In the year 1798, Henry Cavendish would determine the value of the gravitational constant, G, through an experiment where he had measured the small force between two spheres made of lead connected by a rod, also, to note, he measured this small force with a device known as the torsion balance, which was able to detect and measure small forces. His experiment consisted of using light and the rod with the two spheres, with the wire attached to the rod. To clarify first, Isaac Newton developed his law of gravitation which stated that the force of gravity is equal to the gravitational constant G (which had not been measured yet until Cavendish) and the product of the two masses all divided by the squared distance between the two masses’ center of mass. Continuing with Cavendish’s experiment, as he began to twist the rod, creating a torsional force from the wire onto the rotational angle of the rod. Realizing this, Cavendish adjusted his device to measure the value between the rotational angle, also considered the torque of the rod, and the torsional force. He had then brought two larger spheres made of lead close to the small spheres which were still attached to the rod, and as the spheres began to attract to each other, the large spheres applied a gravitational force to the small spheres, which caused the rod to twist or spin. As the torsional force of the wire began to equal the gravitational force, the spheres and the rod had stop moving. During his experiment, Cavendish had measured the force of gravity and had known the values of the two masses and the distance between the two masses, and was able to calculate the value of the constant G, which was 6.75 x 10-11 N m2/kg2 ; however, today, the value of the constant G is considered as 6.673 x 10-11 N m2/kg2.
Around the years of 1589-1592, it is said that Galileo Galilei had tested an experiment where he had dropped two spheres, one of a larger mass and the other of a smaller mass, from the Leaning Tower of Pisa. He did this to refute Aristotle’s theory that objects fall at a speed according to their mass and to prove his theory that objects fell in the same acceleration. Prior to his experiment, he had theorized that objects made with the same material, falling through the same medium, would essentially also fall at equal speeds. During his demonstration, he had begun to drop metal balls, wood balls, cannon balls (all of different masses and materials) off of the Leaning Tower of Pisa. They would all reach the ground at the same time, thus proving that gravity accelerates objects at the same speeds, no matter of their mass or what material they’re made of. However, air resistance must be taken into consideration because, actually, the bigger sphere would hit the ground slightly faster than the smaller object. The larger sphere slightly resists the air resistance more than the smaller object (depending how much heavier it is), causing it to accelerate and have a faster terminal speed than the smaller object; however, this amount of air resistance is so small, it can be considered negligible.
In 1842, physicist Christian Doppler had created “The Doppler Effect”, which essentially was where a wave source is moving relative to the observer; the effect is not a result of a change in the wave source’s frequency. The Doppler effect can be found in any wave such as a light wave, radio wave, and more popular, the sound wave. Surrounding a wave source would be the wavefronts it emits, and these wavefronts connect the points of the wave, usually seen as equally spaced and centered to the source. As a wave source moves in some direction, the wave fronts would begin to look different relative to where the observer is standing. For example, the wave source is moving to the east direction, so the wavefronts to the right of the wave source would begin to come closer together, as the wavefronts to the left of the source would begin to become further apart. From the observer’s point of view, who is standing in front of the wave source’s path, would see/hear a higher frequency than before, and vice versa, if the person was observing from behind the wave source’s path, it would see/hear a lower frequency since the wavefronts are further away from the wave source. A popular example of the Doppler Effect is when a siren/vehicle is omitting a certain pitch, depending on the frequency of its wave. As the siren is approaching a person, the pitch would sound higher than the original since the frequency is higher due to the wavefronts nearing the observer, and as the siren is moving away from the person, the pitch would sound lower than the usual pitch since the frequency is lower.
The Millikan oil drop experiment, conducted in 1909 by Robert Millikan, first measured the electric charge of an electron. He calculated this by observing oil drops that had been charged with electricity, and in non-electric field, he was able to measure the velocity of the oil drop falling through a small hole of a metal plate. The only forces acting on the droplet were the gravitational force and drag force which were equal to each other at constant velocity. Since the two forces were dependent on the radius of the oil drop, it was possible for Millikan to calculate the mass and the force of gravity of the oil droplet. He then introduced voltage to the plate and the droplets were in an equilibrium, which meant that the electric force and force of gravity balanced each other and he began to retest his experiment and confirm his calculations. He found the value of an electron to be 1.5924(17)×10−19 C, which was only .6% off of the value that is used today. Prior to this experiment, it was suspected by many scientists and physicists that the electrons were massless, and Millikan was able to disprove this belief.
Charles Coulomb in the 18th century developed the famous Coulomb’s law that measured the electric force between objects with an electric charge. To develop this law he used the torsion balance, by separating the electric charge and having the resulting force convert into torque, and was able to determine that the electric force between the objects by having the constant, k, multiplied by the two charges in the particles given, q1 and q2, and it was then divided by the squared distance between the charges. This equation is quite similar to Newton’s law of gravity. As the distance between the two objects increased, the force and electric field was bound to decrease. The Coulomb constant, k, is dependent on the medium where the two objects are in. In Coulomb’s law he stated that the same charged objects would repel each other, and opposite charged objects would attract.
Isaac Newton’s Laws of Motion have been used for the past 300 years and have been applied to many of the physics that is learned today. His first law states that an object will stay at rest or continue in uniform motion in a straight line unless an unbalanced force in the system enacts on it. This law is also commonly known as the law of inertia. If an object is in constant speed then acceleration would be 0, thus the object would still continue in the same speed and direction of its motion. Newton’s second law is related to when there are unbalanced forces present and it proposes that the net force of the object would equal the product of the mass of the object and the acceleration of the object. The equation would visually be seen as F=m*a. It is the net force that Newton’s second law relates to and the net force is considered to be the total sum of all the forces enacting on the object. Newton’s Second Law can be split into x-y components, depending on the situation given. The direction of the acceleration of an object is also the direction of the total net force given. In Newton’s Third Law of Motion, it states that “for every action, there would be an equal and opposite reaction.” This holds to be true with forces too because, for example, there is a box on a table at rest, and you are to be asked to list all the forces enacting on the box and their magnitudes in the y direction. There is the normal force of the table on the book and the gravitational force of Earth on the book. Since the book is not accelerating in the y-direction, the normal force and gravitational force have to be equal, however in opposite direction.
Johannes Kepler, around the 1600s, developed three laws of planetary motion. In his law of Ellipses, he stated that the sun is in the center of the planets’ elliptical path. In the Law of Equal Areas, the radius from the center of a planet to the center of the sun will sweep out equal areas in the same amount of time. In this law, he refers to the speed of the planets revolving around the Sun, since the speed of planets moving through space is always changing. In his Law of Harmonies, it is that the periods of two planets squared would be equivalent to the two planets’ distances from the Sun cubed. His third law shows a precise calculation of the period and distance for the planets’ orbits around the Sun.
Albert Einstein, in 1905, stated that the laws of physics remain the same in all non-accelerating reference frame. He also confirmed that the speed of light that is in a vacuum is identical for all its observers. In special relativity, a person cannot see uniform motion or constant speed unless it is relative to surrounding objects. If a person was moving at constant velocity or was at rest, and observes a car moving at constant motion it is impossible to tell if the car is moving unless there were background objects that would give away whether or not the car was moving. In general relativity, the idea was that gravity was a “curving or warping of space”. This meant that spacetime was able to curve around objects, and it allowed objects to accelerate, and this was curvature was affected by the matter and energy within the object, and, confusingly, the mass and energy was also affected by the spacetime curvature.
Foucault’s pendulum was developed in 1851, and it was created in order to show how the Earth’s rotation works. At the north or south pole, the pendulum continues to swing back and forth, and as time progressed it was seen that the pendulum’s motion was directly affected by the motion of the Earth. If the pendulum was to be at the equator, then the pendulum would be observed as fixed. Foucault was able to develop an equation for this phenomenon, where the angular speed equals the product of 360 degrees and the sine of the latitude all divided by the number of days.