COLLEGE PHYSICS

Yuxiang Li (yxli@email.sdu.edu.cn)

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.