Download Lecture 14 Monday September 29

Lecture 14
Monday, September 29
Using Newton’s Laws
Harper’s Index
• Minimum number of U.S. cities that have
shortened the yellow light on traffic signals to
under the legal limit:
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6
Today
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Equilibrium FNET = 0
Dynamics FNET ≠ 0
Mass and Weight
Normal forces
Frictional forces
• Chapter 5: 1-5
Equilibrium
FNET   F  0
FNET , x   Fx  F1x  F2 x  ...  0
FNET , y   Fy  F1 y  F2 y  ...  0
• Hence a=0 and v=constant
• If v=0 and is constant, static equilibrium
• If v≠0 and is constant, dynamic equilibrium
300
Find the tensions, T1 and T2 in the two cords if the
mass of the picture is 11.1 kg.
Procedure
• Make free body diagram
• Picture in (static) equilibrium so
T1x  T2 x  0
T1 y  T2 y  mg  0
Example
Macie pulls a 40 kg rolling trunk by a strap angled at 30° from
the horizontal. She pulls with a force of 40 N, and there is a 30
N rolling friction force acting on trunk. What is the trunk’s
acceleration?
Slide 5-17
Mass and Weight
• Freely falling object
FNET  w  ma
FNET , y   w  mg
w  mg
Elevators
• You are standing on a scale in an elevator.
• A) The elevator is moving upwards
(downwards) at a steady speed. If your mass
is 70 kg, what does the scale read?
• B) The elevator which had been moving
upwards, slows to a stop. What does the scale
read during this time?
• Normal force
Quiz
An elevator, lifted by a cable, is going up at a steady speed.
• Identify the forces acting on the elevator. Make a free body
diagram.
• Is T greater than, equal to, or less than w? Or is there not
enough information to tell?
Slide 4-23
Friction
• Static friction
f s ,max   S n
f s  S n
• Opposes motion
Kinetic Friction
• Opposes motion when object is sliding
f k  k n
• µk is the coefficient of kinetic friction
Rolling Friction
• µr is the coefficient of rolling friction
f r  r n
Problem
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5:24 A 23kg child on slide inclined at 380.
What forces act on child?
Make a free body diagram
Magnitude of normal force?
If the coefficient of static friction is 0.62, will
the child start to slide?
Problems due Today
• 4: 13, 14, 17, 18, 20, 25, 29, 30, 31, 33, 35, 38,
65
Problems due Wednesday
• Reading: 5: 6-8 Drag (skip), applying
Newton’s laws
• Problems: 5: CQ1, CQ3, MC24, MC26, MC30,
1, 2, 4, 7, 11, 13, 16, 19, 21