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Forces Newton’s First and Second Laws Lecture 11

Forces Newton’s First and Second Laws Lecture 11. Classical (Newtonian) Mechanics. Describes the relationship between the motion of objects in our everyday world and the forces acting on them. Conditions when Classical Mechanics does not apply very small objects (< atomic sizes)

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Forces Newton’s First and Second Laws Lecture 11

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  1. Forces Newton’s First and Second Laws Lecture 11

  2. Classical (Newtonian) Mechanics • Describes the relationship between the motion of objects in our everyday world and the forces acting on them • Conditions when Classical Mechanics does not apply • very small objects (< atomic sizes) • objects moving with very large speed (close to the speed of light)

  3. Forces • Usually think of a force as a push or pull • Force is a vector quantity

  4. Forces • May be contact (pushing and pulling) or long range (field) force

  5. Units for force • SI unit of force is a Newton (N)

  6. More about forces • What if there are more than one forces acting on the object? • Different forces can be added vectorially • The net force is the vector sum of all the forces acting on the object:

  7. Example: Find the net force? Each grid has 2 N side. A B C F

  8. Newton’s First Law An object moves with a velocity that is constant in magnitude and direction unless a non-zero net force acts on it. If an object is at rest then it will remain at rest or if it is moving along a straight line with uniform speed then it will continue to keep moving unless an external force is applied on it to change its existing state When there are no external forces acting on an object, the acceleration of the object is zero.

  9. Newton’s First Law

  10. Newton’s First Law: An example

  11. Inertia Because of inertia, objects resist changes in their motion

  12. Inertia and Mass • Inertia is the tendency of an object to continue in its original motion • Mass is a measure of the inertia, i.e resistance of an object to changes in its motion due to a force • Recall: mass is a scalar quantity

  13. Newton’s Second Law • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. • Since both the force and acceleration are vector quantities, the acceleration can also be caused by change of the direction of velocity

  14. Newton’s Second Law

  15. Newton’s Second Law • Note: represents the vector sum of all external forces acting on the object. • Since Newton’s second law is a vector equation, we can always write it in terms of components

  16. Units for force

  17. An inertial reference frame Frame A Frame B

  18. Concept test A car rounds a curve while maintaining a constant speed. Is there a net force on the car as it rounds the curve? 1. No—its speed is constant. 2. Yes. 3. It depends on the sharpness of the curve and the speed of the car. 4. It depends on the driving experience of the driver.

  19. Summary: Newton’s First Law An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

  20. Summary: Inertia • Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. • Inertia is proportional to the mass of the object.

  21. Acceleration Summary: Newton’s Second Law • Force equals mass times acceleration

  22. Summary: Newton’s Second Law

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