| Both sides previous revision Previous revision Next revision | Previous revision |
| 183_notes:gravitation [2021/02/04 23:59] – [Newton's 3rd Law] stumptyl | 183_notes:gravitation [2021/02/05 00:01] (current) – [Newton's 3rd Law] stumptyl |
|---|
| The gravitational force provides the first example of [[http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_third_law|Newton's 3rd Law]], which you might have heard colloquially as "For every action, there is an equal and opposite reaction." Unfortunately, [[http://www.wired.com/2013/10/a-closer-look-at-newtons-third-law/|this colloquialism is a terribly inaccurate definition]] that gets applied incorrectly quite often, [[http://scienceblogs.com/dotphysics/2010/05/06/mythbusters-energy-explanation/|even by the Mythbusters]]! | The gravitational force provides the first example of [[http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_third_law|Newton's 3rd Law]], which you might have heard colloquially as "For every action, there is an equal and opposite reaction." Unfortunately, [[http://www.wired.com/2013/10/a-closer-look-at-newtons-third-law/|this colloquialism is a terribly inaccurate definition]] that gets applied incorrectly quite often, [[http://scienceblogs.com/dotphysics/2010/05/06/mythbusters-energy-explanation/|even by the Mythbusters]]! |
| |
| Newton's 3rd Law results from the idea that a [[:183_notes:momentum_principle#net_force|force quantifies the interaction between two objects]]. You can also think of it as an empirical fact, which stems from our definition of force. That is, __//we observe when one object exerts a force on another object, the second object exerts a force on the first object of the same size but opposite in direction.//__ | Newton's 3rd Law results from the idea that a [[:183_notes:momentum_principle#net_force|force quantifies the interaction between two objects]]. You can also think of it as an empirical fact, which stems from our definition of force. That is, **we observe when one object exerts a force on another object, the second object exerts a force on the first object of the same size but opposite in direction.** |
| |
| To be more concrete, you can think about the gravitational interaction between the Earth and the moon (shown in the figure below). The magnitude of these gravitational forces are the same (see the equation above), but the vector direction for each always points directly towards the other object. | To be more concrete, you can think about the gravitational interaction between the Earth and the moon (shown in the figure below). The magnitude of these gravitational forces are the same (see the equation above), but the vector direction for each always points directly towards the other object. |
| ==== (More) Modern Gravitational Models ==== | ==== (More) Modern Gravitational Models ==== |
| |
| Newton's model of the gravitational force was considered one of the simplest and most explanatory models for many years. We have since made observations that no longer fit with Newton's model (e.g., [[http://en.wikipedia.org/wiki/Gravitational_lens|Gravitational lensing]]). Our best model for gravitation, which observations continue to fit, is called [[http://en.wikipedia.org/wiki/General_relativity|"general relativity"]] (GR) and was developed by [[http://en.wikipedia.org/wiki/Albert_Einstein|Albert Einstein]]. While this model provides us with far better predictions and explanations of a variety of observations, we still use Newton's model of the gravitational force for two reasons: (1) it can provide reasonable predictions for many cases, and (2) [[https://en.wikipedia.org/wiki/Mathematics_of_general_relativity|the mathematics that is used in GR]] is sufficiently sophisticated that you will need more physics and mathematics experience to gain deep insight into its use. | Newton's model of the gravitational force was considered one of the simplest and most explanatory models for many years. We have since made observations that no longer fit with Newton's model (e.g., [[http://en.wikipedia.org/wiki/Gravitational_lens|Gravitational lensing]]). Our best model for gravitation, which observations continue to fit, is called [[http://en.wikipedia.org/wiki/General_relativity|"general relativity"]] (GR) and was developed by [[http://en.wikipedia.org/wiki/Albert_Einstein|Albert Einstein]]. While this model provides us with far better predictions and explanations of a variety of observations, we still use Newton's model of the gravitational force for two reasons: (1) //it can provide reasonable predictions for many cases//, and (2) //[[https://en.wikipedia.org/wiki/Mathematics_of_general_relativity|the mathematics that is used in GR]] is sufficiently sophisticated that you will need more physics and mathematics experience to gain deep insight into its use. |
| | // |
| ===== Examples ===== | ===== Examples ===== |
| |
| * [[183_notes:examples:calcGravForce|Calculating the Gravitational Force]] | * [[183_notes:examples:calcGravForce|Calculating the Gravitational Force]] |
| * [[183_notes:examples:videoswk3|Video Example: Gravitational force and Kinematic equations on the Moon]] | * [[183_notes:examples:videoswk3|Video Example: Gravitational force and Kinematic equations on the Moon]] |