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What is Friction? How does it work?

What is Friction? How does it work?. Force on person by box. Force on box by person. Force of friction on box by floor. Force of friction on floor by box. Friction is a Force.

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What is Friction? How does it work?

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  1. What is Friction?How does it work?

  2. Forceon person by box Force on box by person Force of friction on box by floor Force of friction on floor by box Friction is a Force When you push on a box, it pushes back. You need to push harder than the force of friction between the box and the floor to move it!

  3. Friction Mechanism Corrugations in the surfaces grind when things slide. Lubricants fill in the gaps and let things slide more easily.

  4. What Causes Friction? • Friction is the force that opposes the motion between two surfaces that touch. • The surface of any object is rough. • Even an object that feels smooth is covered with tiny hills and valleys. • The contact between the hills of valleys of two surfaces causes them to stick, resulting in friction.

  5. What affects friction? • The amount of friction depends on: • Roughness of the surfaces • Force pushing the surfaces together

  6. Types of Friction • Kinetic friction occurs when force is applied to an object and the object moves. • Examples: • Sliding Friction: pushing an object across a surface • Rolling Friction: between wheels and a surface • Fluid Friction: opposes the motion of objects traveling through a fluid (air or water)

  7. Types of Friction • Static friction occurs when force applied to an object does not cause the object to move.

  8. Affecting Friction • To reduce the amount of friction, apply a lubricant between two surfaces. • Motor oil, wax, and grease are examples. • Friction can also be reduced by rolling, rather than pushing, an object.

  9. What Causes friction? • Even though a surface may look smooth, if you magnify the surface it isn’t smooth. • Everything is made up of molecules. The molecules come together to make bumps. Some surfaces have molecules that leave large bumps and some leave smaller bumps, but all surfaces have bumps. • Microwelds occur when two bumpy surfaces are rubbed up against each other and they stick together

  10. Sticking Together • The greater the force on the object, the greater the force of the microwelds and the greater force is needed to overcome the microwelds to move the object

  11. Static and Sliding (Dynamic) Friction • Static frictional force: when nothing is sliding • Sliding frictional force: when surfaces are sliding • Static frictional forces always greater than sliding ones • Lubrication provides microscopic rollers between surfaces

  12. Big Consequences! (USGS)

  13. Calculating Friction • Remember: The rougher the surface, the greater the force of friction • The more force pressing the two surfaces together, the greater the force of friction • The force of friction • Ff = (a measurement of surface roughness)*(how heavy the object is) • Ff = m*Weight, where m is called the coefficient of friction (USGS)

  14. Coefficient of Friction

  15. Demo static vs. kinetic • Which force is bigger?

  16. Trajectories with Air Resistance • Baseball launched at 45° with v = 50 m/s: • Without air resistance, reaches about 63 m high, 254 m range • With air resistance, about 31 m high, 122 m range Vacuum trajectory vs. air trajectory for 45° launch angle.

  17. Air Trajectories, cont. • Now v = 40 m/s, optimal angle (45° for vacuum, 40° for air): Optimal angle for 40 m/s in air gets 97 m range vs. 164 m for vacuum trajectory. • Golf balls actually have optimal launch of 25-30°, helped by lift • forces associated with spin of the ball.

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