COLLEGE PHYSICS Yuxiang Li ( [email protected] ) Tel : 88391081 - 8213 Office: Room 213, Building 3B , Software Park Campus
COLLEGE PHYSICS
Yuxiang Li ([email protected])
Tel: 88391081-8213
Office: Room 213, Building 3B, Software
Park Campus
Modular Credit: 2.5 MCs
Course Time: 9-10/Tue., 9-10/Wed for Lect.
5-8 Fri. 25 Dec, for Exp.
Assessment: 10% on Attendance
20% on Homework + Pop Quiz
20% on Experiments
50% on Final examination
1. Have you studied displacement, velocity,
acceleration, and motion in one dimension?
2. Have you studied vectors and two-
dimensional motion?
3. Have you studied force (including frictional
force), conservative forces, nonconservative
forces, and Newton’s Law?
4. Have you studied work, power, kinetic energy,
potential energy, work-energy theorem,
energy conservation?
Course Syllabus
1. Introduction to Physics
Introduction to Physics, review of classical
mechanics
2. Solids and Fluids (Chapter 9)
Fluids in motion, applications of fluid
dynamics, surface tension
3. Vibration and Waves (Chapter 13)
Simple harmonic motion, wave motion,
Interference of waves
5. Electric Forces and Electric Fields (Chapter 15)
Electric charges, Coulomb’s law, the electric
field, electric flux and Gauss’s law
4. Sound (Chapter 14)
Sound waves, Doppler effect, interference of
sound wave, quality of sound, and ear
Textbook
1. “College Physics”, 9th Ed. Raymond A. Serway &
Chris Vuille, Brooks/Cole, Cengage Learning, 2012.
Other References
1. “College Physics”, 2nd Ed. Paul Peter Urone,
Thomson, 2002
2. “University Physics I & II”, 1st Ed. Ronald Lane Reese,
Thomson, 2002
3. “Conceptual Physics”, 11th Ed. Paul G. Hewitt,
Addison-Wesley, 2010.
Review of Classical Physics
• Quantities
• Vectors
• Mass and weight
• Distance and displacement
• Speed and velocity
• Acceleration
• Force, work and energy
• Kinematic and dynamic motion
Some Physical QuantitiesVectors - quantity with both magnitude (size) and
direction.
Scalars - quantity with magnitude only.
Vectors:
• Displacement
• Velocity
• Acceleration
• Force
• Electric field
• Magnetic field
Scalars:
• Distance
• Speed
• Time
• Mass
• Work
• Energy
Vectors
The length of the
arrow represents
the magnitude (how
far, how fast, how
strong, etc,
depending on the
type of vector).
The arrow points in
the directions of the
force, motion,
displacement, etc. It
is often specified by
an angle.
Vectors are represented with arrows
42°
5 m/s
Mass vs. Weight
On the moon, your mass would be the same,
but the magnitude of your weight would be
less.
Mass:
• Scalar (no direction)
• The quantity of matter in an object
Weight:
• Vector (points toward center of Earth)
• The force upon an object due to gravity
Units
Quantity . . . Unit (symbol)
Mass . . . kilogram (kg)
Displacement & Distance . . . meter (m)
Time . . . second (s)
Velocity & Speed . . . (m/s)
Acceleration . . . (m/s2)
Force . . .Newton (N)
Energy . . . Joule (J)
Units are not the same as quantities!
Basic Units
In the SI system of units, there are seven
basic units
Name Symbol for
quantity
SI base unit
Length l Meter, m
Time t Second, s
Mass m Kilogram, kg
Electrical current I Ampere, A
Thermodynamic
temperatureT kelvin, K
Amount of
substance
n mole, mol
Luminous intensity Iv candela, cd
SI Prefixes
pico p 10-12
nano n 10-9
micro µ 10-6
milli m 10-3
centi c 10-2
kilo k 103
mega M 106
giga G 109
tera T 1012
Little Guys Big Guys
Kinematics
Kinematics – branch of physics; study of
motion without regard of the cause.
Position ( ) – where you are located.
Distance (d ) – how far you have traveled,
regardless of direction.
Displacement ( ) – where you are in
relation to where you started.
Distance vs. Displacement
You drive the path, and your odometer goes up by
8 miles (your distance).
Your displacement is the shorter directed from
start to stop (green arrow).
What if you drove in a circle?
start
stop
Speed, Velocity, & Acceleration
Speed (v) – how fast you go.
Speed is the magnitude of velocity.
Velocity ( ) – how fast and which way: the rate at which position changes.
Average speed ( ) – distance/time
Acceleration ( ) – how fast you speed up, slow down, or change direction: the rate at which velocity changes.
v
Speed vs. Velocity During your 14 km trip, which took 10 min, your
speedometer displays your instantaneous speed,
which varies throughout the trip.
v
The faster you go, the
longer your velocity vector.
Your average speed is 84 km/h (total distance
covered by time interval).
Your average velocity is 60
km/h in a SE direction.
At any point in time, your
velocity vector points
tangent to your path.
AccelerationAcceleration – how quickly velocity changes in speed
and/or in direction.
a = +2 m/s2a = -3
m/ss
= -3 m/s2
t (s) v (m/s)
0 55
1 57
2 59
3 61
t (s) v (m/s)
0 34
1 31
2 28
3 25
Acceleration due to Gravity
9.8 m/s2
Near the surface of the
Earth, all objects
accelerate at the same
rate (ignoring air
resistance).
a = -g = -9.8 m/s2
Interpretation: Velocity decreases by 9.8 m/s each
second, meaning velocity is becoming less positive or
more negative. Less positive means slowing down
while going up. More negative means speeding up
while going down.
This acceleration
vector is the same
on the way up, at
the top, and on the
way down!
Kinematics Formula Summary
• vf = vi + a t
• vavg = (vf + vi )/2
• x = vit + ½ at2
• vf2 – vi
2 = 2 ax
2
1
For 1-D motion with constant acceleration:
Sample Problems
1. You’re riding a unicorn at 25 m/s and come to
a uniform stop at a red light 20 m away. What’s
your acceleration?
2. A brick is dropped from 100 m up. Find its
impact velocity and air time.
3. An arrow is shot straight up from a pit 12 m
below ground at 38 m/s.
a. Find its max height above ground.
b. At what times is it at ground level?
2m/s6.15a
m/s3.44v
s5.4t
m7.61h
sor 43.733.0t
Multi-step Problems
1. How fast should you throw a kumquat straight down from 40 m up so that its impact speed would be the same as a mango’s dropped from 60 m?
2. A dune buggy accelerates uniformly at 1.5 m/s2 from rest to 22 m/s. Then the brakes are applied and it stops 2.5 s later. Find the total distance traveled.
19.8 m/s
188.83 m
Answer:
Answer:
1. Newton’s first law of motion: inertial law
If the net force , the object continues in
its original state of motion. That is, an object at
rest remains at rest, and an object moving with
some velocity continues with that same
velocity, unless acted on by a net external force.
Formula: )0( vectorconstant Fv
"keep on doing what they're doing"
It states that there is a cause (net external
force) for any change in velocity.
Dynamics: Newton’s law
Applications* Blood rushes from your head to your feet
while quickly stopping when riding on a
descending elevator.
* The head of a hammer can be tightened
onto the wooden handle by banging the bottom
of the handle against a hard surface.
* Headrests are placed in cars to prevent
whiplash injuries during rear-end collisions.
* While riding a skateboard (or wagon or
bicycle), you fly forward off the board when
hitting a curb or rock or other object which
abruptly halts the motion of the skateboard.
Check Your Understanding
Answer: According to Newton's first law,
the rock will continue in motion in the same
direction at constant speed.
1. Imagine a place in the cosmos far from all
gravitational and frictional influences. Suppose
that you visit that place (just suppose) and
throw a rock. The rock will:
a. gradually stop.
b. continue in motion in the same direction
at constant speed.
2. A 2-kg object is moving horizontally with
a speed of 4 m/s. How much net force is
required to keep the object moving at this
speed and in this direction?
Answer: 0 N. An object in motion will
maintain its state of motion. The presence of
an unbalanced force changes the velocity of
the object.
3. Supposing you were in space in a
weightless environment, would it require a
force to set an object in motion?
Answer: Absolutely yes! Even in space
objects have mass. And if they have mass, they
have inertia. That is, an object in space resists
changes in its state of motion. A force must be
applied to set a stationary object in motion.
Newton's laws rule - everywhere!
2. Newton's Second Law of Motion
The acceleration of an object is directly
proportional to the magnitude of the net external
force acting on it, inversely proportional to its
mass, and in the same direction as the net
external force.
It is a cause and effect relationship
among three quantities.m
Fa
A more familiar form amF
The net force is equated to the product of
the mass times the acceleration.
n
=i
ii FFFFF1
21
In rectangular coordinate system:
Fx = max ,Fy = may ,Fz = maz
The net force is the vector sum of all the
individual forces
PROBLEM:
An airboat with mass 3.50×102 kg, including the
passenger, has an engine that produces a net
horizontal force of 7.70×102 N, after accounting
for forces of resistance.
(a) Find the acceleration of the airboat.
(b) Starting from rest, how long does it take the
airboat to reach a speed of 12.0 m/s?
(c) After reaching that speed, the pilot turns off
the engine and drifts to a stop over a distance
of 50.0 m. Find the resistance force, assuming
it’s constant.
(a) a = 2.20 m/s2 (b) t = 5.45 s (c) -504 N
Whenever one object exerts a force on a
second object, the first object experiences a
force that is equal in magnitude and opposite
in direction to the one it exerts.
3. Newton's Third Law of Motion
In every interaction, there is a pair of forces
acting on the two interacting objects.
Forces always come in pairs - equal and
opposite action-reaction force pairs.
2112 FF
Dynamics: Work and Energy
1. Work: The work done on an object by
a constant force ( ) is defined to be
the dot product of the force and the
displacement.
According
Work is a scalar
The SI units of work are Joules (J)(1 J = 1 Nm)
Note:
Holding a heavy box, or pushing against
a wall
(1) If
(2) When and are in the same
direction, thus
(3) When and are in the opposite
direction, thus
2. Kinetic energy: The kinetic energy (KE )
of an object with mass m that is moving with
velocity v is
Note:
* Kinetic energy is a scalar.
* The unit of KE is the same as for the work
(i.e. Joules, J).
Relation between KE and Wnet:
The work done on an object equals the
change in kinetic energy:
3. Potential Energy: PE
Potential energy is associated with the
position of an object within some system.
System: A collection of objects or
particles interacting via forces.
a) Gravitational potential energy of a
system consisting of Earth and an object
of mass m near Earth’s surface is given
by
Reference levels for gravitational
potential energy: a location at which to
set that gravitational potential energy
equals to zero
b) Elastic potential energy: Energy stored
in a spring
Hooke’s law: The force exerted by the
spring must be proportional to the
displacement x,
F is often called restoring force
The energy stored in a stretched or
compressed spring or other elastic
material is called elastic potential energy
given by
Reference levels for elastic potential
energy: the equilibrium position of a
spring, at which elastic potential energy
equals to zero
Conservative forces:
gravitational force, elastic force, and
electrostatic force.
Work done by conservative forces on an object
depends only on the initial and final positions of
the object and is independent of the path
connecting the two positions.
Potential energy corresponding to each
conservative force:
gravitational potential energy, elastic potential
energy, and electrical potential energy.
Conservation of mechanical energy: In any
isolated system of objects that interact only
through conservative forces, the total mechanical
energy of the system remains constant.
In general, the work Wc done on a moving
object by a conservative force is equal to the
initial value of the potential energy minus the
final value.
Nonconservative forces: Frictional force
Work done by nonconservative forces on an
object depends on not only the initial and final
positions of the object but also on the path
connecting the two positions.
If there is a nonconservative force is
present in the system, the final mechanical
energy does not equal the initial mechanical
energy, the change
Homework:
1. Read Chapter 2 to Chapter 5 on your
textbook and preview Section 9.2, Section
9.4, Section 9.5, and Section 9.6 in
Chapter 9.