Physics Problems & Solutions: Classical Mechanics

An electric sander has a continuous belt that rubs against a wood surface as shown schematically above. The sander is 100 percent efficient and draws current of 9 amperes from a 120-volt line. The belt speed is 10 meters per second. If the sander is pushing against the wood with a normal force of 100 newtons, the coefficient of friction is most nearly

A. 0.02
B. 0.2
C. 0.4
D. 1.1
E. 10

(GR8677 #93)

Solution:

Efficiency:

$\eta =\frac{W_\text{done}}{W_\text{input}}=\frac{P_\text{done}}{P_\text{input}}=1$

$P_\text{done}=P_\text{input}$

$\\P_\text{done}=Fv=\mu N v=\mu(100)(10)=1000\mu\\ P_\text{input}= VI=(9)(120)=1080$

$1080=1000\mu \rightarrow \mu=1.08$

Answer: D

Physics Problems & Solutions

Classical Mechanics - Efficiency

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