Transcript the law of motion

THE LAW OF MOTION
• Force: a “push or pull” of an
object
• Force is a vector quantity
(we can control the
magnitude and direction)
• Contact forces- they result
from physical contact
between two objects (ex.)
• Field forces
- Force between subatomic particles
- The electromagnetic forces between
electric charges
- The weak nuclear force , which
arises in certain radioactive decay
process
- The gravitational force between two
forces (explain)
• Newton First Law of motion (LAW OF
INERTIA)
An object moves with a velocity that is
constant in magnitude and direction unless
acted by a nonzero net force
(every body continues its state of rest or of uniform
speed in a straight line unless acted on by a
nonzero net force)
The net force on an object is defined as the vector
sum of all external forces.
The tendency of an object to continue in its original
state of motion is called INERTIA
• MASS –is the measure of
the object’s resistance to
change in its motion due
to a force (is the quantity
of matter)
• Mass – measure of the
inertia of a body
• Inertia is used in seatbelt
mechanism.
In an accident, the car undergoes a
large acceleration, and come to rest.
Because of inertia, the block
continue to slide, the pin causes the
rod to pivot about its center and
engage the ratchet wheel
• Newton Second Law of Motion
The acceleration a of an object is directly
proportional to the net force acting on
it and inversely proportional to its mass
a =Σ F/m
ΣF=ma
SI unit of force: NEWTON 1N= 1kg m/s2
(1N= 0.225 lb; 1lb= slug ft/s2)
When 1N force acts on an object that has
mass of 1kg, it produce an acceleration of
1m/s2
• Gravitational Force is the mutual force of
attraction between any two objects in the
Universe
• Newton’s Law of Universal Gravitation :
every particle in the Universe attracts every
other particle with a fore that is directly
proportional to the product of the masses of
the particles and inversely proportional to
the square of the distance between
Fg = G (m1m2/ r2)
G= 6,67 x 10 -11 N m2/kg2
• WEIGHT: the magnitude of the
gravitational force acting on an object
of mass m near Earth’s surface
w = mg
SI unit: N
• Newton's Third Law
If an object a and object 2 interact, the
force F12 exerted by the object 1 on object
2 is equal in magnitude but opposite in
direction to the force F21 exerted by the
object 2 on object 1
The action force is equal in magnitude to the
reaction force and opposite direction
( push your hand on the edge of the desk,
what you will observe?)
• The Earth exerts a force Fg on any object,
if the object is a book rest on a desk, the
reaction force to Fg is the force the book
exerts on the Earth Fg’. The desk exerts
an upward force n called normal force
(contact force)
• The normal force is an elastic force
arising from the cohesion of matter and is
electromagnetic in origin.
• The reaction to n is the force exerted by
the book on the desk n’ (draw the sketch)
• Forces on friction
An object moving on a surface or through a
viscous medium, encounters resistance as
it interacts with its surroundings. This
resistance is called friction.
Force of static friction fs - the force that
contracts F and keeps object from moving,
opposite direction of F
If the object isn’t moving fs = - F
If F increase, fs increases
• I f we increase F, we call the friction force
for an object in motion force of kinetic
friction fk
• If F = fk, the acceleration is zero, the object
is moving with constant speed
• Experimentally, both fk and fsmax for an
object on a surface are proportional with
the normal force exerted by the surface on
the object
- The magnitude of the force of static
friction, between any two forces on contact
can have the values:
fs ≤ μs n
μs– coefficient of static friction;
n- magnitude of the normal force by one
surface to another
fs =fs max= μs when the object is on the
verge of slipping.
This situation is called impending motion
- The magnitude of fk acting between two
forces is:
fk = μk n
μk -is the coefficient of kinetic friction
- μk and μs depend of the nature of the
surface (μk< μs )
-the direction of friction force is opposite the
actual motion
- The coefficients of friction are nearly
independent of the area of contact
between the surfaces
• Applications on Newton’s Laws
• Tension-the magnitude of the force exerted
along a rope
-If the rope has mass m,, Then Newton’s secon
Law applied to the rope gives T-T’ = ma
-If the mass is taken negligible, then T=T’
• Consider a crate beeing
pulled to the right on a
frictionless, horizontal
surface
- When we apply
Newton's law on an
object, we are
interested only in those
forces which act on the
object.
- n, Fg, T
- The diagram is called
free-body diagram
• The reactions:
- the force exerted by rope on the
hand doing the pooling
-the force exerted by the crate on
Earth
- the force exerted by the crate on the
floor
aren’t included in free body diagram,
because they act on other objects, not on
the crtate
• Newton’s IInd Law applied on the crate in
the x and y direction :
max= T
may = n-mg = 0; n= mg
Objects in equilibrium- objects that are
either at rest or moving with a constant
velocity
a= 0
ΣF = 0 ; ΣFx = 0; ΣFy=0
• Problem-solving strategy
1. Read the pb
2. Draw a picture of the system, indicate
forces with arrows
3. Label each force
4. Draw a free-body diagram of the object
of interest
5. Apply Newton’s IInd Law (x and y
components)
6. Solve