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4.1 Forces 5/22/2017 Contact forces – push or pull Field forces – gravitation, electric, magnetic APHY201 1 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. Inertia, Mass and Weight 5/22/2017 APHY201 2 4.4 Newton’s Second Law of Motion Observations led Newton to state that: 1 a F and a m F ma Unit: Newtons (N) 1 N = 1 kg m/s2 What is ΣF for constant velocity? 5/22/2017 APHY201 3 4.5 Newton’s Third Law of Motion Forces always exist in pairs (action-reaction) and they act on different objects. 5/22/2017 APHY201 4 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 0 35.0 m s 2 0.110 m 2 5568 m s 2 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. 5/22/2017 APHY201 5