Class 18: Newton’s First Law Frictionkwng/spring2015/PHY231/lecture... · Newton’s first law v...
Transcript of Class 18: Newton’s First Law Frictionkwng/spring2015/PHY231/lecture... · Newton’s first law v...
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Class 18: Newton’s First LawFriction
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Test 2
1. Next Wednesday (March 4) 11:00‐11:50 in this class room.
2. Newton’s Law of gravitation, Hooke’s Law, Newton’s Law of motion.3. No formula or cheat sheet.4. 8 multiple choice problems (5 points each) and 2 long (30 points each) problems. Total 100 points.5. Calculators allowed, but not the program function (though I don’t think it will help).6. Please bring photo ID.7. No reschedule of test even though you have more than two tests that day.
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Newton’s first law
0 F constant v
For a single particle:
0. Newton’s first law is a particular case of Newton’s second law, with a = 0.1. This law holds only in “inertial frames”.2. An observing frame moving with a constant velocity relative to an inertial frame is also an inertial frame. 3. As a beginner, you should always do Newtonian mechanics in an inertial frame. 4. For daily life examples, standing still on ground is a good way to do Newtonian mechanics.
0. Newton’s first law is a particular case of Newton’s second law, with a = 0.1. This law holds only in “inertial frames”.2. An observing frame moving with a constant velocity relative to an inertial frame is also an inertial frame. 3. As a beginner, you should always do Newtonian mechanics in an inertial frame. 4. For daily life examples, standing still on ground is a good way to do Newtonian mechanics.
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Example
m
a
If the train is moving forward with an acceleration a, determine and the tension in the string.
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Example
A student of mass M is standing on a bathroom scale inside an elevator. The elevator is moving up but slowing down with a deceleration a. What will be the reading of the bathroom scale in Newtons?
M
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Static friction
Fn
mgfs
Fx
y
If the block is not moving. Equations of motion:
a=0
mg N (0) m mg - N :direction F f (0) m f - F :direction ss
yx
fs depends on F to keep the block in stationary.
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When Fn is becoming too large
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Fn
mgf
F x
y
Block will start sliding.
How large is large? F f :When nss
When fs is becoming too large
a
Note: This is a magnitude equation, not a vector equation. fs and Fn are not in the same direction!
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Fn
mgfk
F x
y
When the block is sliding,
F f and f Friction nkkk
Kinetic friction
a
1. This is a magnitude equation, not a vector equation. fk and Fn are not in the same direction!2. In general, s k
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In summaryFn
mgf
Fx
y
Friction f
External force F
k |Fn|s |Fn|
Static Sliding
|fk |= k |Fn|
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Example
m
If coefficient of static friction is s, at what angle will the block start sliding down?
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Example
m
The brick is moving forward in the direction as shown. If coefficient of kinetic friction is k, calculate the acceleration and the normal force from the floor to the brick.
F