NewtonsLaws

# NewtonsLaws - N-1 Newton's Laws Forces and Motion A force...

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N-1 Newton's Laws: Forces and Motion A force is a push or a pull. Force is a vector . It has a size and a direction. Forces add like vectors, not like scalars. Example: Two forces, labeled F 1 and F 2 , are both acting on the same object. The forces have the same magnitude 12 FF == GG F and are 90 o apart in direction: net total 1 2 F = + G G F F net ( means "sum of all the forces on the object") F F net F2 ⇒= F t G (NOT 2F) 45 1 45 1 Isaac Newton (British, 1642-1727) first figured out the precise relationship between forces and motion. Newton's First Law (NI): If the net force acting on an object is zero, then it has constant velocity. G net F0v c o n s t a n =⇔ = Newton's Second Law (NII): net Fm a = G G 2 (Notice! F net , not F, in this equation. There may be many forces acting on an object, but there is only one net force.) The net force on an object causes the object to accelerate (change its velocity). Units of force: [ ] [ ][ ] 2 F m a kg m/s 1 newton = 1 N = A force of 1N is about 0.22 pounds. Phys2010 Notes, 9/29/2006 © University of Colorado at Boulder

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N-2 Things to notice about NII: If F net = 0, then a = 0 and velocity = constant. (1 st Law). The vector has the same direction as a G net F G . Magnitude of acceleration a is proportional to 1/m (at constant F net ). is a vector equation net FF m == GG G a y xxy Fm a ,Fm a ⇒= = ∑∑ Before Newton, everyone thought (incorrectly!): "Force causes motion." WRONG! (You can have motion without any force causing the motion. Example: glider on an air track.) After Newton, "Force causes changes in motion." RIGHT! ********************* DEFINITION: The WEIGHT of an object = the force of gravity on the object. W = mg Why? Recall this experimental fact: when object is in free-fall, meaning F net = F gravity , then a = g. So in this situation (free-fall), F net = ma F grav = mg mass m a = g F grav = W How big a force is 1 N? If m = 1 kg, W = F grav = mg = (1 kg)(9.8 m/s 2 ) = 9.8 N. A kilogram mass has a weight of about 10 N, which is about 2.2 lbs. The pound (lb) is the English unit of force: 1 lb = 4.44 N. Example: Glider on an air track with v = constant OR book at rest on a table. In both cases, a = 0 F net = 0 N = force exerted on book by table OR force on glider by air track N = "normal" force W = mg "normal" means "perpendicular" Since F net = 0 N = mg (upward force on book exactly cancels downward force on book.) Phys2010 Notes, 9/29/2006 © University of Colorado at Boulder
N-3 Rules for drawing "Free-body diagram" or force diagram : 0) Draw a blob representing the object. 1) Draw only the forces acting on the object (not the forces which the object exerts on others). 2) Indicate strength and direction of forces on the object by drawing arrows coming out of the object. 3) Use symbols to represent the magnitudes of the forces (Don't worry about +/- signs. The forces arrows show the directions of the forces already.) Example: Object with mass m, hanging by a cord.

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## This note was uploaded on 10/05/2009 for the course PHYS 2010 at Colorado.

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NewtonsLaws - N-1 Newton's Laws Forces and Motion A force...

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