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Explore the two-force equilibrium experiment
1. What factors affect the experimental results when exploring the problem of the balance of the two forces?
Answer: Friction
2. Which experiment do you think is more plausible in the picture on the right?
Answer: A, it should be that object B is subjected to a large friction force, which has a great impact on the experimental effect.
3. How to tell if an object is in equilibrium?
Answer: When the object remains at rest or in a uniform linear motion, the object can be judged to be in equilibrium.
4. How to change the tensile force in the experiment?
Answer: By changing the number of weights, the pulling force on the trolley can be changed.
5. What is the function of the fixed pulley?
Answer: Change the direction of the pulling force.
6. How to explore two forces acting on the same object?
A: Cut the cardboard from the middle and observe if the cardboard is still in equilibrium.
7. How to explore two forces in the same straight line?
Answer: Turn the cardboard at an angle, then let go and observe the movement of the trolley.
8. Experimental conclusion:
Two force equilibrium conditions: homogeneous, equal large, directional, collinear.
9. Experimental Methods:
Control variable method
10. Reasons for choosing a stationary state:
The constant motion state is not easy to control
11. The purpose of conducting multiple experiments:
Derive universal laws and avoid chances.
Explore what factors are involved in sliding friction
(1) Measuring principle:
Two forces are balanced
(2) Measurement method:
Put the wooden block on the horizontal long plank, pull the wooden block horizontally with a spring dynamometer, make the wooden block move at a uniform speed, and read out that the tensile force at this time is equal to the magnitude of the sliding friction force.
(3) Conclusion:
When the roughness of the contact surface is the same, the greater the pressure, the greater the sliding friction. When the pressure is the same, the rougher the contact surface, the greater the sliding friction. The study used a control variable approach.
(4) Application of conversion method:
The magnitude of friction is reflected by the magnitude of the pulling force
(5) From the first two conclusions, it can be summarized as follows:
The magnitude of sliding friction is related to the magnitude of the pressure and the roughness of the contact surface. The sliding friction can also be studied in experiments regardless of the size of the contact surface, the speed of movement, the size of the tensile force, etc.
Experiments to explore the relationship between force and movement
1. How to slide down when you get off the car in this experiment? What is the purpose?
Answer: Let the trolley slide down from the same height of the inclined plane from the standstill; The aim is to make the speed of the trolley the same when it reaches the horizontal plane.
2. How do you know that the smaller the resistance, the slower the motion of the object?
Answer: The distance of the trolley's movement, the less resistance, and the farther the trolley moves, the slower the object's motion decreases.
3. What is the function of this experimental slope?
A: Make the trolley slide down at the same speed.
4. Experimental conclusion:
The less resistance the object is subjected to, the farther the object will move, and if the object is not resisted, it will keep moving in a straight line and keep moving.
5. What is the method used in this experiment?
Answer: Control variable method conversion method
6. Can Newton's first law be explored experimentally?
Answer: No, it can only be deduced on an experimental basis, because there is no such thing as an object that is not subject to force.
7. Which experiment can be done with a slight modification of this experiment?
Answer: (1) The inclined plane is longer, and the scale and stopwatch can be used to explore the experiment of speed change (2) To ensure that the material of the plane is the same, the experiment of adding a wooden block can explore the factors related to the size of kinetic energy.
Newton's First Law:
Explanation: A. Newton's first law is generalized through further reasoning on the basis of a large number of empirical facts, so it is impossible to directly prove Newton's first law by experiment.
- The connotation of Newton's first law: if the object is not subjected to force, the original stationary object will remain at rest, and the original moving object, no matter what the original motion is, the object will move in a uniform straight line.
- Newton's first law tells us that an object can move in a uniform linear motion without force, that is, force has nothing to do with motion, so force is not the cause of generating or maintaining the motion of the object.
- Force is not what sustains the motion of an object, but rather what changes the state of motion of the object.
Explore the factors that are involved in the effects of stress
1. The methods used in this experiment are:
Control variable method, conversion method.
2. Why did you use a sponge instead of a wooden board for this experiment?
Answer: The wooden board is not easy to deform, while the sponge is easy to deform, which is convenient for observing the experimental phenomenon.
3. Comparing Figure A and Figure B, we can get the following conclusions:
When the force area is constant, the greater the pressure, the more obvious the effect of the pressure. (It cannot be said that the pressure is more obvious, because the concept of pressure has not been proposed when this experiment was explored.)
4. Comparing Figure B and Figure C, the experimental conclusion can be obtained:
When the pressure is constant, the larger the force area, the more obvious the effect of the pressure. The size of the sponge deformation is used to measure the size of the pressure effect, which is the conversion method; In order to explore which factors affect the effect of pressure, the method of controlling variables is adopted.
5. If the sponge in Figure B is replaced with a wooden plank, will the effect of pressure be the same as that of the sponge?
Answer: the same, because the effect of pressure is related to the pressure and the force area, there are other factors have nothing to do, after changing to a wooden board, we just can't observe the effect.
6. During the experiment, Xiao Ming cut object B into two pieces of different sizes along the vertical direction, as shown in the figure. He found that they had the same effect on the pressure of the sponge, from which he concluded:
The effect of pressure is independent of the force area. Do you think he did the right thing in the inquiry? The reason: she did not control the same amount of pressure.
Find out what factors are involved in liquid pressure
1. From Figure 1 and Figure 2, we can know that the causes of liquid pressure are:
Liquids are subjected to gravity; Liquids are fluid. (Therefore, in the case of weightlessness in space, the liquid does not generate pressure)
2. What methods are used in the experiment to explore what factors are related to liquid pressure?
Answer: Control variable method, conversion method
3. By observing what is open, do you know how strong the liquid pressure is?
Answer: The height difference of the pages in the U-shaped tube, the height difference is also large, indicating that the pressure generated by the liquid is also large."
4. Two operations before the experiment:
(1) Check whether the liquid level on the left and right sides of the U-shaped tube is flat.
(2) Check the air tightness of the device: (press the rubber film on the metal box by hand to observe whether the liquid level in the U-shaped tube changes, if the change is obvious, the air tightness is good)
5. What is the reason why the height difference in the U-shaped tube has not changed during the experiment? How to fix it?
A: The air tightness is not good, remove it and reinstall it.
6. Is the U-shaped tube used a feed-through?
A: No, it is not
7. Why is the liquid in the U-shaped tube dyed red in this experiment?
Answer: Make the experimental effect obvious and easy to observe.
8. The conclusion of the experiment is that when the density of the liquid is constant, the deeper the depth, the greater the pressure generated by the liquid.
The conclusion of the comparative ethylene-propylene experiment is that when the depth of the liquid is the same, the higher the density of the liquid, the greater the pressure generated by the liquid.
9. As shown in Figure A and B, what is the relationship between the depth of the metal box in the water and the height difference of the page in the U-shaped tube? Why?
Answer: Equal, because the pressure generated on both sides is equal and the density of the liquid is equal, so the depth is also equal.
10. As shown in Figure C, what is the relationship between the depth of the metal box on the left side and the height difference of the page in the U-shaped tube? Why?
A: The height difference in the U-shaped tube is larger than the depth of the metal box. Because the density of salt water is greater than that of water, and the pressure on both sides is equal, the height difference in the U-shaped tube is larger than the depth of the metal box.
11. Measure the height difference of the pages in the U-shaped tube, can you calculate the pressure of the metal box in the liquid on the left side?
A: Yes, because the pressure on both sides is equal.
Verify the Archimedes' principle experiment
1. The more appropriate operation sequence of the experiment is:
b a c d
2. In the experiment, the overflow cup must be poured with water before the experiment can be done, otherwise what will be the result?
Answer: There will be a buoyant force greater than the gravitational force of the object discharging water
3. The numerical relationship of this experimental spring dynamometer is:
Fa-Fc=Fd-Fb
4. Experimental conclusion:
The buoyant force experienced by the object is equal to the gravitational force of the object discharging the liquid
Explore the leverage equilibrium conditions
1. Adjustment of the lever before the experiment:
Left high left tone, right high right tone. After equilibration, the balance nut cannot be adjusted during the experiment.
2. The purpose of adjusting the leverage to horizontal balance is to:
It is convenient for measuring the force arm
3. Advantages of choosing the end point of leverage as a fulcrum:
Eliminate the effect of the lever's own gravity on the experiment.
4. The advantages of replacing weights with dynamometers are:
The tensile force can be directly measured, and the experiment is convenient to operate.
5. The disadvantages of replacing weights with dynamometers are:
The dynamometer itself has a weight, which has a certain influence on the experiment, making the indication of the spring dynamometer larger.
6. As shown in the figure above, in Figure B and Figure C, how does the indication of the spring dynamometer change:
The reason for the change from large to small is that the strength arm changes from small to large.
7. Which diagram do you think is more reasonable?
Answer: Ding Tu, the experiment is not affected by the gravity of the spring dynamometer.
8. The ultimate purpose of using a spring dynamometer instead of a hook code is:
More correct understanding of the power arm.
9. The purpose of multiple experiments is to:
To avoid the contingency of experiments, the conclusion has a universal law.
Explore the mechanical efficiency of pulley blocks
(1) Principle: useful work/total work
(2) The physical quantity that should be measured: the gravity of the hook code G, the height h of the hook code lifting, the tensile force F, and the distance S of the free end of the rope
(3) Equipment: In addition to hook codes, iron frames, pulleys, and fine wires, scales and spring dynamometers are also required.
(4) Steps: The spring dynamometer must be pulled at a uniform speed to raise the hook code, and the purpose is to ensure that the size of the dynamometer remains unchanged.
(5) Conclusion: The main factors affecting the mechanical efficiency of pulley block are:
A: The heavier the movable pulley, the more the number of additional pulleys, the more additional work there is.
B. The heavier the lifting weight, the more useful work will be done.
C friction, if the friction is larger, the extra work will be done.
The same pulley block: the winding method and the lifting height of the heavy object do not affect the mechanical efficiency of the pulley.
(6) Why can't the spring dynamometer be stationary when measuring mechanical efficiency? What about mechanical efficiency?
Answer: When the spring dynamometer is stationary, there is no friction between the rope and the pulley, the tensile force indicator will become smaller, the proportion of useful work in the total work will increase, and the mechanical efficiency of the pulley block will be larger.
Explore the factors that determine the amount of kinetic energy
(1) Conjecture: The magnitude of kinetic energy is related to the mass and velocity of the object;
(2) Experimental research:
Subject: Small steel balls
Methods: Control variable method; Conversion method
How to judge the size of kinetic energy: look at the distance of the wooden block pushed by the small steel ball
The purpose of releasing steel balls of different masses from the same height at rest: so that the initial velocity of the balls when they reach the horizontal plane is the same;
How to change the speed of the steel ball: make the steel ball roll down from different heights;
(3) Analysis and induction: when the mass of the steel ball remains unchanged, the conclusion is that when the mass of the moving object is the same, the greater the velocity, the greater the kinetic energy;
When the velocity of the steel ball is kept constant, the conclusion is that when the velocity of the moving object is the same, the greater the mass, the greater the kinetic energy.
(4) Conclude that the kinetic energy of an object is related to mass and velocity; The greater the speed, the greater the kinetic energy, and the greater the mass, the greater the kinetic energy.
(5) The function of the inclined plane: it is the object that has velocity and changes the magnitude of the object's velocity.
(6) The function of the horizontal plane: the object is balanced by force in the vertical direction, and only by friction in the horizontal direction.
(7) Energy conversion: the process of mechanical energy conversion on the inclined plane: the conversion of gravitational potential energy into kinetic energy, and the process of ability conversion on the horizontal plane: the conversion of kinetic energy into internal energy.
(8) The reason why the wooden block finally stopped: friction on the horizontal plane.
(9) Experimental reasoning and hypothesis: When the horizontal plane is absolutely smooth, the ball will be made at a uniform speed and a straight line with a constant velocity, which cannot achieve the purpose of exploration.
(10) Judgment of overload and overspeed: when overspeeding, the mass remains unchanged, and the greater the speed, the greater the kinetic energy; When overloaded, the velocity remains the same, and the greater the mass, the greater the kinetic energy.
(11) The disadvantage of using the ball in the experiment is that it cannot be guaranteed that the ball moves in a straight line on the horizontal plane.
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