- #1
cdbowman42
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1. A man sharpens a knife by pushing it against the rim of a grindstone. The 30-cm diameter stone is spinning at 200 rpm and has a mass of 28 kg. The coefficient of kinetic friction between the knife and the stone is 0.20. If the stone loses 10 % of its speed in 10s of grinding, what is the force with which the man presses the knife against the stone.
2. friction force=Normal Force(coefficient of static friction)
friction force=Torque/r
Torque=angular acceleration(a)*I
I=1/2Mr^2
angular acceleration(a)=(final angular velocity(w2)-initial angular velocity(w1))/t
3. The force he applies to the stone has no affect on slowing it down because it is directed toward the axis of rotation. It is the friction force that slows it down, because it is perpendicular to the moment arm.
w1=200rpm*2pi/60=20.9rad/s
w2=200rpm-.10(200rpm)=180rpm
180rpm*2pi/60=18.8rad/s
a=(18.8-20.9)/10= -.21rad/s^2
I=1/2(28)(.15)^2
I=.315kg*m^2
Torque= -.21(.315)=.06615 N*m
Friction force= -.06615/.15=-.441 N
Normal force of man=.441/.2= 2.205 N
Final answer: -2.205 N
The book says 9.9 N
Where am i messing up?
2. friction force=Normal Force(coefficient of static friction)
friction force=Torque/r
Torque=angular acceleration(a)*I
I=1/2Mr^2
angular acceleration(a)=(final angular velocity(w2)-initial angular velocity(w1))/t
3. The force he applies to the stone has no affect on slowing it down because it is directed toward the axis of rotation. It is the friction force that slows it down, because it is perpendicular to the moment arm.
w1=200rpm*2pi/60=20.9rad/s
w2=200rpm-.10(200rpm)=180rpm
180rpm*2pi/60=18.8rad/s
a=(18.8-20.9)/10= -.21rad/s^2
I=1/2(28)(.15)^2
I=.315kg*m^2
Torque= -.21(.315)=.06615 N*m
Friction force= -.06615/.15=-.441 N
Normal force of man=.441/.2= 2.205 N
Final answer: -2.205 N
The book says 9.9 N
Where am i messing up?