Lecture5 - PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 5...

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PHYSICS 231 INTRODUCTORY PHYSICS I Lecture 5
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Range (y i =y f ): Relative velocity: Newton’s Laws: 1. If Σ F = 0, velocity doesn’t change. 2. 3. Main points of last lecture R = v i 2 g sin2 ! v be = v br + v re r F ! = m r a r F 12 = ! r F 21
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Newton’s Third Law Single isolated force cannot exist For every action there is an equal and opposite reaction Action and Reaction Forces act on different objects Force on “1” due to “2” r F 12 = ! r F 21
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Free-body Diagram Draw arrows for all forces acting ON object If the object is in “equilibrium” (at rest or constant v) Otherwise, find acceleration by Ignore rotational motion for now. Treat object as a particle. r F ! = 0 r F ! = m r a
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Example 4.1 Two blocks sit on a frictionless table. The masses are M 1 =2 kg and M 2 =3 Kg. A horizontal force F=5 N is applied to Block 1. 1. What is the acceleration of the blocks? 2. What is the force of block 1 on block 2? 1. a = 1 m/s 2 2. F 21 = 3 N M 1 M 2 F
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Mechanical Forces Gravity: w=mg (down) Normal forces Strings, ropes and Pulleys Friction Springs (later)
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Rules for Ropes and Pulleys Force from rope points AWAY from object (Rope can only pull) Magnitude of the force is Tension Tension is same everywhere in the rope Tension does not change when going over pulley Approximations: Neglect mass of rope and pulley, neglect friction in pulley
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Example 4.2 I pull a 5 kg mass up with a rope, so that it accelerates 2 m/s 2 . What is the tension in the rope?
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