1 / 16

Friction

Friction. Friction in machines wastes energy Wastes energy and can be transferred into heat. Advantages: Starts/stops machines Wet or icy roads Sand or gravel. Three Factors Determine Friction. Primary Cause: minute irregularities in rubbing surfaces Force pressing two surfaces together.

red
Download Presentation

Friction

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Friction • Friction in machines wastes energy • Wastes energy and can be transferred into heat. • Advantages: Starts/stops machines • Wet or icy roads • Sand or gravel

  2. Three Factors Determine Friction Primary Cause: minute irregularities in rubbing surfaces • Force pressing two surfaces together. • Nature of substances in contact with each other- Coefficient of friction • Kind of motion involved

  3. Static Friction • Inertia that has to be overcome to start object moving • Condition of surface plays big role in how much starting friction is • “Coefficient of Friction” has been established for various materials • Typically found in table form

  4. Static Friction • Equation: • Ex: 60# block, k=0.3 static force is parallel to friction • What is the static force?

  5. Static Force – more examples • Force due to weight parallel to friction • Holding something in a vise • Static force perpendicular to wt and friction • Equation: • 12 lb block supported in vise, k=0.18 • Force exerted on each face of block?

  6. Sliding Friction • Force needed to keep an object moving on surface • Amount of sliding resistance dependent upon nature of surface and normal force (weight) between the object and surface • Equation:

  7. Sliding Friction • Put a wedge under a block • u=0.09, block = 100#, wedge 12 ft long and 2 ft high • Break it down into two parts • Force to put block on wedge • Force sliding block

  8. Rolling Friction • Rolling friction greatly reduced because of the action of roller and bearings • Coefficient usually low: • Tire on concrete: u=0.02 • Roller bearing: u=0.001 – 0.003 • Equation:

  9. Rolling Friction Example • Wheelbarrow = 500#, tire: u = 0.02 F = 500 x 0.02 = 10#

  10. Power • NOT ability to move large objects • Power is the rate of doing work • Digging a trench: 1 man = 60days; 2 men = 30 days, 1 backhoe = 1 day • Same work, different power

  11. Power example • Engine = 3500# hoisted vertically 7 ft in 3 mins. Calculate the power. • What if 1 min?

  12. Horsepower • Power is often expressed in “horsepower” • Equation: • Equation:

  13. Work Equivalents: BTUs • Heat energy can be transformed to mechanical energy • Mechanical energy can be converted to heat energy • Pilot slows airplane after touchdown, applying brakes. Kinetic energy of moving aircraft changes to heat energy – hot disks & hot tires

  14. Work – BTU Conversion • 1 BTU = 778 ft-lbs of work • Example: 1500# weight lifted vertically 300 ft in 45 minutes • Work done lifting the weight? • How many BTU’s? • Power?

  15. Same example, continued • Horsepower? • Now rework with t = 45 sec. • 45 sec = 0.75 hr

  16. Another Example • What if we had a 350# aircraft engine 2.5 ft above the engine mount on a hoist, and we move the hoist horizontally 15 ft in 15 sec. • How much work was accomplished? • How much Power? • How much Horsepower? • How many BTU’s?

More Related