Friction

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# Friction - PowerPoint PPT Presentation

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.

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Presentation Transcript
Friction
• Friction in machines wastes energy
• Wastes energy and can be transferred into heat.
• Sand or gravel
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
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
Static Friction
• Equation:
• Ex: 60# block, k=0.3 static force is parallel to friction
• What is the static force?
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?
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:
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
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:
Rolling Friction Example
• Wheelbarrow = 500#, tire: u = 0.02

F = 500 x 0.02 = 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
Power example
• Engine = 3500# hoisted vertically 7 ft in 3 mins. Calculate the power.
• What if 1 min?
Horsepower
• Power is often expressed in “horsepower”
• Equation:
• Equation:
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
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?
Same example, continued
• Horsepower?
• Now rework with t = 45 sec.
• 45 sec = 0.75 hr
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?