Transcript Approximate Coefficents of Friction

A 100kg wooden box is standing on a
wooden floor. You push the box with a
force of 100N to the right. The box
does not move. Draw a complete free
body diagram of the box and label all
forces.
FN=1000N
F=100N
FW=1000N
FN=1000N
Ffr.=100N
F=100N
FW=1000N
FN=1000N
F=200N
FW=1000N
FN=1000N
Ffr.=200N
F=200N
FW=1000N
FN=1000N
F=300N
FW=1000N
FN=1000N
Ffr.=300N
F=300N
FW=1000N
FN=1000N
Maximum Ffr.= ?
Which force will make the
box accelerate?
FW=1000N
Static Friction, ƒs
• Static friction acts to
keep the object from
moving
• If F increases, so does ƒs
• If F decreases, so does ƒs
• Static friction has a
maximum: ƒs  µs FN
µs….coefficient of static friction
More About Friction
• Friction is proportional to the normal force
• The coefficient of friction (µ) depends on the
surfaces in contact
• The direction of the frictional force is opposite the
direction of the applied force
• The coefficients of friction are nearly independent of
the area of contact
FN=1000N
Which force will make the
box accelerate?
Maximum Ffr.= μ·FN
FW=1000N
FN=1000N
Which force will make the
box accelerate?
Maximum Ffr.= μ·FN
Maximum Ffr.= 0.42·1000N
FW=1000N
FN=1000N
Which force will make the
box accelerate?
Maximum Ffr.= μ·FN
Maximum Ffr.= 0.42·1000N
Maximum Ffr.= 420N
FW=1000N
FN=1000N
Which force will make the
box accelerate? – Any force
that is larger than 420N.
Maximum Ffr.= 420N
FW=1000N
FN=1000N
F=500N
Maximum Ffr.= 420N
Fnet= 80N
a=0.8m/s2
FW=1000N
1. When a 12-newton horizontal force is applied to a box on a horizontal tabletop, the box
remains at rest. What is the force of static friction acting on the box?
2. According to your reference table, Approximate Coefficents of Friction, what is the minimum
horizontal force needed to start a 300. N steel block on a steel table in motion?
3. According to your reference table, Approximate Coefficents of Friction, what is the minimum
horizontal force needed to start a 300. kilogram steel block on a steel table in motion?
4. Jim wishes to push a 100. N wood crate across a wood floor. According to your table,
Approximate Coefficents of Friction, what is the minimum horizontal force that would be
required to start the crate moving?
5. According to your table, Approximate Coefficents of Friction, which road surface would offer
the greatest traction for the rubber tires of a 1000kg car? Which force would be required to
make the car accelerate on each surface?
a. dry concrete
b. wet concrete
c. dry asphalt
d. wet asphalt
6. A 100kg wooden box is at rest on a wooden floor. How much force is necessary to make the
box accelerate?
7. A 100kg wooden box is at rest on a wooden floor. Your friend is pressing the box straight
downwards with a force of 500N. How much force is necessary to make the box accelerate?
8. A 100kg wooden box is at rest on a wooden floor. Your friend is lifting the box straight
upwards with a force of 500N. How much force is necessary to make the box accelerate?