Key Concepts: Lecture 9
Newton
Newton’s Laws of Motion
More on Kepler’s Laws
Newton’s Law of Universal Gravitation
Newton 16421727
•
Only child, posthumous son of an illiterate
yeoman
–
born prematurely  sickly as child
–
raised by maternal grandmother
–
as a child he built clocks & sundials
–
practical joker
•
Trinity College, Cambridge University at 18
–
studied mathematics & astrology
–
encouraged to study physics by Barrow
•
University closed in 1665 due to plague
–
Invented calculus, studied gravity, optics
•
Barrow resigns & gives Newton his post at
Cambridge
Describing Motion
• Position
• Velocity
– Rate of change of position (speed & direction)
• Acceleration
– Rate of change of velocity
2 examples of acceleration:
Newton’s Laws of Motion
•
Law I:
Law of Inertia
–
A body at rest or in motion at a
constant velocity along a straight
line remains in that state of rest or
motion unless acted on by a net
outside force.
•
Takes next logical step beyond
Galileo’s definition of inertia (
tendency
of a body to keep moving after all forces stop
acting on it
)
•
Uniform motion is just as natural a
state for a body as being at rest
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Laws of Motion
•
Law II  The Force Law
–
The acceleration (a) due to an applied
force (F) is in the same direction as the
force & is proportional to the strength
of the force & is inversely proportional
to the object’s mass (m)
•
The units of force are chosen so the
constant is 1. So we write a = F/m
•
To have acceleration there must be a force
•
Force & acceleration always work in the
same direction
•
Given the same force, a more massive
object accelerates more slowly than a less
massive one
Force=Mass xAcceleration
a
!
F / m
a = constant x F /m
constant = 1
a = F/m
We can write this as
F = m a
Examples of the Second Law
• Friction
– Hockey puck on ice vs. on a street
•
Impact of a bat on a baseball
– The bat imparts a force to the ball and sends it
flying in the opposite direction
Question?
•
A ball is attached to a string and I spin it
abound my head in a circle
– Is the ball accelerating?
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 Spring '07
 Dr.HaywoodSmith,Jr
 Force, Mass, General Relativity

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