7.4. Friction

Data

weight of block = _____________________

Part I

 
Side
Edge
Fstart 
    
Fslide 
    

Part II

 
Side
Edge
Height
    
Length
    

Part III

Force (FA) = hanging mass × 9.8

= _______________ × 9.8 = _______________

Height = _______________ Length = _______________

Calculations

(For all calculations, mg = the weight of the block.)

Part I – Calculate µstart and µslide by using:

µ = FA / mg (4 times)

Part II – Calculate µ by using:

µ = Height / Length (2 times)

Part III – Calculate µ by using:

1) Height / Length, then shift tan to get θ

2) FA – mg sin θ – µmg cos θ = ma

Results

list all the µ’s you calculated in table form

Questions

1) How did µstart compare to µslide? How should it? (Part I)

2 )How did surface area affect friction? How should it? (Parts I & II)

3) Which method was most accurate? Why? (discuss errors)

Machines – make work easier by multiplying the applied force (but the lifter must move a farther distance).

Definitions:

Resistance = R = the weight being lifted

Effort = E = the force being applied

IMA = ideal mechanical advantage – the theoretical maximum multiplier a machine can provide (if the machine works perfectly with no friction). It depends on the way the machine is constructed.

Inclined Plane 

 
 
IMA = Length / Height 
= L / H 
 
Lever
 
IMA = Earm / Rarm 

Pulleys IMA = # of support segments (all segments count except for an outside one pointing down)

   
IMA 
 
I = 1
 
II = 3
 
III = 3
 
IV = 4
 

The IMA tells how much further the lifter (effort) has to move, compared to how far the weight (resistance) moves.

IMA = 
Effort distance
     Resistance     
=  
Edist 
   Rdist   
 (no units)  

AMA = actual mechanical advantage = actual multiplier the machine provides. This must be determined by experiment.

AMA =
Resistance
      Effort      
 = 
R
 E 
   (no units)

*** The AMA must be < the IMA because friction causes some of the energy to be converted into heat.

Efficiency – tells how well the machine works

Efficiency =
AMA
   IMA   
 × 100  (units = %) 

*** The Efficiency must be < 100% due to energy loss caused by friction.

Sample Problem

For the lever shown, 5.0 N of force is used to lift a 10 N object. Find the AMA, IMA and Efficiency of the machine. If the weight is to be lifted a distance of 2.0 m, how far would the Effort have to move?

IMA = Earm / Rarm = 12 / 4.0 = 3.0

AMA = R / E = 10 / 5.0 = 2.0

Efficiency = AMA / IMA × 100 = 2.0 / 3.0 × 100

Efficiency = 6.7 × 101 %

IMA = Edist / Rdist

3.0 = Edist / 2.0 m

Edist = 6.0 m