the box)

Normal Force : This force usually acts in the upward direction
Example: You place a box on the floor. The forces act on the Box are Gravity ( F1= mg, pulling down) and the Normal force ( F2 = F1 but opposite direction with F1)

Shear Force: This force acts in parallel between the surfaces of the object internally ( This force you will mostly see in Mechanics & Dynamics motion )

External Force ( I mean force applied by us on the object and this force is usually given ) : F
example: How much force in term of Newton unit we apply to push the box across the floor etc..

Gravitational Force between 2 particles with masses m1 and m2 separated by a distance r:
F = G* m1 * m2/ r^2; where G = gravitational constant = 6.67*10^(-11)
Example: This is obvious if considering the Earth and ourselves


Electrostatic Force: I think this is beyond the limit..but I just want to illustrate the concept of the Forcesssss
This is the force acting between 2 or many electric charges
Example: If we have 2 opposite electric charges; Q+ and Q-, the force acting on Q+ as well as on Q- is:
F = absolute of (Q+ time Q-)/(4* pi * e * r^2); where e= permittivity constant = 8.85* 10^(-12) C^2/N*m^2, r= distance between 2 charges, Q = 1.6* 10^(-19)Coulomb
FBD: Q+------------> <-------------- Q- (extract each other)
Q+ <---------- ---------------> Q+ ( Repel each other, 2 charges have the same sign will repel each other)

[Huyenthoai99]

Ah Còn 1 Force nữa mà HT quên.... nhưng mà bạn nào chưa có nguoi iêu giống như HT thì có lẻ chưa cảm nhận được đâu... :love: :love: mà hỏng cảm nhận đuoc thì làm sao biết để nói fải hỏng neeeeeeeeè......:15: :15:

[Chris-T]

Come on, go on - Tha^`y HT, list them all out (j/k). Anyway, HT,
you do still have very good memory :cool: Khâm phục. Thanks

[coffeechocolat]

thanks, i don't know why ii'm good in math but in physic huuuuuuuuuuuuuuu ,

[yeudoi]

I'll give you an example of Newton's 3rd law.

Third law says that ' for every action there is a reaction'.

Assume you're standing, you'll have the gravitational force of mg acting 'down' on the earth surface. The earth will have an oppose force mg acting 'up' on you.