The movement of a body thrown vertically upward is weightless briefly. Free fall. Weightlessness. Graphic representation of movement
Lesson 15. Movement of a body thrown vertically upward. Weightlessness (Fedosova O.A.)
Lesson text
Abstract
Name of the subject - physics Class - 9 UMK (name of the textbook, author, year of publication) - Physics. 9th grade: textbook / A.V. Peryshkin, E.M. Gutnik. - M.: Bustard, 2014. Level of training (basic, advanced, specialized) - basic Lesson topic - Movement of a body thrown vertically upward. Weightlessness Laboratory work No. 2 “Measuring the acceleration of free fall.” The total number of hours allocated to study the topic - 1 Place of the lesson in the system of lessons on the topic - 15/15 The purpose of the lesson is to identify and prove what determines the free fall of bodies and the movement of a body thrown vertically upward, using Galileo's formula. Objectives of the lesson - To give the concept of the free fall of bodies and its features. Study the history of the discovery of the laws of this movement. Learn how to perform calculations when bodies are in free fall. Explain the significance of G. Galileo’s experiments. Continue to develop the ability to express conclusions; Development of independence in judgment; Development of logical thinking; develop the ability to conduct thought experiments; develop students' memory and attention; develop the ability to solve high-quality problems. Continue to develop an attitude towards physics as an interesting and necessary science; To instill in children respect and goodwill towards each other, the ability to listen to a friend’s answer; Encourage students to be neat when working with notes in notebooks. Planned results - - Observe experiments indicating the state of weightlessness of bodies; - draw a conclusion about the conditions under which bodies are in a state of weightlessness; -measure the acceleration of free fall; Technical support for the lesson - computer, multimedia projector Additional methodological and didactic support for the lesson (links to Internet resources are possible) - presentation for the lesson from the disk “Physics 9th grade” from VIDEOUROKI.NET https://videouroki.net/look/diski/fizika9 /index.html, test 15 “Free fall” Author: © 2014, COMPEDU LLC, http://compedu.ru Lesson content 1. Organizational stage 1. Mutual greeting of the teacher and students; checking absentees using the log. 2. Updating the subjective experience of students Training tasks and questions: 1. Free fall is called_ 2. Free fall by its nature is_ 3. Acceleration of free fall g = _ 4. Do all bodies fall with the same acceleration? Why?_ 5. Why does a pellet fly faster than a feather in a room if they fall from the same height?_ 6. How long will it take a body to fall from a height of h = 11.25 m? _ 3. Laboratory work No. 2 “Measuring the acceleration of free fall” Purpose: to measure the acceleration of free fall using a device for studying the motion of bodies. Contents of work To conduct experiments, use guide plane 1, carriage 2, sensors 3, electronic stopwatch 4, plastic mat 5 (Fig.). The acceleration due to gravity can be determined by measuring the distance and time of movement from rest. To accurately measure the time of the fall, an electronic stopwatch 4 with magnetic sensors 3 is used. Starting and stopping the electronic stopwatch can be done either by pressing “Start/Stop” or using magnetically controlled reed contacts - in remote sensors 3. The reed switch (sealed contact) consists of two close located elastic metal contacts, which, when introduced into a magnetic field or when approached, are magnetized and attracted to each other. As a result, the section of the electrical circuit connected to the reed switch terminals is closed. The circuit of the electronic stopwatch is designed so that the first time the electrical contacts at its input are closed, the stopwatch starts, the next time it closes, the stopwatch stops. The reed switches are controlled by a small permanent magnet mounted in the middle of the outer side of the carriage 2. Procedure for performing the task Set the guide plane almost vertically to reducing the influence of friction force. Using magnetic holders, attach the sensors to the guide plane, one at its upper edge, the other at the lower edge. By pressing the “Reset” button, set zero on the scale of the electronic stopwatch. Check the operation of the stopwatch by alternately applying the carriage magnet first to the first sensor, then to the second sensor. The stopwatch should begin measuring time when the magnet is brought to the top sensor and stop measuring when the magnet is brought to the bottom sensor. The numbers on the scale before the dot indicate seconds, the numbers after the dot indicate tenths and hundredths of a second. Measure the distance s between the sensors. Release the carriage and measure the time t of its free fall. Repeat measurements 5 times. Calculate the acceleration of gravity: g = 2s /t 2 Find the arithmetic mean value of the acceleration of gravity. No. Time of movement t, s Path s, m Gravity acceleration g, m/s 2 1 2 3 4 5 Determine the deviation of the g value you obtained from the actual value equal to 9.8 m/s2 (i.e. find the difference between them ). Calculate what part (in percent) this difference is from the actual value of g. This ratio is called the relative error ε. The smaller the relative error, the higher the measurement accuracy. ε =| g avg – g| /g 4. Studying new knowledge and methods of activity (working with presentation slides) And he soared under the clouds; For a moment he disappeared - and Noisy, from above, flies towards the prince again. The nimble knight flew away And into the snow with a fatal swing the Sorcerer fell - and sat there... A. S. Pushkin (Ruslan and Lyudmila) An interesting example of rectilinear uniformly accelerated motion is the free fall of a body and the movement of a body thrown vertically. Such movements of bodies were studied by the famous Italian scientist Galileo Galilei. He established that these movements are uniformly accelerated. Measurements have shown that during such movements the acceleration is directed vertically downwards and in absolute value is equal to approximately 9.8 meters divided per second squared. What is especially surprising and for a long time was a mystery is that this acceleration is the same for all bodies. When solving problems involving the free fall of bodies, it is natural to direct the coordinate axis vertically up or down, and choose the Earth as the reference body. The coordinate of a point on the axis is its height above the Earth's surface (or depth below the Earth's surface). The formulas for the speed, displacement and coordinates of a freely falling body and a body thrown vertically are no different from the formulas for rectilinear uniformly accelerated motion. In these formulas, zhe is the projection onto the coordinate axis of the acceleration vector of the free fall of bodies; it is positive and equal to + 9.8 meters divided per second squared if the coordinate axis is directed downward, and –9.8 meters divided per second squared if the coordinate axis is directed upward. Let's consider the most common movements of bodies under the influence of gravity - the free fall of bodies along a rectilinear and curvilinear trajectory. Free fall of bodies along a rectilinear trajectory Let us solve the following problem. Problem 1. A body falls freely without initial speed from a height h above the Earth's surface. Determine the time of movement and the speed of the body at the last moment of movement. As we already know, the free fall of bodies is uniformly accelerated motion, therefore, to solve this problem, we will use the formulas for uniformly accelerated motion for the body’s coordinates and velocity. Let us write down the initial conditions of motion. And let's substitute them into the equation of motion. From the resulting equation of motion it is easy to determine the time of flight of the body; it is equal to the square root of twice the height divided by the acceleration of gravity. If we now substitute the resulting value of the time interval into the speed equation, we can easily obtain a formula for calculating the speed at the last moment of movement. As we can see, the speed is equal to minus the square root of two. The minus sign indicates that the movement of the body, in our case, occurs against the coordinate axis. Problem 2. The ball was thrown upward with an initial speed ve zero directed vertically upward. Determine: the time of the entire movement; speed at the last moment of movement, and also to what maximum height will the body rise? As in the previous problem, we will use the formulas for uniformly accelerated motion of a body. We write down the initial conditions of motion. And we substitute them into the equation of motion. Then the equations of motion will be written in the form: yrek equals ve zero te minus je te square hit; and ve is equal to ve zero minus je te. Let's find the entire time of movement of the ball, taking into account that at the last moment of its movement its coordinate is equal to zero: Zero is equal to ve zero te minus je te squared in half. Solving the resulting quadratic equation for te, we find its roots. The root of the equation equal to zero corresponds to the initial moment of time. Thus, the time of the entire flight of the ball is determined by the formula: te is equal to two ve zero divided by zhe The speed of the body at the last moment of movement will be determined from the speed equation for uniformly accelerated motion, substituting the flight time of the ball into it. It turns out that at what speed we throw the ball vertically upward, at the same speed it will come back. To determine the maximum height of the ball, we need to determine the period of time during which the ball will rise to this height. From the velocity equation it can be seen that the ball moves uniformly slow upward until it reaches its maximum height, then stops for a moment and then begins to move uniformly accelerated downward. Considering that at the top point of the trajectory the speed of the ball is zero, we determine the rise time. As we see, it is equal to half the time of the entire movement. Now, if we substitute the resulting value of the time interval into the equation of motion, then we will determine the maximum flight height of the ball. The weight of a body moving under the influence of gravity alone is zero. This can be verified using the experiments shown in Figure 31. A metal ball is suspended from a homemade dynamometer. According to the readings of the dynamometer at rest, the weight of the ball (Fig. 31, a) is 0.5 N. If the thread holding the dynamometer is cut, then it will fall freely (air resistance in this case can be neglected). At the same time, its pointer will move to the zero mark, indicating that the weight of the ball is zero (Fig. 31, b). The weight of a freely falling dynamometer is also zero. In this case, both the ball and the dynamometer move with the same acceleration, without exerting any influence on each other. In other words, both the dynamometer and the ball are in a state of weightlessness. In the experiment considered, the dynamometer and the ball fell freely from a state of rest. Now let’s make sure that the body will be weightless even if its initial speed is not zero. To do this, take a plastic bag and fill it about 1/3 with water; then remove the air from the bag by twisting it top part into a tourniquet and tied in a knot (Fig. 31, c). If you take the bag by the lower part filled with water and turn it over, then the part of the bag twisted into a rope under the influence of the weight of the water will unwind and fill with water (Fig. 31, d). If, when turning the bag over, you hold the tourniquet, not allowing it to unwind (Fig. 31, e), and then throw the bag up, then both during the rise and during the fall the tourniquet will not unwind (Fig. 31, f). This indicates that during the flight the water does not exert its weight on the bag, as it becomes weightless. You can throw this package to each other, then it will fly along a parabolic trajectory. But even in this case, the package will retain its shape in flight, which it was given when thrown. 5. Fastening the material in the form of a test with mutual verification: 1. What is the body free from during free fall? a) from mass b) from gravity c) from air resistance d) from all of the above 2. In the tube from which the air has been pumped out, there are a pellet, a cork and a bird feather at the same height. Which of these bodies will be the last to reach the bottom of the tube when they fall freely from the same height? a) pellet b) cork c) bird feather d) all three bodies will reach the bottom of the tube at the same time 3. In the absence of air resistance, the speed of a freely falling body in the fifth second of fall increases by a) 10 m/s b) 15 m/s c) 30 m/s d) 45 m/s 4. A stone begins to fall freely from a high steep cliff. What speed will it have 3s after the start of the fall? Air resistance is negligible. a) 30 m/s b) 10 m/s c) 3 m/s d) 2 m/s 5. An icicle, falling from the edge of the roof, flew to the Earth in 3 s. The distance of the icicle is approximately a) 12 m b) 24 m c) 30 m d) 45 m Check your answers. Question number 1 2 3 4 5 Answers in d a a d 6. Homework §14, test 15 “Free fall”
Agreement
Rules for registering users on the website "QUALITY MARK":
It is prohibited to register users with nicknames similar to: 111111, 123456, ytsukenb, lox, etc.;
It is prohibited to re-register on the site (create duplicate accounts);
It is prohibited to use other people's data;
It is prohibited to use other people's e-mail addresses;
Rules of conduct on the site, forum and in comments:
1.2. Publication of personal data of other users in the profile.
1.3. Any destructive actions in relation to this resource (destructive scripts, password guessing, violation of the security system, etc.).
1.4. Using obscene words and expressions as a nickname; expressions that violate laws Russian Federation, standards of ethics and morality; words and phrases similar to the nicknames of the administration and moderators.
4. Violations of the 2nd category: Punishable by a complete ban on sending any types of messages for up to 7 days. 4.1. Posting information that falls under the Criminal Code of the Russian Federation, the Administrative Code of the Russian Federation and is contrary to the Constitution of the Russian Federation.
4.2. Propaganda in any form of extremism, violence, cruelty, fascism, Nazism, terrorism, racism; inciting interethnic, interreligious and social hatred.
4.3. Incorrect discussion of work and insults to the authors of texts and notes published on the pages of the "SIGN OF QUALITY".
4.4. Threats against forum participants.
4.5. Posting deliberately false information, slander and other information discrediting the honor and dignity of both users and other people.
4.6. Pornography in avatars, messages and quotes, as well as links to pornographic images and resources.
4.7. Open discussion of the actions of the administration and moderators.
4.8. Public discussion and assessment of current rules in any form.
5.1. Swearing and profanity.
5.2. Provocations (personal attacks, personal discredit, formation of negative emotional reaction) and bullying of discussion participants (systematic use of provocations towards one or more participants).
5.3. Provoking users to conflict with each other.
5.4. Rudeness and rudeness towards interlocutors.
5.5. Getting personal and clarifying personal relationships on forum threads.
5.6. Flooding (identical or meaningless messages).
5.7. Intentionally misspelling nicknames or names of other users in an offensive manner.
5.8. Editing of quoted messages, distorting their meaning.
5.9. Publication of personal correspondence without explicit expressed consent interlocutor.
5.11. Destructive trolling is the purposeful transformation of a discussion into a skirmish.
6.1. Overquoting (excessive quoting) of messages.
6.2. Use of a red font intended for corrections and comments by moderators.
6.3. Continuation of discussion of topics closed by a moderator or administrator.
6.4. Creating topics that do not carry semantic content or are provocative in content.
6.5. Creating a topic or message title in whole or in part in capital letters or foreign language. An exception is made for titles of permanent topics and topics opened by moderators.
6.6. Create a signature in a font larger than the post font, and use more than one palette color in the signature.
7. Sanctions applied to violators of the Forum Rules
7.1. Temporary or permanent ban on access to the Forum.
7.4. Deleting an account.
7.5. IP blocking.
8. Notes
8.1. Sanctions may be applied by moderators and administration without explanation.
8.2. Changes may be made to these rules, which will be communicated to all site participants.
8.3. Users are prohibited from using clones during the period of time when the main nickname is blocked. In this case, the clone is blocked indefinitely, and the main nickname will receive an additional day.
8.4 Message containing obscene language, can be edited by a moderator or administrator.
9. Administration The administration of the site "SIGN OF QUALITY" reserves the right to delete any messages and topics without explanation. The site administration reserves the right to edit messages and the user's profile if the information in them only partially violates the forum rules. These powers apply to moderators and administrators. The administration reserves the right to change or supplement these Rules as necessary. Ignorance of the rules does not relieve the user from responsibility for violating them. The site administration is not able to verify all information published by users. All messages reflect only the opinion of the author and cannot be used to evaluate the opinions of all forum participants as a whole. Messages from site employees and moderators are an expression of their personal opinions and may not coincide with the opinions of the editors and management of the site.
Subject: Free fall. Weightlessness
- Lesson type: combined.
- The purpose of the lesson: give students an idea of the free fall of bodies, as a special case of uniform motion, in which the magnitude of the acceleration vector is a constant value for all bodies; develop the ability to calculate the coordinate and speed of a body at any time of a free falling body; give the concept of weightlessness.
- Equipment for the lesson: ball, sheet of paper, paper ball, metal coin, paper coin, balls of various masses, Newton tube, PC and ID.
- 1. Preparation for the perception of the main material.
- 2. Studying new material.
- 3. Fixing the material.
- 4. Lesson summary.
- 5. Homework.
- 1. Independent work:
- Option 1. 1) What is the mass of a body to which a force of 10 N imparts an acceleration of 2 m/s2?
- 2) What can be the modulus of the resultant forces of 25 N and 10 N?
- Option 2.1) What acceleration does a force of 20 N impart to a body weighing 2 kg?
- 2) One of the forces acting on the body is equal to 15 N. What is the value of the second force if the modulus of the resultant of these forces is equal to 5 N?
- 1) Read and write Newton’s third law mathematically.
- 2) How does uniformly accelerated motion differ from uniform motion?
- 3) Write down the formula for determining speed during uniformly accelerated motion.
- 4) Write down the formula for determining displacement during uniformly accelerated motion.
- 5) What patterns are inherent in uniformly accelerated motion?
- 6) Name the features of Newton’s third law
- Since the force of gravity acting on all bodies near the Earth’s surface is constant, a freely falling body must move with constant acceleration, that is, uniformly accelerated.
1.Historical information.
- Aristotle's theory: The heavier the body, the faster it falls.
- contradiction: if a light body falls slower than a heavy one, will the light body and the heavy one fall more slowly(?), or faster since one is heavier?
- 1) Falling sheet of paper
- and a paper ball. 2)
- 2) Drop various
- by mass of balls.
- 3) Paper drop and
- metal coin 3)
- separately and together.
- Experiments with balls of different masses dropped from the Leaning Tower of Pisa.
- The balls landed almost simultaneously.
- Consequently, if air resistance can be neglected, all falling bodies move uniformly with the same acceleration.
- We come to the same conclusion when studying stroboscopic photographs.
- - photographing a falling ball at regular intervals (page 53 of the textbook), the photos prove that the movement of the ball is uniformly accelerated and the acceleration of gravity g = 9.8 m/s 2
- denoted by the letter g from the Latin word gravitas (“gravitas”), which means “heaviness.”
- Experiments carried out using a Newton tube
confirm that the acceleration of gravity at a given point on the Earth does not depend on the mass, density and shape of falling bodies.
5. Explanation of the fall of bodies of different masses at different speeds .
- F 1 =F t + F c F 2 =F t + F c
- F c F c
- F 1 F t
- F t F t =mg=m . 9.8m/s 2
Formulas characterizing uniformly accelerated motion
Uniformly accelerated motion
Free fall
V x =V ox +a x t
Movement of a body thrown upward
S x =V ox t+(a x t 2)/2
S y =V oy t+(gt 2)/2
V y =V o y -gt
X = X 0 +V x0 t+(a x t 2)/2
S=V oy t-(gt 2)/2
У=У 0 +V 0y t+(g y t 2)/2
У= V 0y t-(g y t 2)/2
3. Dependence of the speed and coordinates of a falling body on time.
3. Dependence of the speed and coordinates of a body thrown vertically upward on time.
- Let the initial position of the body be the origin of coordinates, let the OU axis be directed downward, then the graphs V y (t) and Y (t) :
Weightlessness is a state in which the weight of a body is zero.
- This state occurs if only the force of gravity acts on the body; the body moves translationally with the acceleration of free fall.
- That is, a body suspended on a spring does not cause any deformation of the spring, and a body lying motionless on a support does not exert any force on it.
- x P= m (g - a) g=a P=0
- 1.Ex. 13 (2) A pencil falls from a table 80 cm high to the floor. Determine the time of its fall.
- 2. Will the time of free fall of different bodies from the same height be the same?
- 3. The stone fell from one cliff in 2s, and from the other in 6s. How many times is the second rock higher than the first?
- Homework:
- § 13, 14, ex.13 (1.3); No. 192, 204, 207.
- Answer the questions after the paragraph, know the abstracts written in the notebook.
This video tutorial is intended for self-study topic “Motion of a body thrown vertically upward.” In this lesson, students will gain an understanding of the motion of a body in free fall. The teacher will talk about the movement of a body thrown vertically upward.
In the previous lesson, we looked at the issue of the movement of a body that was in free fall. Let us recall that free fall (Fig. 1) is a movement that occurs under the influence of gravity. The force of gravity is directed vertically downwards along the radius towards the center of the Earth, acceleration of gravity at the same time equal to .
Rice. 1. Free fall
How will the movement of a body thrown vertically up differ? It will differ in that the initial speed will be directed vertically upward, i.e., it can also be counted along the radius, but not towards the center of the Earth, but, on the contrary, from the center of the Earth upward (Fig. 2). But the acceleration of free fall, as you know, is directed vertically downward. This means that we can say the following: the upward movement of a body in the first part of the path will be a slow motion, and this slow motion will also occur with the acceleration of free fall and also under the influence of gravity.
Rice. 2 Movement of a body thrown vertically upward
Let's look at the picture and see how the vectors are directed and how this fits into the reference frame.
Rice. 3. Movement of a body thrown vertically upward
In this case, the reference frame is connected to the ground. Axis Oy is directed vertically upward, just like the initial velocity vector. The body is acted upon by a force of gravity directed downward, which imparts to the body the acceleration of free fall, which will also be directed downward.
The following thing can be noted: the body will move slowly, will rise to a certain height, and then will start quickly fall down.
We have indicated the maximum height.
The motion of a body thrown vertically upward occurs near the Earth's surface, when the acceleration of free fall can be considered constant (Fig. 4).
Rice. 4. Near the Earth's surface
Let us turn to the equations that make it possible to determine the speed, instantaneous speed and distance traveled during the movement in question. The first equation is the velocity equation: . The second equation is the equation of motion for uniformly accelerated motion: .
Rice. 5. Axis Oy upward
Let's consider the first frame of reference - the frame of reference associated with the Earth, the axis Oy directed vertically upward (Fig. 5). The initial speed is also directed vertically upward. In the previous lesson, we already said that the acceleration of gravity is directed downwards along the radius towards the center of the Earth. So, if we now bring the velocity equation to this reference frame, we get the following: .
This is a projection of speed at a certain point in time. The equation of motion in this case has the form:
.
Rice. 6. Axle Oy pointing down
Let's consider another frame of reference, when the axis Oy directed vertically downwards (Fig. 6). What will change from this?
. The projection of the initial velocity will have a minus sign, since its vector is directed upward, and the axis of the selected reference system is directed downward. In this case, the acceleration of gravity will have a plus sign, because it is directed downward. Equation of motion: 
.Another very important concept to consider is the concept of weightlessness.
Definition.Weightlessness- a state in which a body moves only under the influence of gravity.
Definition. Weight- the force with which a body acts on a support or suspension due to attraction to the Earth.
Rice. 7 Illustration for determining weight
If a body near the Earth or at a short distance from the Earth’s surface moves only under the influence of gravity, then it will not affect the support or suspension. This state is called weightlessness. Very often, weightlessness is confused with the concept of the absence of gravity. In this case, it is necessary to remember that weight is the action on the support, and weightlessness- this is when there is no effect on the support. Gravity is a force that always acts near the surface of the Earth. This strength is the result gravitational interaction with the Earth.
Let's pay attention to one more important point, associated with the free fall of bodies and movement vertically upward. When a body moves upward and moves with acceleration (Fig. 8), an action occurs that leads to the fact that the force with which the body acts on the support exceeds the force of gravity. When this happens, the state of the body is called overload, or the body itself is said to be under overload.
Rice. 8. Overload
Conclusion
The state of weightlessness, the state of overload are extreme cases. Basically, when a body moves on a horizontal surface, the weight of the body and the force of gravity most often remain equal to each other.
Bibliography
- Kikoin I.K., Kikoin A.K. Physics: Textbook. for 9th grade. avg. school - M.: Education, 1992. - 191 p.
- Sivukhin D.V. General physics course. - M.: State Publishing House of Technology
- theoretical literature, 2005. - T. 1. Mechanics. - P. 372.
- Sokolovich Yu.A., Bogdanova G.S. Physics: A reference book with examples of problem solving. - 2nd edition, revision. - X.: Vesta: Ranok Publishing House, 2005. - 464 p.
- Internet portal “eduspb.com” ()
- Internet portal “physbook.ru” ()
- Internet portal “phscs.ru” ()
Homework
The force of gravity acts on all bodies on Earth: resting and moving, located on the surface of the Earth and near it.
A body freely falling to the ground moves uniformly accelerated with increasing speed, since its speed is co-directed with the force of gravity and the acceleration of gravity.
A body thrown up, in the absence of air resistance, also moves with constant acceleration caused by the action of gravity. But in this case, the initial speed v0, which was given to the body during the throw, is directed upward, i.e., opposite to the force of gravity and the acceleration of free fall. Therefore, the speed of the body decreases (for each second - by an amount numerically equal to the module of acceleration of free fall, i.e. by 9.8 m/s).
After a certain time, the body reaches its greatest height and stops at some point, i.e. its speed becomes zero. It is clear that the greater the initial speed of the body when thrown, the longer the rise time will be and the greater the height it will rise by the time it stops.
Then, under the influence of gravity, the body begins to fall down uniformly.
When solving problems on the upward movement of a body under the influence of only gravity, the same formulas are used as for rectilinear uniformly accelerated motion with an initial speed v0, only ax is replaced by gx:
It is taken into account that when moving upward, the velocity vector of the body and the acceleration vector of free fall are directed in opposite sides, so their projections always have different signs.
If, for example, the X axis is directed vertically upward, i.e., co-directed with the velocity vector, then v x > 0, which means v x = v, a g x< 0, значит, g x = -g = -9,8 м/с 2 (где v - модуль вектора мгновенной скорости, a g - модуль вектора ускорения).
If the X axis is directed vertically downward, then v x< 0, т. е. v х = -v, a g x >0, i.e. g x = g = 9.8 m/s 2 .
The weight of a body moving under the influence of gravity alone is zero. This can be verified using the experiments shown in Figure 31.
Rice. 31. Demonstration of weightlessness of bodies in free fall
A metal ball is suspended from a homemade dynamometer. According to the readings of the dynamometer at rest, the weight of the ball (Fig. 31, a) is 0.5 N. If the thread holding the dynamometer is cut, then it will fall freely (air resistance in this case can be neglected). At the same time, its pointer will move to the zero mark, indicating that the weight of the ball is zero (Fig. 31, b). The weight of a freely falling dynamometer is also zero. In this case, both the ball and the dynamometer move with the same acceleration, without exerting any influence on each other. In other words, both the dynamometer and the ball are in a state of weightlessness.
In the experiment considered, the dynamometer and the ball fell freely from a state of rest.
Now let’s make sure that the body will be weightless even if its initial speed is not zero. To do this, take a plastic bag and fill it about 1/3 with water; then remove the air from the bag by twisting its upper part into a rope and tying it in a knot (Fig. 31, c). If you take the bag by the lower part filled with water and turn it over, then the part of the bag twisted into a rope under the influence of the weight of the water will unwind and fill with water (Fig. 31, d). If, when turning the bag over, you hold the tourniquet, not allowing it to unwind (Fig. 31, e), and then throw the bag up, then both during the rise and during the fall the tourniquet will not unwind (Fig. 31, f). This indicates that during the flight the water does not exert its weight on the bag, as it becomes weightless.
You can throw this package to each other, then it will fly along a parabolic trajectory. But even in this case, the package will retain its shape in flight, which it was given when thrown.
Questions
- Does the force of gravity act on a body thrown upward during its ascent?
- With what acceleration does a body thrown upward move in the absence of friction? How does the speed of the body change in this case?
- What determines the maximum height of lift of a body thrown upward in the case when air resistance can be neglected?
- What can be said about the signs of the projections of the vectors of the instantaneous velocity of a body and the acceleration of gravity during the free upward movement of this body?
- Tell us about the course of the experiments shown in Figure 31. What conclusion follows from them?
Exercise 14
A tennis ball is thrown vertically upward with an initial speed of 9.8 m/s. After what period of time will the speed of the rising ball decrease to zero? How much movement will the ball make from the point of throw?
Loading...
