Practice 4C: | 1 | 2 | 3 | Go up
Coefficients of friction- by Krista Heim, 2001

1. Once the crate in sample problem 4C is in motion, a horizontal force of 53 N keeps the crate moving with a constant velocity.  Find k, the coefficient of kinetic friction, between the crate and the floor.  

Here's what you know, m=24 kg, Ff=53 N, g=9.8 m/s^2, use the formula Fg = mg,   Fg=24*9.8, solve for Fg, Fg=235.2 N, then use the formula Ff = ตkFn, 53=235.2ตk, solve for ตk, ตk=.23
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2.   A 25 kg chair initially at rest on a horizontal floor requires a 365 N horizontal force to set it in motion.  Once the chair is in motion, a 327 N horizontal force keeps it moving at a constant velocity.  

        A.  Find the coefficient of static friction between the chair and the floor.  

        Here's what you know, m=25 kg, Ff=365 N, g=9.8 m/s^2, use the formula Fg = mg, Fg =25*9.8, Fg =245 N, then use the formula Ff <= ตsFn, 365=245ตs, solve for ตs,  ตs=1.5.

        B.  Find the coefficient of kinetic friction between the chair and the floor.   

Here's what ya know, m=25 kg, Ff=327 N, the normal force of gravity will be the same as in part A.  Now use the formula Ff = kFn, 327=245 k, solve for k, k=1.3.
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3.  A museum curator moves artifacts into place on many different display surfaces.  Use the values in Table 4-2 to find the max force of static friction and the force of kinetic friction for the following situations:

 

        A.  moving a 145 kg aluminum sculpture across a horizontal steel platform

        Here's what you know, m=145 kg, ตs =.61, k=.47, g= 9.8 m/s^2, find the normal force by using the formula Fg = mg, Fg=145*9.8, solve,     Fg=1421 N, now use Ff <= ตsFn, Static Ff =.61*1421, solve, Static F=870 N, then use Ff = ตkFn, Kinetic Ff= .47*1421, solve, Kinetic F=670 N. 

        B.  pulling a 15 kg steel sword across a horizontal steel shield

        Here's what you know, m=15 kg, ตs =.74, k=.57, g= 9.8 m/s^2, find the normal force by using the formula Fg = mg, Fg=15*9.8, solve,     Fg=147 N, now use Ff <= ตsFn, Static Ff =.74*147, solve, Static F=110 N, then use Ff = ตkFn, Kinetic Ff= .57*147, solve, Kinetic F=84 N.

        C.  pushing a 250 kg wood bed on a wood floor

        Here's what you know, m=250 kg, ตs =.4, k=.2, g= 9.8 m/s^2, find the normal force by using the formula Fg = mg, Fg=250*9.8, solve,     Fg=2450 N, now use Ff <= ตsFn, Static Ff =.4*2450, solve, Static F=1000 N, then use Ff = ตkFn, Kinetic Ff= .2*2450, solve, Kinetic F=500 N.

        D.  sliding a .55 kg glass amulet on a glass display case  

Here's what you know, m=.55 kg, ตs =.9, k=.4, g= 9.8 m/s^2, find the normal force by using the formula Fg = mg, Fg=.55*9.8,solve, Fg=5.39 N, now use Ff <= ตsFn, Static Ff =.9*5.39, solve, Static F=5 N, then use Ff = ตkFn, Kinetic Ff= .4*5.39, solve, Kinetic F=2 N.
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