Inertia and Newton st Law

							                                           2/6/2011




            Inertia and Newton’s 1st
                      Law




                                       1




What will we learn?
Inertia and Newton’s 1st Law
Read Chapter 2




                                       2
topic(s): inertia, center-of-mass




                                                 1
                                                            2/6/2011




Question
If you were to place dishes on a tablecloth and
                    tablecloth,
quickly remove the tablecloth what would happen
to the dishes? Why?




                                                        3
topics: inertia




Inertia
is the tendency of an object to resist changes to its
                    object s
velocity; it is the object’s resistance to forces
impressed upon it.




                                                        4
topics: inertia




                                                                  2
                                                                     2/6/2011




Mass
is a measure of an object’s inertia; it is a scalar
                                     object s
quantity that is independent of the object’s
position.




The SI unit of mass is the kilogram (kg).




                                                              5
topics: inertia




A force . . .
• is a single interaction between two objects.
                   p         p              j
• acts as either a push or a pull on each object.
• is equal in magnitude, but opposite in direction on each object.
• acts in a straight line on each object.
• is a vector.
The SI unit of force is the Newton (N): 1 N = 1 kg m/s2




                                                              6




                                                                           3
                                                            2/6/2011




Newton’s 1st Law
Newton first quantified the idea of force, inertia,
     q
and equilibrium in his 1st Law of Motion:

1) A body will remain in a state of equilibrium until
   acted upon by an applied net external force.

In other words, a body’s velocity cannot change
                                         it.
without an external net force to change it




                                                        7
topics: Newton’s 1st Law




Mechanical Equilibrium
A body is said to be in a
state of mechanical                     ∑F = 0
equilibrium when the net
force acting on it is zero.               a=0

Static equilibrium is the state when a body is at
rest, while dynamic equilibrium is the state when
                                 velocity.
the body is moving at constant velocity
Therefore, a body is in equilibrium when it is not
accelerating.


                                                        8




                                                                  4
                                                                           2/6/2011




Exercise
Identify the magnitude and direction of the net
force in each of the following situations. Which
b          in    ilib i ?      l i
boxes are i equilibrium? Explain.



                                            5N
      3N             6N
 a)                                                          5N
                                b)


      23 N        20 N               43 N             25 N
 c)
                                d)
                                                        18 N
                                                                  9
                8N




A force diagram,
(also called “free-body” diagram), is a diagram that
represents each force that acts on a single body.
                                                 Fy

The center-of-mass of the
body is represented as a
particle at the origin of the
                                     FdW
diagram.                                                              Fx
Each force vector is drawn
with its tail placed at the
origin.                                           FdS

                                                               10




                                                                                 5
                                                             2/6/2011




Exercise
An airplane is in level flight at a speed of 100 km/h and
headed east. A strong (westward) headwind imparts a
horizontal d
h i              force of 700 N onto th airplane. Th
        t l drag f       f          t the i l       The
airplane’s wings generate an upward lifting force of 9600
N. If the airplane has a mass of 1000 kg,
a) what is the magnitude and direction of the force that
    the airplane’s engine must produce in order to
    maintain level flight at 100 km/h into the wind?
b) Is the airplane in equilibrium with that force?
    Explain.



                                                        11




Newton’s 2nd Law
Newton quantified the idea of force, inertia, and
acceleration in his 2nd Law of Motion:

2) An object will accelerate in the direction of the
   net external force applied to it. That is, it will
   change its velocity only in the direction of the
   net external force that causes it.



                           a=
                                ∑F
                                m

                                                        12
topics: Newton’s 2nd Law




                                                                   6
                                                                   2/6/2011




Example
You use a rope to pull a heavy block across a horizontal
rough surface. Friction exerts a force on the block of
10 N west; gravity pulls down on the block with 20 N of
force; the ground pushes up on the block with 3.0 N of
force; the rope pulls with 20.0 N of force eastward at
60° above the horizontal.

a) Draw the block’s free-body diagram.
         Newton s
b) Write Newton’s 2nd Law for the block in component
   form.
c) What is the block’s acceleration?


                                                              13




Example
A sailboat finds itself in a strong eastward ocean current
                                           g
that exerts 5000 N of force on it at an angle of 53.1°
north of east. A gust of wind blowing from the south
exerts 4000 N of force on the boat’s sails. Is the sailboat
in equilibrium? If not, what is its acceleration?




                                                              14




                                                                         7
                                                             2/6/2011




Points to Remember!
Newton was the first to quantify the concept of force
in his Laws of motion:

1) A body will remain in a state of equilibrium until
   acted upon by an applied net external force.

2) An object will accelerate in the direction of the
                                  it.
   net external force applied to it

3) The force of interaction between two bodies is
   always equal in magnitude and opposite in
   direction.
                                                        15




                                                                   8