Transcript Powerpoint

Physics 151 Week 10 Day 2
Topics: Apparent Weight & Conservation of Energy
 Apparent Weight
 Conservation of Energy
Scales and Elevators (Apparent Weight)
Consider a person with a mass of 60 kg is in an elevator
standing on a scale. The elevator is accelerating upward.
• Draw a system schema and 2 force diagrams:
One for the person and one for the scale
• What does the scale read?
• Use Newton’s 2nd law to determine what the scale
reads (This is apparent weight)
Slide 4-19
Scales and Elevators (Apparent Weight)
Suppose a person with a mass of 60 kg is in an elevator
standing on a scale.
Use the system schema and force diagrams of the scale and
the person to determine what the scale would read for the
following situations:
A. If the elevator is descending at 4.9 m/s.
B. If the elevator has a downward acceleration of
4.9 m/s/s.
C. If the elevator has an upward acceleration of
4.9 m/s/s.
Slide 4-19
Apparent Weight
Slide 5-24
Example Problem
A 50 kg student gets in a 1000 kg elevator at rest. As the elevator
begins to move, she has an apparent weight of 600 N for the first 3
s. How far has the elevator moved, and in which direction, at the
end of 3 s?
Slide 5-25
Clicker Question
The apparent weight of an object is
A. the pull of gravity on the object.
B. the object’s mass times the acceleration of gravity.
C. the magnitude of the contact force that supports the
object.
D. the pull of gravity on an object that is accelerating
upward.
Slide 5-7
Answer
2. The apparent weight of an object is
A. the pull of gravity on the object.
B. the object’s mass times the acceleration of gravity.
C. the magnitude of the contact force that supports the
object.
D. the pull of gravity on an object that is accelerating
upward.
Slide 5-8
Energy Model
Types of Energy
Kinetic Energy KE = 1/2 mv2
Potential Energy:
Gravitational Potential Energy
Spring Potential Energy
PEg = mgy
Pes = 1/2 kL2
Conservation of Energy (Closed System)
Before
After
KEi  PEgi  PEsi  Esys  KE f  PEgf  PEsf  Eth
Visualizations:
• Energy Bar Charts
The Basic Equation
Kf  Uf  Eth  Ki  Ui
A few things to note:
• Work can be positive (work in) or negative (work out)
• We are, for now, ignoring heat.
• Thermal energy is…special. When energy changes to
thermal energy, this change is irreversible.
Slide 10-24
Conceptual Example Problem
A car sits at rest at the top of a hill. A small push sends it rolling
down a hill. After its height has dropped by 5.0 m, it is moving at
a good clip. Write down the equation for conservation of energy,
noting the choice of system, the initial and final states, and what
energy transformation has taken place.
Slide 10-25
Checking Understanding
Three balls are thrown off a cliff with the same speed, but in
different directions. Which ball has the greatest speed just
before it hits the ground?
A.
B.
C.
D.
Ball A
Ball B
Ball C
All balls have
the same speed
Slide 10-26
Answer
Three balls are thrown off a cliff with the same speed, but in
different directions. Which ball has the greatest speed just
before it hits the ground?
A.
B.
C.
D.
Ball A
Ball B
Ball C
All balls have
the same speed
Slide 10-27
Additional Questions
Trucks with the noted masses moving at the noted speeds crash
into barriers that bring them to rest with a constant force. Which
truck compresses the barrier by the largest distance?
Slide 10-54
Answer
Trucks with the noted masses moving at the noted speeds crash
into barriers that bring them to rest with a constant force. Which
truck compresses the barrier by the largest distance?
E.
Slide 10-55