Chapter VII
force
I. Strength
1
Concept of force: Force is the action of objects on objects.
2
Unit of force: Newton, abbreviated as cattle, belt
ordinary
Express delivery. Perceptual knowledge of force: the force used to take two eggs is about
1N
three
Function of force: Force can change the shape of an object, and force can change the motion state of an object.
Note: Whether the motion state of an object changes generally refers to whether the speed of the object changes (speed change) and
Has the direction of motion of the object changed?
four
Three elements of force: the magnitude, direction and action point of force;
They can all affect the effect.
five
Schematic diagram of force: the magnitude, direction and action point of force are represented by line segments with arrows.
,
If there is no size
,
Does not mean that
,
In the same figure
,
The greater the power.
,
The longer the line segment should be.
six
Conditions for force generation: ① There must be two or more objects. (2) There must be interaction between objects (no contact)
seven
The essence of force: the forces between objects are mutual.
When two objects interact, the force applying object is also a force applying object, and vice versa.
Second, flexibility.
1
, elasticity
① Elasticity
：
The property that an object deforms when stressed and returns to its original state when it is not stressed is called elasticity.
② Plasticity
：
The property that an object is deformed by force and cannot recover its original shape after deformation is called plasticity.
③ Elasticity
：
The force on an object due to elastic deformation is called elasticity.
,
Elastic force is related to elastic deformation.
Important conditions for the generation of elastic force
：
Elastic deformation occurs
Two objects touch each other.
;
Elasticity in life
：
pulling force
,
Support strength
,
pressure
,
drive
;
2
: spring dynamometer
① Structure: spring, hook, pointer, scale and shell.
② Function: Force measurement.
③ Principle: Within the elastic limit, the greater the tension the spring receives, the longer its elongation.
(Within the elastic limit, the elongation of the spring is directly proportional to the tensile force. )
(4) Use a spring dynamometer
( 1)
See clearly
measuring range
and
partition value
(2)
Check whether the pointer points to the zero scale.
,
otherwise
,
You must set it to zero.
;
(3)
Gently pull the scale hook several times to see if the pointer returns to zero scale after each release;
(4)
When using, the force should be along the axis of the spring, and be careful not to let the pointer and the spring touch the scale shell. When measuring force, it cannot exceed.
Range of spring dynamometer.
(5)
When reading, the line of sight is perpendicular to the scale surface.
Description: In the physical experiment.
,
Some physical quantities are not suitable for direct observation.
Yes, but when it changes, it will cause changes in other physical quantities, which is easy to observe and use.
One idea of manufacturing measuring instruments is that observable quantity means unobservable quantity. This scientific method is called "transformation method"
. Made in this way.
Our instruments include: thermometer, spring dynamometer, etc.
Third, gravity.
、
1
Concept of gravity: The force exerted by the earth's gravity on an object is called gravity. The object of gravity is the earth.
2
The magnitude of gravity is called weight, and the gravity on an object is directly proportional to the mass.
direct ratio
Formula:
G = mg
In ...
G = 9.8N Newton/kg
It represents.
1 kg
The gravity of an object is
9.8N
. In cases where the requirements are not very precise, you can
take
g= 10N/kg
three
Gravity direction: vertical downward.
. Its application is to check whether the wall is vertical and the desktop is horizontal with vertical line and level respectively.
four
Center of gravity-center of gravity
The point where gravity acts on an object is called the center of gravity. The center of gravity of an object with uniform texture and regular shape is at its geometric center.
If the center of gravity of a uniform thin rod is at its midpoint, the center of gravity of the ball is at the center of the ball. The center of gravity of a square thin plate is at the intersection of two diagonal lines.
Chapter VIII
Force and motion
First of all, Newton's first law
1
Newton's first law:
(1) Newton summed up the research results of Galileo and others, and obtained Newton's first law, the content of which is:
All objects are always in a state of static or uniform linear motion without force.
(2) Description:
A
Newton's first law is further inferred and summarized on the basis of a large number of empirical facts. It has withstood the test of practice, so it has always been
Become one of the basic laws of mechanics recognized by everyone. but
There can be no pressure around us, so it is impossible to directly prove that Newton is the first by experiments.
Law.
B
The connotation of Newton's first law: If an object is not stressed, it will remain at rest.
,
The first moving object
,
No matter what sports you used to play.
,
Everything will move in a straight line at a constant speed.
.
C
Newton's first law tells us
：
When an object moves in a straight line at a uniform speed, no force is needed, that is, the force has nothing to do with the state of motion, so the force does not produce or maintain motion.
The reason for the exercise.
2
Inertia: (1) Definition: The property that an object keeps its original motion state unchanged is called inertia.
⑵ Description: Inertia is an attribute of an object. All objects have inertia in any case, and the size of inertia is only related to the mass of the object, and whether the object
Pressure, the size of pressure, whether to move, the speed of movement and so on. Doesn't matter.
Using inertia: the run-up of long jumpers; Hard work can throw stones far away; A few pedals will make the bike slide.
Prevent the harm caused by inertia: the front passengers of small passenger cars should wear seat belts; Keep your distance while driving.
Two, two force balance
1
Definition: When an object is acted by two forces, it is said that the two forces are balanced if it can maintain a static state or a state of uniform linear motion.
2
The equilibrium condition of two forces: two forces act on the same object, with equal magnitude and opposite directions, and the two forces are in a straight line.
three
An object will remain stationary or move in a straight line at a constant speed without force or balance force. Namely, the equilibrium state
.
four
Comparison of balance force and interaction force:
Similarities: ① equal in size; ② The direction is opposite; ③ Linear action.
Difference: the balance force acts on an object, which can be different forces; Interaction forces act on different objects and are forces of the same nature.
five
The relationship between force and motion state;
Force state of an object
Object motion state
explain
equilibrant
Force is not the cause of exercise.
Under unbalanced force
Force is the reason to change the motion state of an object.
The change of motion state of an object refers to the change of speed and direction of motion.
Third, sliding friction.
1
Definition: When two objects are in contact with each other and slide relatively, there will be a force on the contact surface that hinders the relative motion. This force is called
Sliding friction.
2
Friction classification: static friction, sliding friction and rolling friction.
three
Direction of friction: the direction of friction is opposite to the direction of relative motion of objects.
four
、
Under the same conditions (contact surface pressure and roughness are the same), rolling friction is much smaller than sliding friction.
five
Sliding friction: ① measurement principle: two-force balance condition
② Measurement method: Put the wood block on a horizontal long board, and pull the wood block horizontally with a spring dynamometer to make the wood block move at a uniform speed. At this time, the reading tension is equal to sliding.
The magnitude of dynamic friction.
③
Conclusion: When the roughness of contact surface is the same, the greater the pressure, the greater the sliding friction. Under the same pressure, the rougher the contact surface, the greater the sliding friction.
In this study, the control variable method is used.
The first two conclusions can be summarized as follows:
The magnitude of sliding friction is related to the magnitude of pressure and the roughness of contact surface.
experiment
It can also be found that the sliding friction has nothing to do with the size of the contact surface and the speed of motion.
seven
, application:
① The methods of increasing friction are: increasing pressure, rough contact surface and changing rolling friction into sliding friction.
② The methods to reduce friction are: reducing pressure, smoothing contact surface and changing sliding into rolling (rolling bearing).
Separate the contact surfaces from each other (add lubricating oil,
Air cushion, magnetic levitation)
Chapter 9
pressure
I. Stress
1
, pressure:
⑴
Definition: The force vertically pressing on the surface of an object is called pressure.
Note: Not all stress is caused by gravity. Usually, when an object is placed on a horizontal plane, if it is not affected by other forces, then
F = G
⑵ Direction: The direction of pressure always points to the pressed object perpendicular to the supporting surface.
2
Experimental study on the factors affecting the pressure effect;
(1) textbook
P30
draw
9. 1
—
three
Description in Chinese, A and B: When the stress area is the same, the greater the pressure, the more obvious the pressure effect. Description of b and c: pressure
At the same time, the smaller the stress area, the more obvious the pressure effect.
The conclusion of these two experiments is that the effect of pressure is related to the size of pressure and stress area. In this experiment, the control variable method is used to study this problem.
three
Pressure: (1)
Definition: The ratio of the pressure on an object to the stress area is called pressure.
(2) Formula:
p =
S
F
Derived formula:
F = PS
、
S=
P
F
⑶ Unit: Pressure
F
Unit: Newton (
ordinary
)
, area
S
Unit: meter
2
(
m2)
, pressure
p
Unit: Pascal (
protactinium
)
(
four
) application: decompression. Such as: sleepers are laid on the rails, tanks are installed on the rails, and schoolbags are wider.
Increase the pressure For example, the sewing needle is very thin, and the blade of the kitchen knife is very thin.
Second, the pressure of the liquid
keep one's body unmoved
Uniform motion
transport
move
shape
form
Unchanged
Change of moving speed
Change of direction of motion
Motion state change
Liquid depth: A point in a liquid.
The distance to the liquid surface is called a point.
Depth in liquid
1
Characteristics of liquid pressure:
⑴
The liquid has pressure on the bottom and side walls of the container,
(2) There is pressure in all directions inside the liquid;
⑶
The pressure of liquid increases with the increase of depth; At the same depth, the pressure of liquid in all directions is equal;
⑷
The pressure of different liquids is related to the density of liquids.
2
, the calculation formula of liquid pressure:
p=
ρ
somatotropin
When using this formula to solve the problem, the secret
degree
ρ
Unit use
Kg/m
three
, pressure
p
The unit is Pascal (
protactinium
)
press
strong
formula
p
=
ρ
g h
application area
General formula: general solid
Ordinary liquid
General idea
Horizontal plane:
F = G
p
=
S
F
first
p
=
ρ
g h
and
F = PS
Special train of thought
Cylindrical object
p
=
ρ
g h
Ordinary liquid container
F = G
p
=
S
F
three
, connector:
⑴ Definition: A container with an open top and a connected bottom.
⑵ Principle: The communicator is filled with liquid. When the liquid does not flow, the liquid level of each container is kept level.
⑶ Application: teapot, ship lock, boiler water level gauge, automatic water feeder for cows, etc. All the work is based on the principle of communication equipment.
Third, atmospheric pressure.
1
The existence of atmospheric pressure-the experiment proved to be a famous experiment in history-the Madeborg Hemisphere Experiment.
2
Measurement of atmospheric pressure: Torricelli experiment.
( 1)
Experimental process: for a long time,
1m
Fill one end of the glass tube with mercury, plug the nozzle, then insert the glass tube backwards in the mercury tank, and loosen the finger blocking the nozzle.
After that, the mercury level in the tube will not drop if it drops slightly. At this point, the mercury column height difference inside and outside the tube is about
760 mm
(2)
Principle analysis: the liquid level inside and outside the pipe is in phase.
Put a liquid sheet on a flat place, because the liquid is motionless, so the liquid sheet is balanced by the upper and lower pressures. That is, it is big upward.
atmospheric pressure
=
The pressure generated by the mercury column.
(3)
Conclusion: Atmospheric pressure
p
= 760 mmhg = 76 cmhg = 1.0 1
×
10
five
Pa(
Its value changes with the change of external atmospheric pressure.
)
A
The purpose of filling the glass tube with mercury before the experiment is: after the glass tube is inverted, there is a vacuum above the mercury; If it is not filled, the measurement result is very small.
B
In this experiment, if mercury becomes water, the length of the glass tube needs to be
10.3m
C
Gently lift or press the glass tube, and the height difference between the inside and outside of the tube remains unchanged. Tilt the glass tube, the height remains the same and the length becomes longer.
D
Standard atmospheric pressure:
support
76 cm
The atmospheric pressure of mercury column is called standard atmospheric pressure.
1
standard atmospheric pressure
= 760mmhg = 76cmh
g= 1.0 1
×
10
five
protactinium
three
Atmospheric pressure measuring tool: barometer. Classification: Mercury barometer and aneroid barometer.
four
Characteristics of atmospheric pressure: there is pressure in all directions in the air; Atmospheric pressure decreases with height.
five
Relationship between boiling point and air pressure: The boiling point of all liquids is when the air pressure decreases.
decrease
When the air pressure increases,
rise
six
Purpose: Piston pump and centrifugal pump.
Fourthly, the relationship between fluid pressure and flow.
1:
In gas and liquid, the higher the speed, the lower the pressure.
Aircraft lift: When the aircraft is moving forward, due to the asymmetry of the wing, the air above the wing has a high speed and a low pressure, while the air below has a low speed and a low pressure.
Larger, there is a pressure difference between the upper and lower surfaces of the wing, which produces upward lift.
Chapter 10
buoyancy
First, buoyancy
1:
Buoyancy: All objects immersed in liquid or gas are subjected to vertical upward force from liquid or gas, which is called buoyancy.
The reason of buoyancy is that the object immersed in liquid is subjected to the pressure difference between the upper and lower parts of the liquid.
Buoyancy direction: always vertical upward. Force application object: liquid (gas)
Second, Archimedes principle
1
.
Archimedes principle:
An object immersed in a liquid is subjected to upward buoyancy, which is equal to the gravity of the liquid it displaces.
2
.
Direction: vertical upward
three
.
Archimedes principle formula:
row
flotage
G
F
III. Conditions and Applications of Undulating Objects
Motion state of an object
Object motion direction
Power relation
V
row
and
V
thing
Density relation
water channel
downwards
F
flotage
& ltG
thing
V
row
=V
thing
ρ
thing
& lt
ρ
liquid
pause
Stay still in a liquid.
F
flotage
= G
thing
ρ
thing
=
ρ
liquid
grow
up
F
flotage
& gtG
thing
ρ
thing
& gt
ρ
liquid
flotage
Stay on the liquid surface
F
flotage
= G
thing
V
row
& ltV
thing
ρ
thing
& gt
ρ
liquid
4.
According to Archimedes' principle, buoyancy only depends on
liquid density
、
Volume of liquid discharged by an object (volume of liquid immersed by an object)
And objects.
about
Shape, density, mass, volume,
It has nothing to do with the depth and motion state of the liquid.
10.3
Application of floating and sinking conditions of objects;
1.
Application of buoyancy
1)
The ship is made up of
hole
To increase buoyancy. Ship displacement:
Quality of boiling water discharged when the ship is fully loaded.
. The ship sailed from the river to the sea.
Because the density of water becomes larger, the volume of the boat immersed in the water will become smaller, so it will float a little, but the buoyancy will remain the same (always equal to the wheel
Gravity of the ship)
2)
Submarine dependence
Change one's focus
Floating or diving.
3)
Balloons and airships rely on
Fill with a gas with a density less than.
To change buoyancy.
4)
Densitometer is
flotage
Working on the liquid surface, its scale is
"Up small and down big"
2
, buoyancy calculation:
1
) differential pressure method:
F
flotage
=F
up
-F
downwards
2
) weighing method:
F
flotage
=G
thing
-F
Pull
(This method is generally used when the conditions of spring dynamometer appear in the topic. )
three
) suspension suspension method:
F
flotage
=G
thing
four
)
Archimedes method
F
flotage
=G
row
=
ρ
liquid
Raw foaming
row
(This method is generally selected when there is a volume condition in the topic. )
Bankruptcy reorganization protection
Work and mechanical energy
I. Advantages
1
Significance of doing work: If a force acts on an object and the object moves a distance along the direction of this force, the effect of this force will be manifested.
If it works, mechanics says that this force has done work. The work mentioned in mechanics includes two necessary factors: one is the force acting on the object, and the other is the object.
The direction of the force and the distance it moves. There are three situations when no work is done: strong without distance, weak with distance, vertical force and distance.
2
Calculation of work: The greater the force acting on an object, the greater the distance the object moves, and the more significant the effect of this force, indicating that this force does more work. thing
In Neo-Confucianism, the product of force and distance moving in the direction of force is called work:
success
=
force
×
The direction of the force and the distance it moves.
Expressed by formula:
W
=
Air force staff sergeant
Meaning and unit of symbol:
W
——
success
——
Joule (
J
)
F
——
force
——
Newton (
ordinary
)
S
——
distance
——
Meters (
m
)