Download PHYSICS 51: Introduction

PHYSICS 50: Lecture 7-1
RICHARD CRAIG
Homework #6
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Read Chapter 7
Exercises and Problems: 7.5, 7.14,
7.29,7.38, 7.46, 7.55
Due Thursday, 3/13
Potential Energy
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Instead of considering how the work
form gravity changes the kinetic energy
of a system I can consider gravity a
form of energy
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Gravitational Potential energy
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Ug = mgh
Conservation of Energy
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When no “outside” forces or friction then
mechanical energy is conserved
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Kinetic + Gravitational + Elastic = Constant
Total energy is always conserved
Q7.4
Concept Question
The two ramps shown are both frictionless. The heights y1 and y2 are the same for
each ramp. A block of mass m is released from rest at the left-hand end of each
ramp. Which block arrives at the right-hand end with the greater speed?
A. the block on the curved track
B. the block on the straight track
C. Both blocks arrive at the right-hand end with the same speed.
D. The answer depends on the shape of the curved track.
Work and energy along a curved path
• Notice the same
expression for
gravitational
potential energy is
the same along
either a curved or
straight path.
Shoot bullet at same speed
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Consider the
speed of a
projectile as it
traverses its
parabola in the
absence of air
resistance.
Conservative forces
Q7.2
Concept Questions #2
You toss a 0.150-kg baseball straight upward
so that it leaves your hand moving at 20.0 m/s.
The ball reaches a maximum height y2.
v2 = 0
y2
What is the speed of the ball when it is at a
height of y2/2? Ignore air resistance.
v1 = 20.0 m/s
m = 0.150 kg
A. 10.0 m/s
B. less than 10.0 m/s but more than zero
C. more than 10.0 m/s
D. not enough information given to decide
y1 = 0
Conservation of Mechanical Energy
Around the Loop
Diver on a spring Board