Hewitt problems
13,14,17,19,31,32,43,54, (55)
13. What is the value of the Earth’s gravitational field at
the center of Earth? Zero, at the center of the Earth,
gravity is pulling you in every direction equally. Also,
the equation becomes undefined when the
denominator becomes zero
14. If you stepped into a hole that passed completely
through Earth, you’d oscillate down and up. How long
would a one-way trip take? How long would a round
trip take? The book says a one way trip would take 45
minutes, so a round trip would take 90 minutes
17. Why are ocean tides higher at the time of a full moon?
Full moons only happen when the sun Earth and moon are
all lined up, that means that the tides they cause are also
all lined up.
• 19. Why are black holes black? Because the gravity is
so strong, even light cannot escape it. (warping of the
space-time continuum)
• 31. Stephan says a crumpled paper weighs more than
a normal one. Gravity is the same for both pieces of
paper. There is just less wind resistance on the
crumpled one.
• 32. Earth and the Moon are gravitationally attracted to
each other. Does the more massive Earth attract the
moon with a grater force, the same force, or less force
than the moon attracts Earth?
The force is equal between both the moon and
Earth, just in opposite directions (Newton’s Third Law).
Newton’s Universal Law of Gravitation provides for only
one force between two objects, so they must have the
same magnitude.
• 43. The Sun exerts almost 200 times more force on the
oceans of Earth than the Moon does. Why then, is the
Moon more effective in raising tides? Because the
moon is closer, the difference in gravity (Δg) between
the sides due to the moons pull is much larger than
change (Δg) the Sun’s pull.
• 54. Isabella drops a chunk of iron of mass m from the
roof of her high school and it accelerates at g. Then
she ties two chunks of iron together, of mass 2m.
Show that when she drops the double chink, the
acceleration of fall is also g. When Isabella doubles the
mass, f=ma shows that double the force would be
needed to accelerate it at the same rate. So no matter
what, objects on Earth accelerate at -9.8 m/s2.
• 55. 1 Newton= 1kgm/s2 , so N/Kg = kgm/s2/kg = m/s2