Transcript 12.1 Gravity is a force exerted by masses

Chapter 3: Gravity, Friction,
and Pressure
12.1 Gravity is a force exerted by masses
12.2 Friction is a force that opposes motion
12.3 Pressure depends on force and area
12.4 Fluids can exert a force on objects
12.1 Gravity is a force exerted by
masses
• Masses attract each other
– Gravity is the force that objects exert on each
other because of their masses
– Gravity accelerates all masses equally
• Gravity is a universal force: acts between
any two masses anywhere in the universe
– Between the earth and moon, sun and moon
– Between dust and gas particles in space
helped form the solar system
The force of gravity
• Recall: the net force on you
determines how your motion
changes
– Force between you and the
desk is extremely small
compared with other forces
constantly acting on you
(friction, muscles, Earth’s
gravity, other objects)
• Strength of the gravitational
force between two objects
depends on mass and
distance
• Greater mass results in greater
force
• Smaller distance results in
greater force
Gravity on Earth
• Gravity acts on both masses equally, even if the
masses are very different
• Every action force has an equal and opposite
reaction force
– Earth’s gravity exerts a force on a coin, the coin
exerts an equal upward force on Earth
• Small coin mass means it can easily be accelerated
• Large Earth mass means it is much more difficult to
accelerate
• g = acceleration due to gravity = 9.8 m/s2 at
Earth’s surface (moon: 1.6 m/s2)
– Newton’s 2nd law: F=ma F=mg
Gravity
• In a vacuum:
– Quarter vs penny falling
• Quarter has more mass so gravity exerts more force on it
• But it also has more inertia, so the greater force does NOT
produce a larger acceleration
• Therefore: objects with different masses fall with the same
acceleration
http://www.classzone.com/books/ml_science_share/vis
im/mfm05_pg79_vacuum/mfm05_pg79_vacuum.html
At a given location on the earth and in the absence of
air resistance, all objects fall with the same uniform
acceleration. Thus, two objects of different sizes and
weights, dropped from the same height, will hit the
ground at the same time.
Gravity
• In a vacuum:
– Coin dropped falls at the same rate as one thrown
forward
• Horizontal velocity does not affect acceleration due to gravity
• Gravity is directed downward so it changes only the
downward velocity of the coin, not its forward velocity
An object is controlled by two independent motions. So
an object projected horizontally will reach the ground in
the same time as an object dropped vertically. No matter
how large the horizontal velocity is, the downward pull of
gravity is always the same.
Gravity
At the instant a cannon fires a cannonball horizontally over
a level range, another cannonball held at the side of the
cannon is released and drops to the ground.
Which strikes the ground first?
Distance Fallen
Distance fallen in
one second is
5 meters.
This distance
fallen is the
same whether
falling straight
down or in
projectile motion.
0.5 s
5 meters
1.0 s
1.5 s
• http://scienceblogs.com/dotphysics/2009/1
0/mythbusters-bringing-on-the-physicsbullet-drop.php
• http://www.youtube.com/watch?v=D_JbgP
8mpsE
Weight and Mass
• Mass is a measure of how much matter an
object contains
– Same no matter where the object is located
• Weight is the force of gravity on that object
– Depends on the force of gravity acting upon it
• On Earth: Mass = 50 kg, Weight = 490 N
• On Moon: Mass = 50 kg, Weight = 82 N
Gravity keeps objects in orbit
• Orbit: elliptical path one body follows
around another body due to the influence
of gravity
• Centripetal force keeps one object in orbit
around another due to the gravitational
pull
– Between Earth and Moon, Earth and Sun
– “falling around Earth”
Projectile Motion & Curvature
For initial speeds that are faster and faster, the range of
the projectile is farther and farther.
For very large speeds, the curvature of Earth starts to be
noticeable.
Earth’s Curvature
Curvature of the Earth is about 5 meters
over a distance of 8000 meters (which is
about 5 yards over 5 miles).
Missing the Ground
Suppose you throw a ball at a speed of 8000 m/s (about 18,000 mph).
After one second, ball travels 8000 meters and falls 5 meters.
In that distance, Earth curves by same amount (5 meters).
If nothing stops
the ball, what
happens?
8000 m
5m
NOT to
Scale
Orbits and Centripetal Force
Gravity provides the centripetal force
required for a satellite to move in a circle.
Getting into Orbit
-Rocket needs to lift above
the atmosphere and then
fire thrusters to acquire the
required orbital speed of
about 8000 meters (8 km) per
second.
-path of the falling object
matches the curve of Earth’s
surface
-greater than 11,000 m/s and
the spacecraft will escape the
pull of Earth’s gravity!
Elliptical Orbits
For speeds higher than 8 km/s, the orbit is
elliptical instead of circular.
Escape Speed
If speed exceeds
11.2 km/s then
object escapes
Earth because
gravity weakens (as
object gets further
away) and never
slows the object
enough to return it
back towards Earth.
Hyperbolic
Circular
Elliptical
People in Orbit
• Elevator: on a scale during downward
acceleration, you would appear to weigh less –
the scale is also moving downward, and you are
pushing on it less
– If in free fall (fall entirely due to gravity), you would not
press against the scale at all
• Spacecraft in orbit is in free fall (around Earth)
– Astronauts weight does not press against the floor of
the spacecraft, so objects behave as if there were no
gravity
In-class activity p.394
Time (s)
Velocity
(m/s)
0
0
2
18
4
29
6
33
8
35
10
36
12
36
14
36
16
36
18
36
1) For both variables, decide the
scale that each box on your
graph will represent and what
range you will show for each
variable.
2) Determine the dependent and
independent variables.
3) Plot the independent variable
along the horizontal axis, and the
dependent along the vertical axis.
Challenge: alter the scale you chose to
use in #1. Graph again. How do
different scales give different
impressions of the data?
• http://www.edinformatics.com/math_scienc
e/solar_system/gravity_mass_weight.htm