# Mechanics Kinematics and Newtons Laws by ewghwehws

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```									Mechanics
A force is a push or pull.

Contact Forces
Non-contact Forces
such as gravity,
magnetic, and
electric forces.
Newton’s First Law of Motion
An object continues in a state of rest or in
a state of motion at constant speed along
a straight line…
unless compelled to change that
state by a net force.
The mass of an
object is a
quantitative
measure of
inertia. The SI
Unit of mass is
kilogram (kg).
Immediately after I release the spring,
what does the bottom of the spring do?
1.         It begins to fall immediately at the
same rate as the rest of the
0%
spring.
2.         It begins to fall, but at a slower
rate than the top of the spring.
3.         It remains stationary.
4.         It moves upward as the top of the
spring moves downward.

1    2   3    4

1    2     3     4    5    6    7    8    9    10   11   12   13   14   15   16       17   18       19   20
21   22    23    24   25   26   27   28   29   30
Immediately after I release the spring,
what does the bottom of the spring do?
Newton’s Second Law of Motion
When a net external force F acts on
an object of mass m, the acceleration
a that results is directly proportional to
the net force and has a magnitude

F
that is inversely proportional to the
mass.
The direction of the acceleration is
a
the same as the direction of the net
m
force.

SI Unit of Force: kg • m/s2 = newton (N)
Individual Forces                 Net Force

4N           10 N                             6N

The net force on an object is the
vector sum of all forces acting on
that object.
m = 1850 kg

 F  + 275 N + 395 N  560 N = +110 N
a
 F   110 N  0.059 m/s  2

m          1850 kg
If the airplane’s mass is 13 300 kg, what
is the magnitude of the net force that the
catapult and jet engine exert on the
plane?
 F  ma  (13 300 kg)(31m/s )
2

= 4.1105 N
The Normal Force
The Normal Force & Friction
Static Frictional Force

The magnitude of the static frictional force can have
any value from zero up to some maximum value, fsmax,
depending on the applied force.
In equation form, we write:
fs  f   s
m ax

The equality holds only when fs attains its maximum
value, which is
f   s
m ax
  s FN
where µs is the coefficient of static friction and FN
is the magnitude of the normal force.
The Normal Force & Friction
Static Frictional Force - Example

The Force Needed to Start a Sled
Moving

A sled is resting on a horizontal patch of
snow and the coefficient of static friction is
0.350. The sled and its rider have a total
mass of 38.0 kg. Determine the horizontal        fs       F
force needed to start the sled barely
moving.

f   s
max
  s FN   s mg
 (0.350)(38.0 kg)(9.80 m/s )            2

 130 N
Kinetic Frictional Force

The magnitude of the kinetic frictional force is given by

f k   k FN
where k is the coefficient of kinetic friction and FN is the
magnitude of the normal force.
Kinetic Frictional Force - Example

Sled Riding

A sled is traveling at 4.00 m/s along a horizontal stretch of snow. The
coefficient of kinetic friction is 0.0500. How far does the sled go before
stopping?
f k   k FN   k mg       ma
v v
2    2
v v  2    2        a   k g
x      0            0
2a   2( k g )
 (4.00 m/s)      2
                         16.3 m
 2(0.0500)(9.80 m/s )
2
Newton’s Third Law of Motion
Whenever a body
exerts a force on a
second body, the
second body exerts
an oppositely
directed force of
equal magnitude
on the first body.
If an astronaut pushes on the   92 kg
spacecraft with a force P = +36
N, then, according to Newton’s
third law of motion, the
spacecraft simultaneously
pushes back on the astronaut                  11 000 kg
with a force –P.

Acceleration of the spacecraft
P    36 N
as               0.0033 m/s 2
ms 11 000 kg

Acceleration of the astronaut
 P  36 N
aA             0.39 m/s 2
mA   92 kg
Second Law: object tends to remain force or in
Third Law: when one body exerts a force on another
Newton’s First Law: an a non-zero net externalat reston an object
produces second body exerts a force equal in magnitude, but
motion at an acceleration onunless a net proportional to the net
body, the constant velocity that object force acts on that object
oppositely directed on the first body
force and inversely proportional to its mass.

FN           F     y    FN  mg  0
F
F
F     x    F  ma

W = mg
Newton’s Second Law: a non-zero net external force on an object
First Law: when one body exerts a at rest or
Newton’s Third Law:an object tends to remain force on in
produces body, the second that object proportional to the net
another an acceleration on body exerts a force acts on
motion at constant velocity unless a net force equal in that
force and inversely proportional to its mass.
magnitude, but oppositely directed on the first body
object

FN         F   y    FN  mg  0
fS          F
F
F     x    F  fS  0
f

W = mg

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