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Forces

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An introduction to Forces and Newton's First Law.

An introduction to Forces and Newton's First Law.


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  • 1. Forces(Newton’s Laws of Motion)
  • 2. What is a Force?A force is a push or pull that can cause an object to 1. change shape and / or 2. change motion
  • 3. What is a Force?Force is a vector quantity (direction is important) that acts on an object by another object (agent).
  • 4. What is a Force?There are 4 fundamental forces . . .
  • 5. What is a Force?There are 4 fundamental forces . . . but several common ones . . .
  • 6. Weight (Gravity due to Earth)The magnitude of the force depends on mass of the object and strength of gravitational field: W = mg g = 9.81 N kg-1The direction is always down toward the center of the Earth.
  • 7. Other Common ForcesTension - forces from tight ropes etc.Normal, Contact, Reaction – support forces from hard surfaces pushing on each other.Air Resistance - air pushing on moving objects opposite to their direction of motion.Friction - pushing on sliding objects (opposite to direction of motion) or sticking of an object to a surface.(these are all due to electron and proton electromagnetic force)
  • 8. Galileo was first to acknowledge thatmotion would continue without friction.
  • 9. Isaac Newton (1642-1727)Newton built upon Galileo’s thinking and proposed atheory for the causes of motion. The central ideas in histheory are his three laws of motion and his law ofuniversal gravitation.
  • 10. Newton’s First Law of Motion (Law of Inertia) An object remains at rest or continues in constant motion in a straight line if there is no resultant (net) external force.Net (Resultant) Force- The sum of all forces actingon an object. If all of the forces cancel each other outthen the net force is zero and the object’s motion is notchanged.
  • 11. Car and driver are traveling at 60 km h-1.The car is stopped by a force; the driver is not.
  • 12. Newton’s First Law in your worldFor each situation, state how the first law is demonstrated.1. To dry off your toothbrush when you are finished brushing your teeth, you shake or tap it. Why?1. If you drop your headphones while flying in an airplane, it lands at your feet and does not move to the back of the cabin. Why?
  • 13. Free Body Diagrams
  • 14. Translational EquilibriumIf the resultant force on an object is zero, then it is said to be intranslational equilibrium. SF = 0According to Newton’s 1st Law, the object must be at rest or atconstant velocity.Example:
  • 15. Equilibrium 1
  • 16. Equilibrium 2
  • 17. Free-body Diagrams Sketch the free-body diagram for the objects in the following situations at equilibrium. A skydiver falling at terminal A block sliding down a velocity. hill at constant velocity.A car driving at constant velocity. A sign hangs from the ceiling.
  • 18. Newton’s 2nd Law In Newton’s 2nd law the resultant force ≠ 0; the object accelerates.How is the acceleration related to the net force applied? Let’s investigate!