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a physical quantity
Physics is the study of the
material world and the rules
that govern its behavior.
time
• It is based on observation of nature.
• It identifies discernible patterns in nature.
• It allows one to make predictions.
However, it does not explain nature.
It only describes nature, and with limited accuracy.
Questions to think about
• What is the “position” of an object
• What is “time”
• What do we mean by “movement”
Over here !!!
center of mass
A configuration of selected points associated with
an object with respect to other objects in space is
called the position of the object.
Time is a quantity associated
with changes in the universe.
A repetitive process (a clock) is used
to assign a value to each configuration
of the universe (instant).
The second has been defined
(since 1967) as the duration of
9,192,631,770 oscillations of a
particular radiation emitted from
133Cs atoms.
Physics Laboratory, NIST
Boulder, Colorado
René Descartes and the fly
z
z
θ
r
y
René Descartes (1596–1650)
philosopher, mathematician
and physicist
x
ϕ
x
y
Cartesian Coordinates of position:
( x, y, z )
Polar Coordinates of position:
( r, θ, ϕ)
Position is a three dimensional vector quantity.
Movement
y
If the position of an
object changes with
time, the objects moves.
(The object is in motion).
x
z
Comment 1
The motion of an object is relative. Objects always move with
respect to other objects (reference frame).
Comment 2
A universal stationary reference frame does not exist!
Speed and Velocity
0s
Speed (of an object) is
defined as the distance
traveled by the object
(along a path) in an
infinitesimal time interval
divided by the time.
y
2 cm
0.1 s
0.2 s
0.3 s
0.4 s
0.5 s
0.6 s
x
z
Velocity is defined as the
rate at which an object
changes its position. It
contains
information
about the speed of the
object and the direction of
its motion.
speed =
dis tan ce 2cm
cm
=
= 20
time
0.1s
s
velocity = 20
cm
parallel to x - axis
s
Velocity and Position
Velocity of an object makes it possible to predict its future
position.
500 m
100 m/s
750 m
150 m/s
What will be the position of the
plane in five seconds?
Where is the plane going to be in the next
five seconds after changing the velocity to
150 m/s while maintaining its direction?
The Average Value of a (varying) physical quantity is
such a constant value of that quantity that in a considered
amount of time the effect of the average value is equal to
the effect of the varying values.
Example:
In five seconds, a fly completes a circle with a circumference of 100 cm.
What was the average speed of the fly?
1m = v av ⋅ 5s
v av = 20 cm
s
What was its average velocity?
0m = v av ⋅ 5s
v av = 0
m
s
Newton’s First Law of Motion
It is possible to find such a reference frame, that an
object free of external influences moves with constant
velocity or remains at rest.
Such a reference frame is called an
inertial reference frame.
In many practical situations, the
reference frame of the earth can be
considered to be inertial.
The tendency of an object to maintain its
state of motion (in an inertial reference
frame) is called inertia.
v
earth pulls
ice pushes
Newton’s Apple
Sir Isaac Newton (1642-1727)
- English scientist; invented the
mathematical basis of calculus;
formulated the laws of motion;
described gravitational interaction
Sir Isaac Newton (1642 - 1727)
inertia puzzle
A rotary lawn mower spins its blade rapidly over the lawn
and cuts the top of the grass off. Would the blade still cut
the blades of grass if they weren’t attached to the ground?
It would! In fact, when the clippings are not collected,
the blade cuts (mulches) them into smaller pieces.
interactions
The velocity (in an inertial reference frame) of an object can
change only by interaction with other objects.
Several physical quantities (such as force, work, power,
torque) are used to describe interactions.
Changing Velocity
When an object changes its velocity it
accelerates.
The object accelerates when
its speed is increasing or
decreasing.
The objects accelerates
when the direction of its
velocity is changing.
Acceleration
The rate at which the velocity of an object varies is called
the acceleration of the object.
The acceleration of an object makes it possible to
predict the object’s future velocity.
4 m/s
Example.
At 1 s: velocity = 4 m/s to the left
4 m/s
acceleration = 10 m/s2 down
2 m/s
change in velocity in 0.2 s
= 10m/s2 ⋅ 0.2s down = 2m/s down
At 1.2 s:
the velocity of the ball is 4 m/s to the
left and 2 m/s downwards
4 m/s
Puzzle: What will be the
velocity of the ball at 1.4s?
1.2 s
1.4 s
4 m/s
1s
10 m/s2
10 m/s2
May the force be with you!
Force is a measure of interaction between objects.
• Force can only be exerted on an object.
• Only objects can exert a force.
• Force is always associated with a type of interaction.
Example:
W - the earth exerts a gravitational force on the ball.
N
w
N - the floor exerts a support force on the ball.
Force and Acceleration
small force
USA
USA
Lucas Oil
Lucas Oil
large force
If an unbalanced force is exerted on an object, the
stronger the force the greater the object’s acceleration.
Mass is an attribute of an object
describing the object’s inertia.
It is easy to accelerate a
baseball. It has a small mass.
It is difficult to accelerate a bowling
ball. It has a large mass.
Newton’s Second Law of Motion
Fnet = m ⋅ a
The resultant (net) force exerted on an object is equal to
the product of that object’s mass and its acceleration
a
N1
N2
W
W
a
Measure for Measure
A measurement is a procedure in which a numerical
value is assigned to a physical quantity.
one three
, two
four
In every measurement, the given physical quantity is
being compared to a standard.
SI Units
Systéme Internationale d’Unités
SI units are worldwide the most accepted system of
units
• based on powers of ten (decimal)
• constructed on a few basic units (the meter,
the kilogram, and the second)
1 kg