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UConn PHYS 1501Q - Friction
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Phys 1501Q 1st Edition Lecture 9Outline of Last Lecture: Forces and Free Body DiagramsI. Spring ForceII. Hooks LawIII. GravityIV. Newton’s Law of Universal GravitationV. Calculate Mass Using Newton’s Law of GravityVI. Tension CheckpointVII. Spring CheckpointVIII. Elevator AccelerationIX. Block on an Inclined Plane ExampleOutline of Current Lecture: FrictionI. FrictionII. Static FrictionIII. Kinetic FrictionIV. Example: Box on Table (Sitting)V. Example: Box on Table (Sliding)VI. Microscopic FrictionVII. Example: Friction with Inclined PlaneVIII. Example: Masses, Table, and PulleyThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.Current Lecture: FrictionI. Friction- Friction is a force represented as (f)- Friction opposes motion (or would be motion) - Friction can still act if an object is not moving- Friction is due to surface contact- 2 Types of Friction = Static and KineticII. Static Friction- Occurs when surfaces are not sliding- The coefficient of static friction is μS- f = μS∗N (friction is the coefficient of static friction times the Normal force)III. Kinetic Friction- Occurs when surfaces are sliding- Coefficient of static friction is μK- f = μk∗NNOTES: -μS>μk ALWAYS-f is parallel to the surface and perpendicular to NIV. Example: Box on Table (Sitting)Static friction keeps a box from moving when it is being pulled. What is the total force acting on the box?Fnet = 0  f = T  friction = tension pulling on the rope pulling the boxV. Example: Box on Table (Sliding)- Exerting a force (F) onto box- Y acceleration = 0- N = W = mg- Breakaway:If no motion: f= -F (friction is opposite the force we exist) up to μS∗NOnce we exceed this, it’s breakaway and the coefficient of friction now becomesμk- If box is moving:f = μK∗N =−μkmg (for horizontal axis)VI. Microscopic Friction- Microscopically surfaces are not really smooth- Friction is when the contact points on the surface are making and breaking bonds - One has to apply force on an object to break bonds to get it to moveVII. Example: Friction with Inclined PlaneA mass is on an inclined plane with friction1) At what angles will an object go from being stationary to sliding?2) What is the acceleration?X component: f – mgsinθ = ma = 0 (box is not sliding)Y component: f – mgcosθ =ma = 0 (always zero because box never jumps off ramp)Max friction force before slippage: f = μsNμsN= mgsinθ for max θμs(mgcosθ)= mgsinθ cancel mg out to becomeTanθ=μsAfter slippage:f = μkNMa(x component) = -gcosθ(μs−μk¿ acceleration once box starts to slipVIII. Example: Masses, Table, and Pulley1. Find tension in rope.2. What are the acceleration of the two masses?Notes: - Tension increases with friction- T=0 means freefall- T=mg implies no acceleration- T = somewhere between 0 and mg- T1=T2 because it’s the same rope connecting the masses- A1 = A2 both blocks have the same accelerationM2 (No vertical acceleration) M1N = mg mg – T = maT-f = ma T = m(g-a)T- μkmg = ma ^^ Plug this T into M2 equationM1(g-a) - μk(M2)(g) = (M2)aA = (m1-m2)g/(m1+m2)T =


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UConn PHYS 1501Q - Friction

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