普通物理
Lecture 5
Law of Motion
Law of Motion
Sir Isaac Newton
Sir Isaac Newton
Sir Isaac Newton
Sir Isaac Newton
1642
1642 –
– 1727
1727
Formulated basic
Formulated basic
concepts and laws of
concepts and laws of
mechanics
mechanics
Universal Gravitation
Universal Gravitation
Calculus
Calculus
Calculus
Calculus
Newton's Law of Motion
Newton's Law of Motion
i i
i
i
i i
i
i
The branch of physics involving the motion of
The branch of physics involving the motion of
an object and the relationship between that
an object and the relationship between that
motion and other physics concepts
motion and other physics concepts
Kinematics
(
(運動學
運動學):
):
description of motion
Kinematics
(
(運動學
運動學):
):
description of motion
Kinematics
(
(運動學
運動學):
):
description of motion
是在討論物體運動的狀況,不考慮產生運動的原因,只研究
是在討論物體運動的狀況,不考慮產生運動的原因,只研究
位
移、速度、加速度及時間
之間的關係。
之間的關係。
Kinematics
(
(運動學
運動學):
):
description of motion
Dynamics
(
(運動力學
運動力學):
):
cause of the motion
移、速度、加速度及時間
之間的關係。
之間的關係。
Dynamics
(
(運動力學
運動力學):
):
cause of the motion
研究作用於物體的
研究作用於物體的
力
力
、物體
、物體
質量
質量
及物體
及物體
運動狀況
運動狀況
之間所存在的
之間所存在的
關係。可以用來預測某些已知力所造成的運動,或是決定要產
關係。可以用來預測某些已知力所造成的運動,或是決定要產
生某一運動所需的力
生某一運動所需的力
33生某一運動所需的力。
生某一運動所需的力。
Newton's Law of Motion
Newton's Law of Motion
In this chapter we will introduce
Newton’s three laws of
In this chapter we will introduce
Newton s three laws of
motion
which is at the heart of classical mechanics.
Newton’s laws describe physical phenomena of
a vast range
Newton s laws describe physical phenomena of
a vast range
.
Ex:
the motion of stars and planets
Exceptions:
p
When the speed of objects approaches
(1% or more) the speed of
light in vacuum (c = 3×10
8m/s)
In this case we must use
Einstein’s
light in vacuum (c 3×10 m/s).
In this case we must use
Einstein s
special theory of relativity
(1905)
When the objects under study become very small (e g
electrons
Contents
Newton’s First Law
Newton’s First Law
牛頓第一運動定律
牛頓第一運動定律
Newton s First Law
Newton s First Law
牛頓第一運動定律
牛頓第一運動定律
Force
Force
力
力
Newton’s Second Law
Newton’s Second Law
牛頓第二運動定律
牛頓第二運動定律
Inertia & Mass
Inertia & Mass
慣性與質量
慣性與質量
Newton’s Third Law
Newton’s Third Law
牛頓第三運動定律
牛頓第三運動定律
Inertial Reference Frames
Inertial Reference Frames
慣性參考體
慣性參考體
Inertial Reference Frames
Inertial Reference Frames
慣性參考體
慣性參考體
Application of Newton’s Laws
Application of Newton’s Laws
牛頓運動定律的應用
牛頓運動定律的應用
Free Body Diagrams
Free Body Diagrams
自由體圖
自由體圖
Friction
Friction
摩擦力
摩擦力
Drag force and terminal Speed
Drag force and terminal Speed
拖曳力與終端速率
拖曳力與終端速率
Uniform Circular Motion/ Centripetal force Speed
Uniform Circular Motion/ Centripetal force Speed
55
Uniform Circular Motion/ Centripetal force Speed
Uniform Circular Motion/ Centripetal force Speed
等速率圓周運動與向心力
等速率圓周運動與向心力
Newton’s First Law
Newton’s First Law
Before Newton
A force was required in order to keep an object moving at
constant
velocity
An object was in its “natural state” when it was at rest.
After Newton
F i ti
i d t b
f
d i t d
d
Friction
was recognized to be a force and introduced
For example:
Slide an object on a floor with an initial speed, very soon the object will
Newton’s First Law
Newton’s First Law
An object continues in a state of rest
or in a state of motion at a
constant speed
along a straight line unless compelled to
constant speed
along a straight line, unless compelled to
change that state by a net force.
If no force
acts on a body, the body’s velocity cannot change;
that is the body cannot accelerate
Force
Force
A push or pull exerted on an object We can define a force exerted on an A push or pull exerted on an object. We can define a force exerted on an object quantitatively by measuring the acceleration it causes
We place an object of mass m = 1 kg on a frictionless surface and measure
Force
Force
External Force
External Force
Any force that results from the interaction between the object and its
i
environment
Internal Force
Forces that originate within the object itself, and they
cannot change the
object’s velocity
Inertia & Mass
Inertia & Mass
慣性與質量
Inertia
慣性與質量
Inertia
The natural tendency of an object to remain at rest or in motion at a
Mass
y
j
constant speed along a straight line.
Mass
An
intrinsic
characteristic of a body that automatically comes with the
existence of the body A
measure of the resistance
of an object to
existence of the body. A
measure of the resistance
of an object to
changes in its motion due to a force
Inertia & Mass
Inertia & Mass
F
mo
Apply F on a second body of unknown mass
m
Xwhich results in an acceleration
a
X.
The
ao
m
Xwhich results in an acceleration
a
X.
The
ratio of the accelerations is inversely
proportional to the ratio of the masses
F
p p
aX mXo
o
X
X
o
a
a
m
m
m
m
o
a
X
a
X
m
a
a
11 11Newton’s Second Law
Newton’s Second Law
F
m
a
When a net external force acts on an object of mass , the
acceleration that results is directly proportional to the net force
and has a magnitude that is inversely proportional to the mass
Note: If several forces act on a body (say F F and F ) the net force F
(say , , and ) the net force
is defined as: i i th t f A B C net net A B C F F F F F F F F F
i.e. is the vector sum of , , and ne C t A B F F F F
Newton’s Second Law
Newton’s Second Law
The results of the discussions on the relations between the net force
F
The results of the discussions on the relations between the net force
F
netapplied on an object of mass
m
and the resulting acceleration a can be
summarized in the following statement known as:
g
“Newton’s second
law”
The net force on a body is equal to the product of
the body’s mass and its acceleration
Fnet
the body s mass and its acceleration
a m
F
net
ma
F
ma
13 13 , net x xF
ma
F
net y,
ma
yF
net z,
ma
zNewton’s Second Law
Newton’s Second Law
The force that the earth exerts on any object (in the picture
The Gravitational Force
The force that the earth exerts on any object (in the picture a cantaloupe) It is directed towards the center of the earth. Its magnitude is given by Newton’s second law.
y
Mutual force of attraction between any two objects
m
m
Expressed by Newton’s Law of Universal Gravitation:
2 2 1 g
r
m
m
G
F
Newton’s Second Law
Newton’s Second Law
Weight
The magnitude of the gravitational force acting on an object of mass m
near the Earth’s surface is called the weight w of the object
We g
g j
w = m g is a special case of Newton’s Second Law
g is the acceleration due to gravity, can also be found from the Law of
U i l G i i
W
Universal Gravitation
The weight of a body is defined as the magnitude of the force required to prevent the body from falling freely
y g W ,
0
net y yF
ma
W
mg
W
mg
mg 15 15Note: The weight of an object is not its mass. If the object is moved to a location where the acceleration of gravity is different (e.g. the moon where gm = 1.7 m/s2) , the mass does not
Newton’s Second Law
Newton’s Second Law
Contact Forces:
Forces act between two objects that are in contact. The contact forces have two components. One is acting along the normal to the contact surface (normal
force) and a second component that is acting parallel to the contact surface
Newton’s Second Law
Newton’s Second Law
Tension:
Tension:
The force exerted by a rope or a cable attached to an object
Ch t i ti
always directed along the rope
Characteristics
the same value along the rope
always pulling the object
Neglect rope mass compared to the mass of the
Assumptions
the same value along the rope
Neglect rope mass compared to the mass of the object it pulls
Rope does not stretch
17 17
Rope does not stretch
Newton’s Third Law
Newton’s Third Law
Wh
t
b di i t
t b
ti
f
h th
th
When two bodies interact by exerting forces on each other, the
forces are equal in magnitude and opposite in direction
For example consider a book leaning against a bookcase. We label the force exerted the book the case. Using the same convention we label on by the force
BC CB F F exerted the cason e by the book.
CB
Newton's third law can be written as: The book together with the bookcase are known as
a
BC CB
Newton’s Third Law
Newton’s Third Law
A second example is shown in the picture
h l f Th hi d l
f
i
to the left. The third –law force pair
consists of the earth and a cantaloupe.
U i
h
i
b
Using the same convention as above we
can express Newton’s third law as:
F
F
F
CE
F
EC
19 19
Inertial Reference Frames
Inertial Reference Frames
慣性參考體
慣性參考體
慣性參考體
慣性參考體
牛頓認為宇宙中存在一個與任何物體均無相互作用,而且永遠
牛頓認為宇宙中存在一個與任何物體均無相互作用,而且永遠
靜止的空間,稱作 絕對空間(absolute space),所有的物體均
在此絕對空間中運動。
凡是相對於絕對空間靜止,或作等速率直線運動(且不旋轉)的
參考體,即稱之為 慣性參考體 (inertial reference frame)。
牛頓運動定律及其所推展出的力學定理
僅成立於慣性參考體
牛頓運動定律及其所推展出的力學定理,僅成立於慣性參考體
中。
The earth rotates about its axis once every 24 hours and thus it is accelerating with respect to an inertial reference frame. Thus we are making an approximation when we consider the earth to be an inertial reference frame. This approximation is excellent for most small scale phenomena Nevertheless for large scale phenomena small scale phenomena. Nevertheless for large scale phenomena
Application of Newton’s Laws
Application of Newton’s Laws
pp
pp
Free body Diagrams
Frictional force between two objects
The drag force exerted by a fluid on an object
moving through the fluid
moving through the fluid
Uniform circular motion
21 21
Free Body Diagrams
Free Body Diagrams
y
y
g
g
Among the many parts of a given problem we choose one which we call the “system”
Among the many parts of a given problem we choose one which we call the system .
Then we choose axes and enter all the forces that are acting on the system and omitting those acting on objects that were not included in the system.
An example is given in the figure below. This is a problem that involves two blocks labeled "A" and "B" on which an external force Fapp is exerted.
We have the following "system" choices:
a. System = block A + block B. The only horizontal force is Fapp
b. . There are now two horizontal forces: and c. . The only horizontal force
System = block A System = block B is app AB BA F F F
Free Body Diagrams
Free Body Diagrams
y
y
g
g
1. Choose the system to be
studied
2. Make a simple sketch of the
system
3. Choose a convenient
coordinate system
4. Identify all the forces that
act on the system. Label
them on the diagram
5. Apply Newton’s laws of
23 23
Friction
Friction
Friction
Friction
When F reaches a certain limit the crate “breaks away” and accelerates to the left. Once the crate starts moving the force
Once the crate starts moving the force opposing its motion is called the kinetic frictional force , . Thus if we wish the crate to move with constant speed we must decrease F so that it
balances ƒk (frame f) In frame (g) we plot balances ƒk (frame f). In frame (g) we plot ƒ versus time t
25 25
Friction
Friction
FN
Properties of friction:
F
FN
Properties of friction:
The frictional force is acting between two dry un-lubricated surfaces in contact
mg
y
Property 1.
If the two surfaces do not move with respect to each other, then the static frictional force balances the applied force .
Property 2.
s,max s Nf
F
ope y .
The magnitude ƒs of the static friction is not constant but varies from 0 to a maximum value ƒs, max = μSFN The constant μs is known as the coefficient
f t ti f i ti If F d ƒ th t t t t lid of static friction. If F exceeds ƒs,max the crate starts to slide
Friction
Friction
Property3.
Once the crate starts to move the frictional force is known as kinetic friction. Its magnitude is constant and is given by the equation: ƒk = μk FN
μ is known as the coefficient of kinetic friction
μK is known as the coefficient of kinetic friction.
ƒ
k< ƒ
s max FFN
ƒ
kƒ
s,maxNote 1:
The static and kinetic friction acts parallel to the surfaces in contact The direction
mg
the surfaces in contact. The direction
opposes the direction of motion (for kinetic friction) or of attempted motion (in the case of static friction)
Note 2:
27 27
Note 2:
Drag force and terminal Speed
Drag force and terminal Speed
g
g
p
p
拖曳力與終端速率
When an object moves through a fluid (gas or liquid) it experiences an When an object moves through a fluid (gas or liquid) it experiences an
opposing force known as “drag”. Under certain conditions (the moving object must be blunt and must move fast so as the flow of the liquid is turbulent) the magnitude of the drag force is given by the expression:
1
21
2
D
C Av
2
C : drag Coefficient (拖曳係數) d it f th di fl id (流體密度)
: density of the surrounding fluid (流體密度)A : effective cross sectional area of the moving object (物體有效截面積) v : object’s speed (速率)
29 29 v : object s speed (速率)Drag force and terminal Speed
Drag force and terminal Speed
g
g
p
p
拖曳力與終端速率
Consider an object (a cat of mass m in this case) start moving in air Consider an object (a cat of mass m in this case) start moving in air.
Initially D = 0. As the cat accelerates D increases and at a certain speed vt
D = mg At this point the net force and thus the acceleration become zero
and the cat moves with constant speed vt known the the terminal speed
1
2
1
2
t
D
C Av
mg
2
2
tmg
v
t
Example 2:
The terminal speed of a sky diver is 160 km/h in the spread-eagle position and 310 km/h in the nosedive position Assume that the diver’s drag
and 310 km/h in the nosedive position. Assume that the diver s drag
coefficient C does not change from one position to the other, find the ratio of the effective cross-sectional area A in the slower position to that in the faster position? faster position?
2
2
v
C
mg
A
1
2
A
v
C
75
.
3
)
160
/
310
(
)
(
2
2
A
slowv
fastv
)
(
)
(
slow fastv
A
31 31Uniform Circular Motion/
Uniform Circular Motion/
Centripetal force
Centripetal force
Centripetal force
Centripetal force
The direction of the acceleration vector
The direction of the acceleration vector
always points towards the center of
rotation C (thus the name centripetal) Its
(
p
)
magnitude is constant:
2
vv
a
r
r
Apply Newton’s law to analyze uniform circular motion we conclude
that the net force in the direction that points towards C
2
mv
mv
Uniform Circular Motion/
Uniform Circular Motion/
Centripetal force
Centripetal force
Centripetal force
Centripetal force
Recipe
.
C
r
Recipe
Draw the force diagram for the
m
x
y
g
object
Choose one of the coordinate axes
m
v
Choose one of the coordinate axes
(the y-axis in this diagram) to point
towards the orbit center C
Determine
ynetF
Set
2 ynetmv
F
33 33 ynetr
Example 3:
A racing car with m=600 kg travels on a flat track in a circular arc of radius R= 100 m Because of the shape of the car and the wings on it the
R 100 m. Because of the shape of the car and the wings on it, the passing air exerts a negative lift FL downward on the car. Assume μs between the tires and the track is 0.75.
(a) If the car is on the verge of sliding out of the turn when its speed is 28 6 (a) If the car is on the verge of sliding out of the turn when its speed is 28.6
m/s, what is the magnitude of FL?
(b) If , when v=90 m/s, is the car possible to run on the ceiling?FL v2
Example 4:
A racing car with m=600 kg travels on a flat track in a circular arc of radius R= 100 m Because of the shape of the car and the wings on it the
R 100 m. Because of the shape of the car and the wings on it, the passing air exerts a negative lift FL downward on the car. Assume μs between the tires and the track is 0.75.
(a) If the car is on the verge of sliding out of the turn when its speed is 28 6 (a) If the car is on the verge of sliding out of the turn when its speed is 28.6
m/s, what is the magnitude of FL?
(b) If , when v=90 m/s, is the car possible to run on the ceiling?FL v2
35 35
Example 5:
The Rotor is a large hollow cylinder of radius R=2.5m that is rotated rapidly around its central axis with a speed v. A rider of mass m stands on the Rotor floor with his/her back against the Rotor wall. Cylinder and rider begin to turn. When the y g speed v reaches some predetermined value, the Rotor floor abruptly falls away. The rider does not fall but instead
remains pinned against the Rotor wall. The coefficient of remains pinned against the Rotor wall. The coefficient of static friction μs=0.5 between the Rotor wall and the rider is given. What is the minimum speed v ?
2
( 1)
mv F F
The normal reaction FN is the centripetal force.
, , = (eqs.1) , 0 , (eqs.2) x net N y net s s s N s N F F ma R F f mg f
F mg
F 2 2 minIf we combine eqs.1 and eqs.2 we get: mg s mv v Rg v Rg R
Example 6:
In a 1901 circus performance Allo Diavolo introduced the stunt of riding a bicycle in a looping-the-loop The loop is a stunt of riding a bicycle in a looping the loop. The loop is a circle of radius R. We are asked to calculate the minimum speed v that Diavolo should have at the top of the loop and not fall
not fall
37 37
Assignment 4
Assignment 4
Due Date: 10/28/2009
Due Date: 10/28/2009
Due Date: 10/28/2009
Due Date: 10/28/2009
Assignment 4 Assignment 4 Assignment 4 Assignment 4 1.The figure shows acceleration-versus-force graphs for two objects pulled by rubber bands What is the mass ratio m /m ?
pulled by rubber bands. What is the mass ratio m1/m2 ?
2.
The period of a pendulum is proportional to the square root of its length. A 2.0-m-long pendulum has a period of 3.0 s. What is the period of a 3.0-m-long pendulum?
3.
It’s a snowy day and you’re pulling a friend along a level road on a sled. You’ve both been taking physics, so she asks what you think the coefficient of friction between the sled and the snow is. physics, so she asks what you think the coefficient of friction between the sled and the snow is.
Assignment 4 Assignment 4
44.
A ball is shot from a compressed-air gun at twice its terminal speed.
a. What is the ball’s initial acceleration, as a multiple of g, if it is shot straight up? b. What is the ball’s initial acceleration, as a multiple of g, if it is shot straight down? c. Draw a plausible velocity-versus-time graph for the ball that is shot straight down. 5.
A 100 kg basketball player can leap straight up in tile air to a height of 80 cm as shown in the figure A 100 kg basketball player can leap straight up in tile air to a height of 80 cm, as shown in the figure below. You can understand how by analyzing the situation as follows:
a. The player bends his legs until the upper part of his body has dropped by 60 cm, then he begins his jump. Draw separate free-body diagrams for the player and for the floor as he is jumping, but before his feet leave the ground.
b. Is there a net force on the player as he jumps (before his feet leave the ground)? How can that be? Explain.
c With what speed must the player leave the ground to reach a height of 80 c. With what speed must the player leave the ground to reach a height of 80
cm?
d. What was his acceleration, assumed to be constant, as he jumped?
e. Suppose the player jumps while standing on a bathroom scale that reads in newtons. What does the scale read before he jumps, as he is jumping, and after his feet leave the ground?
39 39
Assignment 4 Assignment 4
6. 6.
In an amusement park ride II called The Roundup, passengers stand inside a 16-m-diameter rotating ring. After the ring has acquired sufficient speed, it tilts into a vertical plane, as shown in the figure. a. Suppose the ring rotates once every 4.5 s. If a rider’s mass is 55 kg, with how much force does the
ring push on her at the top of the ride? At the bottom?
b. What is the longest rotation period of the wheel that will prevent the riders from falling off at the top?