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--- 183_notes:friction [2021/02/18 21:20] – [Models of friction] stumptyl
+++ 183_notes:friction [2021/02/18 21:23] (current) – [Friction] stumptyl
@@ Line 3: / Line 3: @@
 ===== Contact Interactions: The Normal Force & Friction =====
-A microscopic perspective of materials helps to explain how contact interactions occur in nature. Contact interactions (forces) are not themselves [[http://en.wikipedia.org/wiki/Fundamental_interaction|fundamental forces of nature]], but they are the result of electrical forces between atoms. Compression and extension of materials occur not only at the [[183_notes:model_of_a_wire#modeling_the_solid_wire|macroscopic level]], but also at the [[183_notes:youngs_modulus#hanging_a_mass_from_a_platinum_wire|microscopic level]]. In these notes, you will read about how these ideas give rise to forces due to contact such as the [[http://en.wikipedia.org/wiki/Normal_force|normal force]] and dry [[http://en.wikipedia.org/wiki/Friction|friction]].
+A microscopic perspective of materials helps to explain how contact interactions occur in nature. Contact interactions (forces) are not themselves [[http://en.wikipedia.org/wiki/Fundamental_interaction|fundamental forces of nature]], but they are the result of electrical forces between atoms. Compression and extension of materials occur not only at the [[183_notes:model_of_a_wire#modeling_the_solid_wire|macroscopic level]], but also at the [[183_notes:youngs_modulus#hanging_a_mass_from_a_platinum_wire|microscopic level]]. **In these notes, you will read about how these ideas give rise to forces due to contact such as the [[http://en.wikipedia.org/wiki/Normal_force|normal force]] and dry [[http://en.wikipedia.org/wiki/Friction|friction]].**
-==== Forces due to contact ====
+===== Forces due to contact =====
-When two objects are in contact, their contact surfaces exert forces on each other. This is quite different from the gravitational force because while it acts on every piece of mass, as you will learn, we consider that it acts at the [[http://en.wikipedia.org/wiki/Center_of_mass|center of the of mass]] of the object. This assumption doesn't often affect our predictions and explanations of motion.  In fact, in all the models that you have used so far, we haven't been concerned about exactly where the force acts on an object only that it does act.
+When two objects are in contact, their contact surfaces exert forces on each other. This is quite different from the gravitational force because while it acts on every piece of mass, as you will learn,__ //_we consider that it acts at the [[http://en.wikipedia.org/wiki/Center_of_mass|center of the of mass]] of the object.//__ This assumption doesn't often affect our predictions and explanations of motion.  In fact, in all the models that you have used so far, we haven't been concerned about exactly where the force acts on an object only that it does act.
 For contact forces, (for a time) you will continue to use the assumption that we can just consider whether a contact force acts or not (And in what direction it acts). In the future, you might need to know precisely where it acts because [[183_notes:pp_vs_real|it might cause the motion to be a bit more complicated]]. For example, you can tip a box over if you push on it at the right location, but below that location it doesn't tip over.
-=== The normal force ===
+==== The Normal force ====
 [{{ 183_notes:mi3e_04-023.png?200|A cell phone placed on a  stable compresses the table, which is why the normal force can change its size}}]
@@ Line 24: / Line 24: @@
 Here, you can choose any cell phone (or computer, or brick!) and as long as the momentum change remains zero, and the table can support it, the "normal" force will be equal to the weight of the cell phone.
-=== Lecture Video ===
+==== Lecture Video ====
 {{youtube>3nJm4NYX23A?large}}
-=== Friction ===
+==== Friction ====
 [{{ 183_notes:mi3e_04-024.png?200|Start pushing a brick on a table and the atoms rearrange a bit; they resist the motion}}]
-Friction is a resistive force that is due the contact between two objects. While the normal force is perpendicular to the contact surfaces, the frictional force is parallel. So, the vector sum of these two forces (when both are acting) is the force that the surface exerts on the object. That is, both the normal and frictional forces are due to the same contact interactions, they are often just separated into parallel and perpendicular components to the surface for convenience.
+Friction is a resistive force that is due the contact between two objects. //While the normal force is perpendicular to the contact surfaces, the frictional force is parallel.// So, the vector sum of these two forces (when both are acting) is the force that the surface exerts on the object. That is, both the normal and frictional forces are due to the same contact interactions, they are often just separated into parallel and perpendicular components to the surface for convenience.
 [{{183_notes:asperities.png?200|The roughness of two materials in contact shown at the microscopic level. }}]
@@ Line 40: / Line 40: @@
 You have certainly observed this in your everyday life. To move an object at constant speed often requires you to supply a constant force. In physics, you are told there's no need for a force to keep an object in motion. Friction helps us deal with this apparent violation to Newton's 1st law. If there's a frictional force opposing the motion, then the motion of the object will slow down unless there's a similarly sized constant force applied in the direction opposite of friction.
-== Friction can act in the direction of motion ==
+=== Friction can act in the direction of motion ===
 Think about the next time you pick up a glass. The frictional force between your fingertips and the glass is in the direction of motion, namely bringing the glass to your mouth.