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PHYS16 – Lecture 10 & 11

PHYS16 – Lecture 10 & 11. Force and Newton’s Laws September 29 and October 1, 2010. http://fashionablygeek.com/wp-content/uploads/2009/09/mass-times-acceleration.gif. Key Concepts: Force. Force Definition and Types Center of Mass Free Body Diagrams Newton’s Laws Friction.

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PHYS16 – Lecture 10 & 11

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  1. PHYS16 – Lecture 10 & 11 Force and Newton’s Laws September 29 and October 1, 2010 http://fashionablygeek.com/wp-content/uploads/2009/09/mass-times-acceleration.gif

  2. Key Concepts: Force • Force • Definition and Types • Center of Mass • Free Body Diagrams • Newton’s Laws • Friction

  3. Newton’s Laws – First Law • A body at rest remains at rest & a body in motion stays in motion unless acted on by a force • Doesn’t seem true because most objects don’t infinitely stay in motion, why? (friction) • Can see this is true if decrease air friction • Example – Brick pushed along ground vs. Brick in cart pushed along ground

  4. Newton’s Laws – Second Law • If there is a NET force then the object is accelerated in the direction of the force. • The magnitude of the acceleration is proportional to the force and inversely proportional to the object’s mass.

  5. What happens if there is no net force? • Scenario #1 – there are forces acting on the object, but they cancel • Scenario #2 – there are no forces acting on the object • If there is no NET force then no acceleration

  6. Mechanical Equilibrium • Mechanical Equilibrium – when FNET=0 • Static Equilibrium • v= 0 • FNET=0 • Example – book on a table • Dynamic Equilibrium • v= constant • FNET=0 • Example – car moving at constant speed

  7. Newton’s Laws – Third Law • Every action has an equal and opposite reaction • Does this seem true? If I push a block does it push back on me? • Yes, I just have friction from floor keeping me in place • Can feel pressure of block on fingers • So all reactionary forces – Normal, Friction, Tension will vary with the amount of applied force

  8. Problem #1 • I hold a 0.5 kg mass by a string so that it is still in space. What is the force I apply? • 5 N, upward

  9. Problem #2 • A mass is connected to a string which goes around a pulley. I hold the string at the other end. The mass is 0.5 kg. How much force do I have to apply? • 5 N, downward

  10. Problem #3 • Panda is in a box with a rope at one end. I pull Panda and box at 8.0 N at a constant velocity. The surface has a kinetic friction coefficient of 0.37. What is the mass of Panda and box? • 2.2 kg (there was a brick in the box) • What if I just pull Panda and box without brick at a constant velocity using a 2 N force, how much is the Panda/box mass?(0.55 kg)

  11. Problem #4 • I have a block with mass m sitting on an incline of θ. Assuming that there is friction, what is the frictional force in terms of m, g, and θ? • |Ff |= mg sin(θ), direction is up the incline parallel to incline surface

  12. Problem #5 • A cart of mass M sits on a frictionless table. There is a amassless rope connected at one end of the cart that is horizontal, parallel to the table. The rope goes around a massless pulley and then is connected to a hanger of mass m hanging vertically from the pulley. The cart and hanger are accelerated at a value given by a. What is a in terms of M, m, and g? • a=mg/(m+M)

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