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Friction – At the next level…

Friction – At the next level…. Warm-up: Try to define friction in your own words. List 3 examples of physics you encountered over the break that involved Friction.

robert-ellis
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Friction – At the next level…

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  1. Friction – At the next level… Warm-up: Try to define friction in your own words. List 3 examples of physics you encountered over the break that involved Friction. Friction: A force that resists the sliding of one surface across another surface. The origin of the force is the interaction of tiny bumps on the surfaces and the “stickiness” (attraction between) of the atoms on the surface of the two materials. Friction always acts in the direction to oppose motion.

  2. Can We Control Friction? Vote… Let's see who is correct!

  3. How To Control Friction? • Brainstorming Time: • What can you do to increase friction? • What can you do to decrease friction? • Can we predict what the friction value would be in a given situation?

  4. Mini-Lab Time • Attach a force probe to the wooden block and determine the force required to do the following: • Compare force required to pull block from rest when felt side is down vs. wood side is down. • With (wood side down), compare force required to get the block moving initially, vs. the force required to keep it moving at a constantvelocity. • For the following, pull (felt side down) at a constantvelocity… • Compare force required to pull block with wide side down vs. narrow side down. • Compare force required to pull block with extra weight on it vs. without extra weight on top of it. • Compare force required to pull block at a slow constant velocity vs. a rapid constant velocity.

  5. Findings? • What, if anything, impacted the friction force? • Friction force from rest “static” is greater than friction force during motion “kinetic”. • Different surfaces have different friction forces. • Adding mass increased friction force… WHY? • By changing mass, we increased Fg AND FN… which is causing friction to change?

  6. Time to Think How can we change FN but keep Fg constant? What happens to FN as you increase the angle? Brainstorming: Experimentally, how could you determine if friction is constant (thus FN isn’t impacting friction) as the slope increases or if it is changing (thus FN is impacting friction). Record your thinking in your notes. Θ

  7. Putting it all together… Finally!!!

  8. Static vs. Kinetic If it isn’t moving but has a force applied to it, there has to be static friction resisting the applied force. If it is moving on a surface that has friction (most from here out) then there must be kinetic friction in the opposite direction of motion. Ffs Fa Ffk Ffk Fa Fa OR

  9. Friction Equations • Static Friction: • Ffs ≤ μsFN • Kinetic Friction: • Ffk = μkFN • General Equation: • Ff = μFN So what two factors determine the friction force???

  10. Mu (μ) Mu (pronounced mew) is the coefficient of friction between two specific materials. Mu does not have a unit and is generally between 0 and 1. The static coefficient of friction is always greater than the kinetic coefficient of friction for the same materials. The smoother the two surfaces, the smaller the coefficient. (generally)

  11. Joke Two cats fall from a tree branch onto the tin roof of a house. Which cat will be able to hang on and not slide off the house to its death? Answer: The one with the larger µ 

  12. Horizontal Application A 7.50kg box is pushed across the floor with constant velocity. If the coefficient of friction is 0.45, what is the friction force? (Don’t forget to set up the x and y equalities) Fg= 7.50 kg · -9.81 m/s2 = -73.6N FN= 73.6N Ff =µFN = 0.45 · -73.6N Ff =Fpush v Ff = -33.1N

  13. Force at an Angle Application • A 54.5kg box accelerates at 0.275m/s2 as it is pulled by a rope making a 24.9˚ angle to the ground. If it is pulled with a force of 338N: • What is the friction force? • -292N • What is the normal force? • 392N • What is the coefficient of friction? • 0.745

  14. Static Friction on a Slope Application • A 19.4kg box sits at rest on a ramp which has an angle of 25.8˚. • What is the friction force keeping it in place? • -82.8N • What is the normal force? • 171N • If the slope increases, what happens to the friction force? Evidence? • Ff = μFN static

  15. Sliding Friction on a Slope Application • A 68.0kg box slides down a 42.0˚ ramp with an acceleration of 0.195m/s2. • What is the friction force? • -433N • What is the coefficient of friction? • 0.873 a

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