Transcript Lecture 2

Chapter 2, Kinematics
Terminology
• Mechanics = Study of objects in motion.
– 2 parts to mechanics.
• Kinematics = Description of HOW objects move.
– Chapters 2 & 4 (Ch. 3 is mostly math!)
• Dynamics = WHY objects move.
– Introduction of the concept of FORCE.
– Causes of motion, Newton’s Laws
– Most of the course after Chapter 4
For a while, assume ideal point masses (no physical size).
Later, extended objects with size.
Brief Overview of the Course
• Translational Motion = Straight line motion.
– Chapters 2,4,5,9,13
• Rotational Motion = Moving (rotating) in a circle.
– Chapters 6,10,11,12
• Oscillations = Moving (vibrating) back & forth in same path.
– Chapter 15
Continuous Media
• Waves, Sound
– Chapters 16,17,18
• Fluids = Liquids & Gases
THE THEME OF
THE COURSE IS
NEWTON’S LAWS
OF MOTION!!
– Chapter 14
Conservation Laws: Energy, Momentum, Angular Momentum
– Just Newton’s Laws expressed in other forms!
Motion in One Dimension
Sects. 1-4: Outline
1. Displacement:
x  x f  xi
2. Average velocity:
x
v
t
2. Time interval:
3. Instantaneous
velocity:
t  t f  ti
v  Limt 0
x dx

t dt
4. Speed is the absolute value of the velocity
5. Average
acceleration:
a
v f  vi
t
6. Instantaneous
acceleration:
a  Limt 0
v dv

t dt
Reference Frames, Coordinate Systems &
Displacement
• Every measurement must be made with
respect to a reference frame.
• Usually, speed is relative to the Earth.
Coordinate Axes
• Define reference frame using a standard
coordinate axes.
• 2 Dimensions (x,y)
• Note, if its convenient,
could reverse + & - !
Coordinate Axes
• 3 Dimensions (x,y,z)
• Define direction using these.
Displacement & Distance
• Distance traveled by an object
 displacement of the object!
• Displacement = change in position of object.
• Displacement is a vector (magnitude & direction).
Distance is a scalar (magnitude).
• Figure: distance = 100 m, displacement = 40 m East
Displacement
x1 = 10 m, x2 = 30 m
Displacement  ∆x = x2 - x1 = 20 m
∆  Greek letter “delta” meaning “change in”
x1 = 30 m, x2 = 10 m
Displacement  ∆x = x2 - x1 = - 20 m
Displacement is a VECTOR
Vectors and Scalars
• Many quantities in physics, like
displacement, have a magnitude and a
direction. Such quantities are called
VECTORS.
– Other quantities which are vectors: velocity,
acceleration, force, momentum, ...
• Many quantities in physics, like distance,
have a magnitude only. Such quantities are
called SCALARS.
– Other quantities which are scalars: speed,
temperature, mass, volume, ...
• The Text uses BOLD letters to denote
vectors.
• I usually denote vectors with arrows over
the symbol.
• In one dimension, we can drop the arrow
and remember that a + sign means the
vector points to right & a minus sign means
the vector points to left.
Average Velocity
Speed  (Distance traveled)/(Time taken)
Vector→Average Velocity  (Displacement)/(Time taken)
• Velocity: Both magnitude & direction
describing how fast an object is moving. A
VECTOR. (Similar to displacement).
• Speed: Magnitude only describing how fast an
object is moving. A SCALAR. (Similar to distance).
• Units: distance/time = m/s
Scalar→Average
Average Velocity, Average Speed
• Displacement from before. Walk for 70 s.
• Average Speed = (100 m)/(70 s) = 1.4 m/s
• Average velocity = (40 m)/(70 s) = 0.57 m/s
• In general:
∆x = x2 - x1 = displacement
∆t = t2 - t1 = elapsed time
Average Velocity:
(∆x)/(∆t) = (x2 - x1)/(t2 - t1)
Example
• Person runs from x1 = 50.0 m to x2 = 30.5 m
in ∆t = 3.0 s. ∆x = -19.5 m
Average velocity =
(∆x)/(∆t)
= -(19.5 m)/(3.0 s) = -6.5 m/s. Negative sign
indicates DIRECTION, (negative x direction)
Instantaneous velocity
 velocity at any instant of time
 average velocity over an
infinitesimally short time
• Mathematically, instantaneous velocity:
v = lim ∆t  0 [(∆x)/(∆t)] ≡ (dx/dt)
lim ∆t  0  ratio (∆x)/(∆t) considered as a whole for smaller & smaller
∆t. As you should know, mathematicians call this a derivative.

Instantaneous velocity
v ≡ time derivative of displacement x
Acceleration
• Velocity can change with time. An object
whose velocity is changing with time is said
to be accelerating.
• Definition: Average acceleration = ratio of
change in velocity to elapsed time.
a  (∆v)/(∆t) = (v2 - v1)/(t2 - t1)
– Acceleration is a vector.
• Instantaneous acceleration
a = lim ∆t  0 [(∆v)/(∆t)]
• Units: velocity/time = distance/(time)2 = m/s2
Example
Conceptual Question
1. Velocity & acceleration are both vectors.
Are the velocity and the acceleration
always in the same direction?
NO!!
If the object is slowing down, the
acceleration vector is in the opposite
direction of the velocity vector!
Example
Deceleration
• “Deceleration”: A word meaning “slowing down”. We
try to avoid using it in physics. Instead (in one dimen.)
talk about positive & negative acceleration.
• This is because (for one dimen. motion) deceleration
does not necessarily mean the acceleration is negative!
Conceptual Question
2. Velocity & acceleration are vectors.
Is it possible for an object to have a zero
acceleration and a non-zero velocity?
YES!!
If the object is moving at a constant
velocity, the acceleration vector is zero!
Conceptual Question
3. Velocity & acceleration are vectors.
Is it possible for an object to have a zero
velocity and a non-zero acceleration?
YES!!
If the object is instantaneously at rest (v = 0) but
is either on the verge of starting to move or is
turning around & changing direction, the
velocity is zero, but the acceleration is not!
Examples
Example 2.1: Calculating Average Velocity & Speed, p. 28
Problem: Use the figure & table to find the displacement & the average
velocity of the car between positions (A) and (F).
Example 2.3: Average & Instantaneous Velocity, p. 25
Problem: A particle moves along the x axis. Its x coordinate varies with time as
x = -4t + 2t2. x is in meters & t is in seconds. The position-time graph for this
motion is in the figure.
a) Determine the displacement
of the particle in the time
intervals t = 0 to t = 1 s &
t = 1 s to t = 3 s (A to B & B to C)
b) Calculate the average velocity
in the time intervals t = 0 to
t = 1 s & t = 1 s to t = 3 s
(A to B & B to C)
c) Calculate the instantaneous
velocity of the particle at
t = 2.5 s (point C) .
Example 2.4: Graphical Relationships between x, vx (v) & ax (a)
Problem: The position of an object moving along the x axis varies with time as in the
figure. Graph the velocity versus time and acceleration versus time curves for the object.
Example 2.5, Average & Instantaneous Acceleration, p. 30
Problem: The velocity of a particle moving along the x axis varies in time
according to the expression v = (44 - 10t 2), where t is in seconds.
a) Find the average acceleration
in the time interval
t = 0 to t = 2.0 s.
b) Find the acceleration at t = 2.0 s.
Ch. 2, Sects. 1-4: Motion in One Dimension
Summary
1. Displacement:
x  x f  xi
2. Average velocity:
x
v
t
2. Time interval:
3. Instantaneous
velocity:
t  t f  ti
v  Limt 0
x dx

t dt
4. Speed is the absolute value of the velocity
5. Average
acceleration:
a
v f  vi
t
6. Instantaneous
acceleration:
a  Limt 0
v dv

t dt