Download Chapter 3 - Nicholls State University

CHAPTER 3
Laws of Motion
Understanding Our Universe
FIRST EDITION
Palen | Kay | Smith | Blumenthal
© 2012 by W. W. Norton & Company
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You read in the paper that a new planet was
found. The article states that when it is
closest to its star, it moves at 31 km/s. When
it is farthest from its star, it moves at 35 km/s.
This story has an error because:
a. Kepler’s 2rd law says the planet has to sweep
out equal areas in equal times, so the speed
of the planet cannot change.
b. Planets stay at a constant distance from their
stars, they don’t move closer or farther away
c. Kepler’s 3rd law says the planet must move
fastest when it’s closest, not when it’s farther
away.
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The figure shows a planet’s orbit that obeys Kepler’s laws.
However, not all of the shaded regions have equal areas.
During which part of the planet’s orbit (A, B, C, or D) would
the planet move with the greatest speed?
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Photo: humbletree on Flickr
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An unbalanced force must be acting when:
A. An object accelerates
B. An object changes direction, but not
speed
C. An object changes speed, but not
direction
D. An object changes speed and direction
E. All of the above
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Your weight equals the force
between you and Earth.
Suppose you weigh 60 kg. Then
the force on Earth is:
A. 60 kg.
B. Much smaller than 60 kg
because your mass is much
less than Earth’s.
C. Exactly zero, since only
massive objects have gravity.
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Imagine you are standing at the top of a tall
tower. You drop 3 objects shaped like a
ball made out of different materials.
Neglecting air resistance, in what order do
they hit the ground?
a. Styrofoam, pumpkin, lead
b. Lead, pumpkin, styrofoam
c. Pumpkin, lead, styrofoam
d. All hit at same time
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Newton’s Universal
Law of Gravitation
Fgrav = G ×
m1 × m2
r2
Suppose you are transported to a planet
with twice the mass of Earth, but the
same radius. Your weight would?
a. Increase
b. Decrease
c. Stay the same
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Earth and the Moon have a gravitational
force between them. The mass of the
Moon is 1.2% of that of the Earth.
Which statement is incorrect?
A. The force on the Moon is much larger
than that on Earth.
B. The forces are equal size, even though
the masses are different.
C. The Moon has a larger acceleration
than Earth.
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The Space Shuttle is in a circular orbit at a
constant speed of 28,000 km/hr. Is
gravity acting on the spacecraft?
A. The spacecraft is not accelerating since
the speed is constant. No force = no
gravity.
B. The direction of the motion is constantly
changing. This counts as an
acceleration. Forces produce gravity.
C. The astronauts inside are weightless. No
weight equals no gravity.
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Summary: Science at Work!
• Newton expanded Galileo’s
observations and results into 3
Laws.
• Newton derived Kepler’s
Laws from his law of gravity.
Newton, Kepler, Galileo
Griffith Observatory. L.A., CA
• Newton’s Laws were tested
by Kepler’s observations,
showing his law of gravity to
be correct.
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