Transcript Law of Universal Gravitation

Law of Universal Gravitation
Chapter 12
November 9/10
Objectives
• Analyze how Earth is falling
• State Newton’s law of universal gravitation
• Evaluate the significance of inverse-square law
Introduction
• All objects have gravity
• Therefore, all objects attract one another
The Falling Apple
• An apple falls to Earth because of gravity
• Does Earth’s gravity pull the moon?
– YES
• Why doesn’t the moon fall into Earth?
– It does but it doesn’t crash because of tangential
speed
The Falling Moon
• Earth’s gravity pulls moon = centripetal force
• Moon falls beneath the straight line it would
follow if no force acted on it
– Below the tangential speed line
• Figure 9.2
• Confirmed with experiment
– Figure 9.4
The Falling Earth
• Earth (and other planets) falls around the sun
• Why don’t planets crash into sun?
– Because of tangential velocities
• What would happen if tangential speeds went
away?
– Crash into sun
– This has happened to asteroids and other matter
without a big enough tangential speed
Check for understanding
• In the picture below use vectors and words to
describe how the Earth orbits the sun
Tangential speed
Sun
Earth
Fc = gravity
Earth stays in orbit as the resultant of the sun’s gravity and Earth’s tangential speed
Newton’s Law of Universal Gravitation
• Newton discovered gravity is universal
– Everything pulls on everything else
• Law of Universal Gravitation
– “Every object attracts every other object with a force
that for any two objects is directly proportional to the
mass of each object”
– Greater masses = greater force of attraction
– Greater distance (farther away) = less force of
attraction
Newton’s Law of Universal Gravitation
(con’t)
• Law can be expressed as an equation
FG = force of gravity (N)
G = universal constant
= 6.7 x 10 -11 Nm2
kg2
m1 = mass of one object (kg)
m2 = mass of other object (kg)
r = radius (m)
Class work- Honors
• Use the equation that relates to the law of
universal gravitation to solve the problem
Inverse Square Law
• When a quantity varies as the inverse square
of its distance from its source
• Figure 9.4 and 9.5
– Force of gravity depends on distance from Earth.
Closing
• An apple falls into Earth. Why doesn’t the
moon fall and crash into Earth?
– It does but it doesn’t crash because of tangential
speed
AND
– Force of gravity is weaker since moon is farther
from Earth’s surface than apple
Homework
• Next class we will:
– Take a quiz on all units and symbols- STUDY!
– review for the unit exam
• The following class is the unit exam on:
– Newton’s 3 laws
– Centripetal force and tangential speed
– Law of universal gravitation