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Dynamics

Dynamics. The study of why objects move. Forces. Sir Isaac Newton studied the causes of acceleration. He came up with three famous laws of motion. In 1687, Sir Isaac Newton published The Principia. Force and Motion. A force is a push or pull on an object .

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Dynamics

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  1. Dynamics The study of why objects move.

  2. Forces • Sir Isaac Newton studied the causes of acceleration. • He came up with three famous laws of motion. • In 1687, Sir Isaac Newton published The Principia.

  3. Force and Motion • A force is a push or pull on an object. • A force has magnitude AND direction so it is a vectorquantity. • In any interaction between an object and its surroundings, the object is considered the “system” and the surroundings that exert forces are the “environment”.

  4. Newton’s First Law • The Law of Inertia • Every body continues in its state of rest or of uniform speed in a straight line unless acted on by a nonzero net force. • That is: • An object in motion will stay in motion with a constant velocity unless acted upon by a net force.

  5. Inertia • The natural tendency of objects to resist changes in their state of motion. • Example, if you wanted to move a SMART car out of a parking spot, could you do it? • What if it was a 1-ton truck, could you still free up a parking spot?

  6. Mass vs. Weight • Your MASS is the amount of MATTER that composes you. • Your WEIGHT is the response of that mass to the pull of gravity. (No air resistance in this free fall). • Your WEIGHT will vary planet to planet due to a varying g, however, your mass will remain constant.

  7. Weighing in an Elevator • Your standing on a scale on the elevator. • What would happen to your weight as you go up to the top floor? • What would happen to your weight as you go down to the bottom floor?

  8. Apparent Weight • Typically a scale reads the support force of an object (the object’s weight which is due to mass and gravity). • If other forces are exerted on the object to “increase or decrease” the downward force, the scale reads the support which is the “apparent weight”. • Example-stand on a scale and push up on the sink at the same time… is your weight more or less?

  9. Break • Recap • Dynamics • Inertia • Mass and weight.

  10. Newton’s second law • Law of Motion • The acceleration of a body is directly proportional to, and in the same direction as, the net force acting on the body, and inversely proportional to its mass. • That is: • F = ma, where F is the net force acting on the object, m is the mass of the object and a is the acceleration of the object.

  11. Units: Would you like a fig Newton? • One unit of force causes a 1-kg mass to accelerate at 1m/s2. • F = m x a, therefore a unit of force is derived from a kg * m/s2. • This is a Newton (N), one force unit.

  12. How does acceleration depend on the object? • Let us keep the same force. • If mass increases, then acceleration is less and vice versa for a decrease in mass. Constant force Acceleration

  13. Force Types • Contact forces-acts on an object by touching it. • Long-range force: acts on an object without touching it. Object is in contact with a force “field” such as the force of gravity. • All forces have “agents” that we can name. The agent is the cause of the force. If you can’t name it, it doesn’t exist!

  14. Types of Forces • Applied force, FA, external force applied on an object. • Weight force, FW, force due to gravitational attraction between to objects. • Normal force, FN, support force exerted by a surface onto an object. • Tension force, FT, force exerted by a string when attached to a body and pulled taunt. • Friction force, Ff, contact force between two surfaces that opposes relative motion.

  15. Drawing Free Body Diagrams FN • Box on a table. Ff FA FW or Fg

  16. Drawing Free Body Diagrams FN • Box hanging on string. FW or Fg

  17. Drawing Free Body Diagrams FN • Two body system. FT Ff FT FW or Fg FW or Fg

  18. Problem solving • Draw a free-body diagram showing the system. • Identify all of the forces acting on the object. • Draw in the force vectors into the system. • Add the force vectors to find Fnet. • Use Newton’s second law to calculate acceleration. • Use kinematics to find velocity and position of the object. • What does an unbalanced force do?

  19. Your turn to Practice • Two horizontal forces, 225N and 165N are exerted in opposite directions on a crate with a mass of 5 kg. What is the net horizontal force on the crate? • What is the net vertical force on the crate? • What is the acceleration of the crate?

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