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PHYSICS 231 Lecture 9: More on forces

PHYSICS 231 Lecture 9: More on forces. Remco Zegers. Newton’s Laws.

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PHYSICS 231 Lecture 9: More on forces

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  1. PHYSICS 231Lecture 9: More on forces Remco Zegers PHY 231

  2. Newton’s Laws • First Law: If the net force exerted on an object is zero the object continues in its original state of motion; if it was at rest, it remains at rest. If it was moving with a certain velocity, it will keep on moving with the same velocity. • Second Law: The acceleration of an object is proportional to the net force acting on it, and inversely proportional to its mass: F=ma • If two objects interact, the force exerted by the first object on the second is equal but opposite in direction to the force exerted by the second object on the first: F12=-F21 PHY 231

  3. General strategy for problems with forces • If not given, make a drawing of the problem. • Put all the relevant forces in the drawing, object by object. • Think about the axis • Think about the signs • Decompose the forces in direction parallel to the motion and perpendicular to it. • Write down Newton’s first law for forces in the parallel direction and perpendicular direction. • Solve for the unknowns. • Check whether your answer makes sense. PHY 231

  4. A 2000 kg sailboat is pushed by the tide of the sea with a force of 3000 N to the East. Because of the wind in its sail it feels a force of 6000 N toward to North-West direction. What is the magnitude and direction of the acceleration? PHY 231

  5. T T 900 1kg A mass of 1 kg is hanging from a rope as shown in the figure. If the angle between the 2 supporting wires is 90 degrees, what is the tension in each rope? ThorR PHY 231

  6. Fs,k n=-FgL Fg//=mgsin FgL=mgcos   Fg=mg Problem A)If s=1.0, what is the angle  for which the block just starts to slide? B)The block starts moving. Given that k=0.5, what is the acceleration of the block? PHY 231

  7. n T T Fk Fg Fg All the forces come together... If a=3.30 m/s2 (the 12kg block is moving downward), what is the value of k? PHY 231

  8. A lawsuit… A passenger in a bus files a lawsuit claiming that when the driver suddenly pushed the brakes a suitcase came flying toward her from the front of the bus and she got injured. You are in the jury; will you award damages? • No, this could not have happened • Yes, this is plausible PHY 231

  9. puck on ice • After being hit by a hockey-player, a puck is moving over • a sheet of ice (frictionless). The forces working on the • puck are: • No force whatsoever • the normal force only • the normal force and the gravitational force • the force that keeps the puck moving • the normal force, the gravitational force and the force • that keeps the puck moving PHY 231

  10. A person is pushing an ice-sledge of 50kg over a frozen lake with a force of 100N to the east. A strong wind is pushing from the south-west and produces a force of20N on the sledge. a) What is the acceleration of the sledge? b) if the coefficient of kinetic friction is 0.05, what is the acceleration? PHY 231

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