Chpater8,13-15- Friction, linear Momentum and Collision

# Chpater8,13-15- Friction, linear Momentum and Collision -...

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Friction and Linear Momentum and Collisions Lectures 21-22 (Chapters 8, 13-15) Dr. Marina Milner-Bolotin [email protected] Check Blackboard for course web site!

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Part I: Lecture 21 Friction Goals: Example: Consider pushing a box as shown here: How can you determine if it will slide, tilt, or stay in static equilibrium? Our goal is to investigate forces of friction in more depth… 2
Friction is defined as a force of resistance acting on a body which prevents or retards slipping of the body relative to a second body. Experiments show that frictional forces act tangent (parallel) to the contacting surface in a direction opposing the relative motion or tendency for motion. For the body shown in the figure to be in equilibrium, the following must be true: Characteristics of Dry Friction (Section 8.1)

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To study the characteristics of the friction force F , let us assume that tipping does not occur (i.e., “ h ” is small or “ a ” is large). Then we gradually increase the magnitude of the force P. Typically, experiments show that the friction force F varies with P , as shown in the right figure above. Characteristics of Dry Friction (Continued)
Characteristics of Dry Friction (Continued) The maximum friction force is attained just before the block begins to move (a situation that is called “impending motion”). The value of Once the block begins to move, the frictional force typically drops and is given by F = μ N . The value of μ k (coefficient of

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A block with weight W is placed on an inclined plane. The plane is slowly tilted until the block just begins to slip. Determining μ s Experimentally y x
Steps for solving equilibrium problems involving dry friction: 1. Draw the necessary FBD. Make sure that you show the friction force in the correct direction (it always opposes the motion or impending motion). 2. Determine the number of unknowns. Do not assume F = μ S N unless the impending motion condition is given. 3. Apply the equations of equilibrium and appropriate frictional equations to solve for the unknowns. Problem Solving Dry Friction (Section 8.2)

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For a given W and h , how can we determine if the block will slide first or tip first? In this case, we have four unknowns ( F, N, x , and P ) and only three E-of-E. Hence, we have to make an assumption to give us another equation. Then we can solve for the unknowns using the three E- of-E. Finally, we need to check if our assumption was correct. Impending Tipping Versus Slipping
Problem Solving: Tipping Vs. Slipping

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Given: A uniform ladder weighs 20 lb. The vertical wall is smooth (no friction).
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Chpater8,13-15- Friction, linear Momentum and Collision -...

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