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4.1 Forces
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Contact forces – push or pull
Field forces – gravitation, electric, magnetic
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4.2 Newton’s First Law of Motion
Aristotle: nature is at rest
vs.
Galileo: nature is at rest or in motion
Newton: An object moves with a velocity that is
constant in magnitude and direction, unless acted on by
an external, nonzero net force.
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Inertia, Mass and Weight
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4.4 Newton’s Second Law of Motion
Observations led Newton to state that:
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a   F and a 
m
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 F  ma
Unit: Newtons (N) 1 N = 1 kg m/s2
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What is ΣF for constant velocity?
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4.5 Newton’s Third Law of Motion
Forces always exist in pairs (action-reaction) and they
act on different objects.
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In class: Question 5 and Problem 7, 67
Other problems ↓
9. Use Eq. 2-11c to find the acceleration of the ball, with v = 0,
vo = 35 m/s and x-xo = 0.11 m.
v v
2
aavg 
2
0
2  x  x0 
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0   35.0 m s 
2  0.110 m 
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2
 5568 m s 2
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Favg  maavg   0.140 kg  5568 m s 2  7.80  10 2 N
Thus the average force on the glove was 780 N, in the direction
of the initial velocity of the ball.
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