### 12.4 Universal Forces

```12.4 Universal Forces
Objectives
• Describe factors affecting gravitational forces.
• Describe centripetal forces.
Introduction/Review
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Who developed the laws of motion?
How many laws of motion exist?
What is Newton’s first law of motion?
What is Newton’s second law?
What is Newton’s third law?
Universal Forces
• Forces that occur naturally everywhere in the
universe are called universal forces.
Universal Gravitation
• Gravitational force is an attractive force that acts
between any two masses.
▫ Newton’s law of universal gravitation states that
every object in the universe attracts every other
object.
▫ The gravitational force between any two
masses is directly proportional to their masses
and inversely proportional to the distance
between them.
Universal Gravitation
Sun
Earth
Gravitational
attraction
• Gravity is the weakest of the universal forces, but
is the most effective force over long distances.
Universal Gravitation
• The gravitational attraction between the
Earth and Moon causes the ocean tides.
▫ The gravitational pull produces a bulge of water in
the ocean on each side of the Earth.
▫ The rotation of the Earth beneath these bulges
produces two high and two low tides each day.
Centripetal Force
• Centripetal force is a center-directed force that
keeps a rotating object moving in a circular path.
Centripetal Force
Circular path
Inertia
Centripetal force
Closure
•TEST TOMORROW!!!
• What will the test look like?
• How many people would rather write on paper,
then put their grades into the clicker?
• What can you study?
```