Transcript Gravity

Gravity
What is Gravity?
• An attractive force between any two
objects that depends on the masses of the
objects and the distance between them
• Force increases as the mass of either
object increases or as the objects move
closer
• Earth has enough mass and is close
enough that you feel its gravity
• Name 3 objects (only 2 can be on Earth)
whose gravity you cannot feel
Four Basic Forces
1. Gravity
You already know about this one!
2. Electromagnetic
Electricity and magnetism are
caused by the electromagnetic
force, as are chemical
interactions between atoms
and molecules
3. Strong nuclear
4. Weak nuclear
Nuclear forces only act on
particles in the nuclei of atoms
Law of Universal Gravitation
• Formulated by Isaac Newton, 1687
• Based on his observations of stars and
planets
Law of Universal Gravitation
What happens to the
gravitational force as the
distance between two
objects increases from 1
m to 2 m?
m1m2
F =G
d2
gravitational force = (constant) x
(mass 1) x (mass 2)
(distance)2
No matter how far apart,
gravitational force never
disappears…called a long
range force.
Finding Other Planets
• 1840s most distant planet known was
Uranus
• But all planets affect motion of all other
planets due to gravity, so…
• Motion of Uranus disagreed with predicted
motion from law of universal gravitation
• Two astronomers independently
calculated this and led to the discovery of
Neptune in 1846
Earth’s Gravitational Acceleration
• If air resistance is the same, two objects of
different masses will fall at the same acceleration
= free fall
• Close to Earth’s surface free fall acceleration is
9.8 m/s2 and is given the symbol g
Gravitational force (N) = mass (kg) x gravitational acceleration (m/s2)
F = mg
Example problem
What is the gravitational force on a skydiver
with a mass of 60 kg?
F=?
F = mg
m = 60 kg
2
2
F
=
60
kg
x
9.8
m/s
g = 9.8 m/s
F = 588 N
Example problem
What is the gravitational force on a 150 kg
piano falling from a skyscraper?
F=?
F = mg
m = 150 kg
2
2
F
=
150
kg
x
9.8
m/s
g = 9.8 m/s
F = 1470 N
Weightlessness and Free Fall
• Weightlessness is when an object does
not experience gravity
• Video of astronauts in orbit
• A typical shuttle mission orbits the Earth at
an altitude of about 400 km
• Gravity inside shuttle is 90% that on Earth
• Why do objects feel weightless?
• If you stand on a scale, net force is zero
• You exert force on scale, scale exerts
equal force upward to you
• Suppose you stand on that scale in an
elevator in free fall
• Scale no longer exerts a force on you, only
force is gravity, scale would read zero
• ONLY FORCE ON AN OBJECT IN FREE
FALL IS GRAVITY
• Space shuttle in orbit is also in free fall
• Falling around the Earth
• Objects in shuttle seem to float because
they all fall with the same acceleration
• How does something orbit the Earth?
• Why doesn’t the moon fall down?
Projectile Motion
• Anything that’s thrown or shot through the
air is called a projectile
• Earth’s gravity causes projectiles to follow
curved path
• Video showing many projectile objects
(4.17 min)
Projectile Motion con’t
• Horizontal and Vertical Motions
– Thrown ball force comes from your hand
pushing ball in horizontal motion
– When it leaves your hand, gravity pulls it
down, vertical motion
– Therefore it appears to travel in a curved path
Projectile Motion con’t
• Horizontal and Vertical DISTANCE
– A thrown ball and one that is dropped from
the same height will hit the ground at the
same time
– Both balls travel the same vertical distance in
same amount of time
– Thrown ball travels farther horizontally
Centripetal Force
• When a ball travels through a curved tube it may
speed up or slow down during straight sections
= acceleration
• BUT even if its speed doesn’t change in the
CURVED parts it is STILL ACCELERATING
because it is a change in the direction of the
velocity
• Change is towards the center of the curved path
= CENTRIPETAL ACCELERATION
Centripetal Force
• Car rounding a curve has centripetal force, this
comes from traction (friction of tires on road)
• If friction is too small, car will move in a straight
line (off the road)
• Anything that travels in a circle is doing so from
centripetal force, accelerating it toward the
center
• Demo: bucket of water, penny on a coat hanger
• Same reason moon orbits Earth