Transcript Link to Notes - Coweta County Schools

Gravity
Chapter 7
Newton
 Noticed the moon followed a curved path
 Knew a force was required to allow an
object to follow a curved path
 Therefore, the Earth must exert a force
on the moon
 Needed to develop calculus to fully
explain and support his theory
Law of Universal
Gravitation
 All objects attract each other. This attraction is
directly proportional to the product of the
masses of the objects and inversely
proportional to the square of the distance
between them.
 OR
Fg = G m1m2
d2
Where G is the universal gravitational constant =
6.67 x 10-11 Nm2/kg2.
So How Can We Use This?
 Can never be 0 because mass can’t be 0
 Since G is so small, gravity is a very
weak force (actually, it’s the weakest
natural force).
 Gravity causes the Earth (and all planets)
to be round because all parts are pulled
equally towards the center.
 Planets pull each other slightly out of
orbit (how most astronomy has
advanced) - perturbation
Gravitational Fields
 The force of gravity causes an acceleration
 Surrounds all large objects with varying
strengths based on mass and radius.
 g = Gm
r2
 Ex. Earth’s g = 10 m/s2
 Exists within the Earth also:
 At the center of the Earth g = 0 because equal force
above and below you cancels out.
Weight
 A force caused by the acceleration due to
gravity acting on your mass (F = ma)
 Measured as the support force the ground must
exert to hold you up
 Accelerating in an elevator effects your weight:
 Going up causes more support force on your feet so
weight increases.
 Going down causes less support force on your feet
so weight decreases.
 You can never truly be weightless because
gravity always exists
 You feel “weightless” because you lack a support
force to push back on you
Tides
 Caused by differences in the pull of the
moon’s gravity on different sides of the
Earth
 Moon matters more because it’s closer, so
the difference in pull is more
 Sun does help though, since it’s mass is so
large
 Larger tides occur when sun and moon line
up (spring tides), smaller ones occur when
they are in opposition (neap tides)
Black Holes
 Stars expand as they grow, then become so
unstable, that they collapse on themselves.
 Since the same mass now takes up little
distance, the Fg is huge.
 It can become so big that the speed required to
escape its gravity is greater than the speed of light =
300000000 km/s (nothing is that fast)
 The bigger the star, the worse the black hole is.
Satellites
 A projectile moving fast enough to fall
continuously around an object
 The curve of it’s path matches the curve of
the object’s surface
 Must go at least 8 km/s to orbit the Earth
 Would take 90 min to orbit the Earth at this
speed
 Needs to be above the atmosphere or
friction would burn it up
Orbits
 It’s the horizontal velocity that has to be a
certain rate, not vertical
 So gravity doesn’t slow down the speed of
the object
 Orbits are often elliptical so speed isn’t
uniform
 Moves fastest when closest to the object
with gravity, slower when far away
Escape Speed
 V = 2Gm
d
 In order to “escape” the curved path that
follows the Earth, the speed needs to be at
least 11.2 km/s
 You haven’t really escaped the gravity, just gotten a
curve big enough that you aren’t pulled back
towards the Earth hard enough to maintain a circle