Name: using pendulums to find gravity
Uploaded: 2023-05-16T08:18:41.000Z
Duration: 1 min 12 s
Channel: JoVE

Un objet dans un champ gravitationnel subit une accélération due à la force gravitationnelle.
À l’aide d’un pendule physique, la valeur de l’accélération due à la gravité peut être mesurée.
Considérons une tige uniforme de longueur L et de masse m suspendue à une extrémité. La distance entre le point de pivot et le centre de masse de la tige est la moitié de la longueur de la tige.
Rappelons l’équation de l’inertie de rotation d’une tige, tournant autour d’un axe perpendiculaire passant par son centre de masse.
À l’aide du théorème de l’axe parallèle, l’inertie de rotation autour d’un axe perpendiculaire passant par une extrémité de la tige est déterminée.
Rappelez-vous la période de temps du pendule physique. En mettant les deux côtés au carré et en substituant la valeur de la distance entre le point de pivot et le centre de masse et le moment d’inertie, la formule de l’accélération due à la gravité est déterminée.
Si la longueur de la tige est d’un mètre et qu’elle a une période de temps de 1,64 seconde, la valeur de g est calculée comme étant de 9,8 mètres par seconde carrée.

<ol>
	<li>Walker, J., Halliday, D., Resnick, R. (2007), Fundamentals of Physics, 10e édition, John Wiley & Sons, Inc. ; section 15.4 ; pages 426-427.</li>
	<li>OpenStax. (2019). Physique universitaire Vol. 1. [Version Web]. Extrait de <a href="https://openstax.org/books/university-physics-volume-1/pages/15-4-pendulums">https://openstax.org/books/university-physics-volume-1/pages/15-4-pendulums</a> section 15.4 ; Pages 764 à 768.</li>
</ol>

mesurer l'accélération due à la gravité

Considérez une tasse à café accrochée à un crochet dans un garde-manger. Si la tasse est renversée, elle oscille d'avant en arrière comme un pendule jusqu'à ce que les oscillations s'arrêtent.

Un pendule simple peut être décrit comme une masse ponctuelle et une corde. Pendant ce temps, un pendule physique est un objet dont les oscillations sont similaires à celles d'un pendule simple, mais ne peuvent pas être modélisées comme une masse ponctuelle sur une corde car sa masse est répartie sur une plus grande surface. Le comportement d'un pendule physique peut être modélisé en utilisant les principes du mouvement de rotation et le concept du moment d'inertie. Pour un pendule simple et un pendule physique, la force de rappel est la force de gravité. Avec un pendule simple, la gravité agit sur le centre de la boule du pendule, tandis que dans le cas d'un pendule physique, la force de gravité agit sur le centre de masse (CM) de l'objet.

La période (T) d'un pendule simple dépend de sa longueur et de l'accélération due à la gravité (g). La période est entièrement indépendante d'autres facteurs, tels que la masse et le déplacement maximal. Étant donné la dépendance de T sur g, si la longueur d'un pendule et la période d'oscillation sont précisément connues, elles peuvent être utilisées pour mesurer l'accélération due à la gravité. Cette méthode de détermination de la gravité peut être très précise.

Un pendule physique peut également être utilisé pour mesurer l'accélération de la chute libre due à la gravité dans un endroit particulier de la surface terrestre, des milliers de mesures ayant été réalisées lors de prospections géophysiques.

Mesurer l'accélération due à la gravité

Prenons l’exemple d’une tasse à café suspendue à un crochet dans un garde-manger. Si la tasse est frappée, elle oscille d’avant en arrière comme un pendule jusqu’à ce que les oscillations s’éteignent 
...

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Unit 1

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Oscillations

KEY TERMS AND CONCEPTS

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Measuring Acceleration Due to Gravity

using pendulums to find gravity

Pendulums can be used to measure the acceleration due to gravity. A simple pendulum is a point mass attached to a string. A physical pendulum is an object that swings in a similar way, but its mass is spread out, so it cannot be treated as a point mass on a string.

A physical pendulum is described using rotational motion and moment of inertia. In both types of pendulum, gravity provides the restoring force that brings the object back toward the middle of its swing. For a simple pendulum, gravity acts at the center of the bob. For a physical pendulum, gravity acts at the center of mass of the object.

The period of a simple pendulum depends on its length and the acceleration due to gravity, g. It does not depend on mass or on the maximum displacement. If the length of a pendulum and its period are known with precision, they can be used to find g.

This method can be very accurate. A physical pendulum can also be used to measure the free-fall acceleration due to gravity at a specific location on Earth’s surface. Thousands of such measurements have been made during geophysical prospecting.

Using Pendulums to Find Gravity

simple harmonic motion

Period and Frequency in Simple Harmonic Motion

Simple harmonic motion describes oscillatory motion when the net force follows Hooke's law. In this type of motion, the force acts toward the equilibrium position. If there is no damping from friction or other non-conservative forces, the oscillator moves with equal displacement on both sides of equilibrium. The period of a simple harmonic oscillator is found from the equation of motion. Period is the time for one complete oscillation. Frequency is the inverse of period, so the two values can...

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characteristics of simple harmonic motion

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Simple Harmonic Motion: Period and Phase

Simple harmonic motion depends on how displacement, force, and acceleration work together. In this motion, the acceleration of the system, and therefore the net force, is proportional to the displacement and points in the opposite direction. That restoring force pulls the object back toward equilibrium. The period and frequency of a simple harmonic oscillator do not depend on amplitude. A diving board shows this idea well. A stiff, thick diving board has a large force constant, so it has a...

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oscillations about an equilibrium position

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Stable Equilibrium and Atomic Oscillation

Stable equilibrium is a key idea in oscillation. When an object sits at a stable equilibrium point, a small push moves it away, but it then tends to oscillate around that point. If the equilibrium point is unstable, a slight disturbance will not bring the object back. The transcript also describes a half-stable equilibrium point. It is still considered unstable, but the name comes from the fact that the force on only one side points toward the equilibrium point. That makes it different from a...

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Simple Harmonic Motion: Energy and Speed

Simple harmonic motion combines potential energy and kinetic energy. In a spring-like oscillator, Hooke’s law explains the potential energy stored when the string or spring is stretched or compressed. Because there are no dissipative forces, the object also has kinetic energy as it moves.

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frequency of spring-mass system

How Mass and Spring Stiffness Set Oscillation

A spring-mass system shows simple harmonic motion, and its oscillation rate depends on the mass and the spring force constant. In this model, the angular frequency, period, and frequency are set by those two values. They do not depend on the motion’s amplitude or on the initial conditions. To find these relationships, the system can be analyzed with Newton’s second law. Imagine a block on a spring on a frictionless surface. Three forces act on the mass: weight, the normal force, and the spring...

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simple harmonic motion and uniform circular motion

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Simple Harmonic Motion from Circular Motion

Simple harmonic motion and uniform circular motion are closely linked. Simple harmonic motion is oscillatory motion in a system where the net force follows Hooke’s law. Uniform circular motion is motion along a circular path at constant speed. A simple way to create simple harmonic motion is to use uniform circular motion. One example is a ball attached to a vertical turntable that spins at a steady rate. If you watch the ball’s shadow on the floor, the shadow, or projection, moves in simple...

problem solving: energy in simple harmonic motion

problem-solving-energy-in-simple-harmonic-motion

Energy Conservation in Simple Harmonic Motion

Energy conservation in simple harmonic motion explains how a spring and mass system shares energy as it moves back and forth around equilibrium. Simple harmonic motion, or SHM, is periodic motion in time and position. The object oscillates with a constant amplitude and frequency. In SHM, kinetic energy and potential energy change into each other continuously. A good example is the spring in a car shock absorber as the car moves on a bumpy road. The wheel attached to the spring oscillates in...

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simple pendulum

Pendulum Motion and Period

A simple pendulum is a small ball, or bob, hanging from a string that has negligible mass and does not stretch. Pendulums appear in everyday life, including clocks, swing sets, and a sinker on a fishing line. Their motion is a useful model for studying repeating movement. The period of a simple pendulum depends on two factors: its length and the acceleration due to gravity. The period does not depend on the bob’s mass or on the maximum displacement. For small angles, the pendulum behaves like...

torsional pendulum

Angular Oscillation in a Torsion String

Angular oscillation in a torsion string is shown by a rigid body suspended from a string that twists as it moves. The twisting of the string provides the restoring force. The string is ideally massless, and in practice its mass is much smaller than the rigid body&amp;#39;s mass, so it is neglected. When the angular displacement is small, the motion can be modeled as linear angular oscillation. The amplitude is measured as an angle. In this system, the role of mass is played by the rigid...

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physical pendulum

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A physical pendulum is a rigid body that hangs from a fixed pivot point and swings when it is displaced. Its motion is similar to a simple pendulum. To describe that motion, physicists use the small-angle approximation, where sin θ ≈ θ. This approximation is valid up to about 14°. The period and angular frequency of a physical pendulum can be found by comparing it with a spring-mass system. The mass moment of inertia is the key quantity that describes how the body’s mass is spread around the...

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measuring acceleration due to gravity

Pendulums can be used to measure the acceleration due to gravity. A simple pendulum is a point mass attached to a string. A physical pendulum is an object that swings in a similar way, but its mass is spread out, so it cannot be treated as a point mass on a string. A physical pendulum is described using rotational motion and moment of inertia. In both types of pendulum, gravity provides the restoring force that brings the object back toward the middle of its swing. For a simple pendulum,...

damped oscillations

Energy Loss in Oscillations

Oscillations in the real world often lose energy as they move. A system that keeps the same amplitude without any loss is called undamped. That kind of motion is rare, because friction and other non-conservative forces usually reduce the motion over time. Common examples show this clearly. A guitar string stops vibrating a few seconds after it is plucked. A swing also needs a person to keep pushing it if the motion is to continue. In some devices, such as car shock absorbers, damping is useful...

types of damping

How Damping Changes Oscillations

Damping changes how a system oscillates by reducing its back-and-forth motion. As damping increases, the period and frequency begin to change because damping opposes motion and lowers the net force in both directions. When damping is very large, the system stops oscillating altogether. It then moves slowly toward equilibrium. An overdamped system reaches equilibrium slowly. An underdamped system reaches equilibrium faster, but it oscillates around the equilibrium point while doing so. A...

forced oscillations

Resonance in Driven Oscillators

Resonance in driven oscillators describes how a periodic force changes an object’s motion. A driven oscillator may first move in a messy way. These early motions are called transients. After the transients die out, the oscillator reaches a steady state. In this state, the motion becomes periodic and the displacement is determined. The amplitude of a driven oscillator can be found from the equations for force and motion. That amplitude depends on the natural frequency of the system and the...

concept of resonance and its characteristics

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Resonance in Music, Cars, and Radios

Resonance is a key feature of driven oscillators, and it depends strongly on damping. Damping is the process that reduces motion by removing energy from the system. When a system must resonate at one specific frequency, very light damping is needed. Piano strings and many other musical instruments are common examples of this kind of response. Heavy damping works better when the goal is small-amplitude oscillations. A car suspension system uses this approach so that bumps do not produce large...

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key terms and concepts

Learn how simple and physical pendulums can be used to measure acceleration due to gravity, using period, length, moment of inertia, and center of mass.

Using Pendulums to Find Gravity | JoVE K12

physics

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Using Pendulums to Find Gravity

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