Junior Freshman
Course: PY1H01
PHYSICS FOR HEALTH SCIENCES
(Dental Science)
Recommended Text:
"Introduction to Biological Physics for the Health
and Life Sciences.“
Authors: Kirsten Franklin et al
Published by Wiley, 2010.
Physics –fundamental science
Knowledge of it is required in many fields:
chemistry, medicine, biology, dentistry etc
Dentistry
Important to understand the principles of physics
A few examples
Biting force impact on dental structures
Bones, teeth fillings, cement crowns,
bridges, implants.
Teeth extraction with minimal effort
.
Orthodontics
move the teeth quickly and efficiently.
Thermal properties of dental materials
Hot liquids
Electricity
Nerves
X-rays
Examination of teeth
Lasers
Early detection of caries
Dimensions and Units
Measuring a physical property
Example, length, time, speed etc
physical properties are called dimensions.
They denote the physical nature of a quantity
Dimensions
Time
Units
seconds, hours, etc
length
metres, feet, miles ,etc
Physical quantity has two parts,
 number
 unit
For example, the length of a football
pitch is 100m.
To say it is 100 has no meaning.
100 inches, 100m or 100 apples?
Units
Almost all measureable quantities can be
expressed as a combination of
dimensions: mass, length and time.
In 1960, international committee agreed on
a standard system of units called
Systéme International (SI)
(SI) also called the Metric System,
Fundamental Physical
Quantities. (Dimensions)
Mass:
(SI units)
kilogram
Length:
metre
Time:
second
SI notation, prefixes & Abbreviations
Multiplying
factor
SI prefix
Scientific
and
abbreviation notation
1 000 000 000 000
tera (T)
1012
1 000 000 000
giga (G)
109
mega (M)
106
1 000
kilo (k)
103
0.001
milli (m)
10-3
0.000 001
micro (µ)
10-6
0.000 000 001
nano (n)
10-9
0.000 000 000 001
pico (p)
10-12
1 000 000
Example:
1 kilometer = 1km = 1000m = 103 m
1 nanometer =1nm = 0.000 000 001m =10-9m
1milligram = 1 mg = 0.001gram = 10-3 gram
1microsecond = 1ms = 0.000 001s = 10-6 s
Equations, Unit Consistency,
Conversions
In physical equations
Each physical quantity has 2 parts,
a number and a unit.
Obviously, numbers on each side of the
equation must equate.
2 = 5 is wrong
Units must also equate. 5 apples ≠ 5 oranges.
Units divide and multiply just like numbers
Example: speed = distance / time.
If a car travels 100 m in 20s, its speed is 5 m/s
That is 100 m / 20 s = (100/20)(m/s) = 5 m/s
Numbers and units must equate
Units and change of units
How to change units?
Example
Convert km/h
m/s
1km = 1000m
1h = 60min = 60*60s = 3600s
1km
1000m
36km / h  36 
 36
 10m / s  10ms 1
1hr
3600s
Convert m/s
km/h
1m
3600
1m / s  1 3600
 1
km / hr  3.6km / hr
1h
1000
Mechanics
Objective
to link: time, displacement, distance,
velocity, speed and acceleration
Study of Motion without regard to its cause is
called kinematics.
The relationship of motion to the forces which
cause it is called dynamics.
In this section we will consider motion in a
straight line.
Displacement
Motion is concerned with the displacement of
an object from one position in space and time
to another.
Displacement (Ds) of an object is defined as
its change in position and is given by
Ds  x2  x1
where x1 is its initial position and x2 is its
final position. (Greek letter delta (D) is used to
denote a change in any physical quantity)
Ds can be positive or negative
positive, in the positive x direction
negative, in the negative x direction
Displacement
A person walks 4km east and then 3 km west.
What is her displacement ?
4km
3km
1km
Answer: Displacement is 1km east.
Even though the distance travelled is 7km,
displacement is 1km east.
Distance and displacement are different.
Distance has magnitude only but no given
direction and is called a scaler.
Displacement has both magnitude and
Direction: it is a vector quantity
Vectors and Scalers
Quantities that can be described by a single
number (with unit) are called scalars, while
quantities also needing directional information
are called vectors.
Scalar Quantities (magnitude but no direction)
e.g. mass, temperature, time etc.
Single number and unit completely
specifies each
Vector Quantities (both magnitude and direction)
e.g. displacement, velocity, acceleration.
magnitude, direction and unit required
Directional information is important, for example;
Orthodontics: teeth must not only be moved
but moved with a particular displacement
Velocity
The velocity is the change in displacement
(Ds) divided by the corresponding change in
time (Dt):
Ds

Dt
SI unit (metres per second)
m/s or ms-1
Velocity can be positive or negative:
80km/h
A
B
-80km/h
Velocity is a vector quantity: it has a
magnitude and a direction
Velocity
Velocity and speed are different
Velocity is a vector quantity
It has magnitude and direction
Example: 30km/hour west.
Speed is a scaler quantity
Example: 30km/hour.
No direction specified.
Acceleration
Acceleration is the change in velocity divided
by the corresponding change in time
D v  v0
a

Dt
t
v0 = initial velocity
v = final velocity
t = time taken
SI units ≡ ms-2
Acceleration can be positive or negative:
Examples:
Accelerating from 0m/s to 20m/s in 10s:
20ms 1  0ms 1
a
 2ms 2
10s
Decelerating from 20m/s to 0m/s in 10s:
0ms 1  20ms 1
a
 2ms 2
10s
Acceleration is a vector quantity (magnitude
and direction)
Linking velocity with acceleration
and time
v  v0
a

t
v  vo  at
Final velocity (v) is initial velocity (vo) plus
change due to acceleration*:
Example:
A runner accelerates at a rate of 8.0 ms-2 in the
first 0.75 s of a race. What is the magnitude of
her velocity at the end of this period?
1
v  0ms  8ms
2
 0.75s   6ms
1
Linking distance, displacement with
velocity, acceleration and time
s
Average velocity = displacement/time v 
t
Distance is average velocity multiplied by time:
 v  v0 
s  vt  
t
 2 
v  vo  at
 vo  (v0  at ) 
s
t
2


1 2
s  v0t  at
2
Example:
A runner has an acceleration of 8.0 m.sec-2 in
the first 0.75 sec of a race. How far has the
runner traveled in the period?
1
s  0  8m sec2  (0.75sec)2  2.25m
2
Linking velocity with
acceleration and distance
s Distance is average
 v0  v 
v
t
velocity multiplied by s  
 2 
t time.
Acceleration is change
in velocity divided by
time.
 v  v0 
a

 t 
2
2
v

v
v

v
v

v


0  0
0
as  
t



2
 t  2 
v  v  2as
2
2
0
Example:
A car accelerates from rest at 16 m.s-2 over a
distance of 400 m; what is the final velocity?
v 2  0  2 16ms 2  400m  12800m2 s 2
v  113.3ms 1
Summary: 4 useful equations
v  v0  at
1 2
s  v0t  at
2
 v  v0 
s  vt  
t
 2 
v  v  2as
2
2
0
Problem solving: Depending on information
given , choose one or more of the 4 equations
Exercise:
If your dentist touches a nerve in your tooth the
nerve impulse generated travels to your brain
in 1ms. Estimate the speed of the nerve impulse.
d
v
t
d
0.1m
2
1
v 
 10 ms
3
t 110 s
Exercise:
Human nerve impulses are propagated at
a rate of 102m/s. Estimate the time it takes for
a nerve impulse, generated when your foot
touches a hot object, to travel to your brain.
d
v
t
d
1.8m
t   2 1  0.018s
v 10 ms
18 ms
Exercise:
In orthodontic treatment a tooth when subjected
to a certain force moves a distance of 2 mm in
a period of 0.75 years. Estimate the average
speed (in ms-1) of the tooth.
d
v
t
Time in seconds
t  0.75 yrs  0.75  365  24  3600sec
t  23.65 106 s
3
2 10 m
v
6
23.65 10 s
12
v  84.57 10 ms
1
Exercise:
An athlete can run at a steady speed of
36km/h (!!) and can stop in 2.5s. What is
the average acceleration of the athlete
while stopping?
v  v0  at
First convert 36km/h to ms-1
1km
1000m
36km / h  36 
 36
 10m / s  10ms 1
1hr
3600s
0  10ms 1  a  2.5s
10ms 1
a
 4ms 2
2.5s
a is negative since athlete is decelerating