Transcript PHY160-19

TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review oscillations and energy
- Look again at the energy stored in a
“spring”
- Begin a final review of force, motion
and energy concepts
THE SUN AND MOON
- Repeat the sun plot analysis for new
data
A game you can play is to give a penny a shove so that it slides across a table, trying to
get it to stop on a target.
A) Describe the role that the friction force plays in this game.
Friction is the force that acts on the penny opposite the direction of motion, and accelerates it
to an eventual stop.
B) Assume that the penny is sliding on the table with a known speed. Explain how
friction and the laws of motion determine how far the penny goes before it stops.
1st Law: The penny keeps moving until friction applies a force
2nd Law: Friction applies a force to accelerate the penny
3rd Law: The penny exerts a force back on your finger
C) Assume that the penny is sliding on the table with a known speed. Explain how the
energy concept explains how far the penny goes before it stops.
The penny begins with a certain amount of kinetic energy - the energy supplied by
friction to stop the penny must equal the initial kinetic energy.
D) The mass of a penny is 0.004 kg. If the initial velocity is 0.5 m/sec and the sliding friction force = 0.002 N,
how far does the penny slide?
Kinetic energy = 1/2 mv2 = 0.5 × 0.004 kg × 0.5 m/s × 0.5 m/s = 0.0005 Joules
0.0005 Joules = Force × distance
distance = 0.0005 J/0.004 N = 0.25 m
OSCILLATIONS
Oscillations can be looked at in terms of
force and acceleration, or in terms of
energy.
OSCILLATIONS
Pendulum: Force and Acceleration
weight and tension are
NOT equal and opposite
here, so there is a
net force, and thus an
ACCELERATION
tension
net force
weight
OSCILLATIONS
Pendulum: Force and Acceleration
tension
At the bottom, tension and weight
cancel - no net force
weight
OSCILLATIONS
Pendulum: Force and Acceleration
on the swing upward,
forces become unbalanced
again, net force reappears
tension
net force
weight
Acceleration in a
pendulum is always
toward the central line,
or equilibrium
position (where it would
hang if stationary.)
OSCILLATIONS
Pendulum: Energy
Potential energy is stored
as pendulum is pulled back;
there is no motion or kinetic
energy yet
OSCILLATIONS
Pendulum: Energy
At the bottom, the potential
energy is gone - but speed and kinetic
energy are highest
OSCILLATIONS
Pendulum: Energy
on the swing upward,
the speed and kinetic energy
lower; potential energy is
again stored
OSCILLATIONS
SPRING: Force and acceleration
When the spring is “pulled
back”, the pull from your
hand and the restoring force
are balanced
equilibrium line
“restoring” force
pull from hand
OSCILLATIONS
SPRING: Force and acceleration
When you release, you
remove the force from your
hand - forces are no longer
balanced - restoring force =
the net force
“restoring” force
net force
SPRING ACCELERATES UP
OSCILLATIONS
SPRING: Force and acceleration
no net force
When the spring returns to
equilibrium position, it is
MOVING quickly, but there is
no more restoring force; NO
ACCELERATION at this
instant
OSCILLATIONS
SPRING: Force and acceleration
net force
“restoring” force
When the spring passes the
equilibrium line, the restoring
force pulls the other way
SPRING ACCELERATES DOWN
OSCILLATIONS
SPRING: Energy
When the spring is pulled
down, POTENTIAL ENERGY
is stored in the spring
OSCILLATIONS
SPRING: Energy
As the spring passes the
equilibrium, there is no more
potential energy; but the
speed is maximum, along
with kinetic energy
OSCILLATIONS
SPRING: Energy
As the spring reaches the
other side, it slows, and
kinetic energy again
becomes potential energy
MASS OF SPRING vs. PENDULUM
SPRING
Mass increases, restoring force stays
same; so acceleration decreases
PENDULUM
Mass increases, restoring force
(weight) also increases, so acceleration
stays the same
Frequency does
not depend on mass
Frequency decreases with mass
WHAT YOU ARE EXPECTED TO KNOW:
- Understand how to view pendulums and springs
in terms of force, acceleration and energy
- How pendulum and spring periods vary with
mass and amplitude
- How to measure frequency
(frequency = 1 / period)
TODAY’S OUTCOMES:
FORCE, MOTION AND ENERGY
- Review oscillations and energy✓
- Look again at the energy stored in a
“spring”
- Begin a final review of force, motion
and energy concepts
THE SUN AND MOON
- Repeat the sun plot analysis for new
data