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Lecture 10

AS101 Lecture Notes - Lecture 10: Net Force

Department
Astronomy
Course Code
AS101
Professor
Victor Arora
Lecture
10

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Lecture 10 – February 9, 2015
Recap: Not Fooling Ourselves
- Kepler’s Laws describe how the planets revolve around the Sun, but not why
- Acceleration is a change in speed and/or direction
- All objects fall at the same acceleration near the Earth (if air resistance is negligible)
Forces
- Force: push or pull in some direction
Proportional Relationships:
- If two things are related to each other, in such a way that changing one value causes the
other to change by the same factor, we say that they are proportional to each other
oDirectly (the “same” change) or inversely (the “opposite”)
- Example:
oTuition Cost = cost per credit x number of credits
- You can figure out the effect of a change by keep the scaled “balanced”
- Example: the cost per credit doubles and you take the same amount of credits; what
happens to tuition cost?
oTuition Cost (2x) = (2x) cost per credit x number of credits (same)
oSo tuition cost and cost per credit are: directly proportional
- Example: the cost per credit doubles but you don’t have extra money; what happens to
number of credits
oTuition Cost (same) = (2x) cost per credit x number of credits (0.5x)
oSo number of credits and cost per credit are: inversely proportional
Acceleration and Net Force
- Newton’s 2nd Law
- The acceleration of an object is proportional to the net force acting on that object, and
inversely proportional to its mass
oOr just: acceleration is net force divided by object’s mass
- Example: if a car experiences twice the force, it will experience twice the acceleration
- Example: if one car is half as massive as another car, it will take half as much force to
give it the same acceleration
Law of Inertia
- Newton’s 1st Law
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