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Static Equilibrium

Static Equilibrium. Still a constant velocity…but this time, it’s zero. Static = not moving. Static Equilibrium. Static (not moving/stationary/at rest) Equilibrium (balanced forces = constant velocity) The sum of the forces acting on the object in any direction is zero. Forces.

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Static Equilibrium

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  1. Static Equilibrium Still a constant velocity…but this time, it’s zero. Static = not moving

  2. Static Equilibrium • Static (not moving/stationary/at rest) Equilibrium (balanced forces = constant velocity) • The sum of the forces acting on the object in any direction is zero.

  3. Forces • A force is a push or pull on an object and is directional, (making it a vector) • When forces are balanced (they add to zero) the velocity of the object is zero. • Units: Newtons = Types of Forces: Weight (W) Tension (T) Normal reaction (contact forces) Drag (drag) Upthrust (upthrust) Friction (f)

  4. Weight (W) • The result of the gravitational attraction between the object in question and the Earth. • If the object were on another planet, then its weight is defined as the gravitational interaction between its mass and that planet’s mass. • On Earth, W = mg; where m is the mass of the object and g is the gravitational field strength of the Earth (a property of the gravitational field of the Earth with units N kg-1).

  5. Tension • A string that is taught is said to be under tension. • Tension is created when two forces are applied in opposite directions at the ends of the string. • This means that any arbitrary point on the string is acted upon by 2 forces. • In most cases, the string is idealized by assuming it is massless. (really

  6. Normal reaction (contact forces) • If a body touches another body, there is a force of reaction or contact force between the two bodies. • This force is perpendicular to the body exerting the force.

  7. Drag • Oppose the motion of a body through a fluid (a gas or liquid). • Typical examples: air resistance on a car or plane, or the resistance force experienced by a steel marble dropped into a jar of honey. • Directed opposite to the velocity of the body and magnitude generally depends on the speed of the body.

  8. Upthrust • Any object placed in a fluid experiences an upward force called upthrust • If then the body will float on the fluid • If then the body will sink. • Caused by the pressure that the fluid exerts on the body

  9. Frictional Force • Oppose the motion of a body. • Arises whenever one body slides over another (kinetic friction) • Friction also arises whenever there is just a tendency for motion, not necessarily motion itself, such as when a block rests on an inclined plane but does not move (static friction)

  10. Force Diagrams • Account for all forces acting in all directions • There’s pretty much always weight in every force diagram… • Is it in contact with something? You need a normal force and probably some friction somewhere unless the problem states frictionless. • Is something holding it up (like a string/rope or spring)? You need a tension force somewhere. • Remember: THE SUM OF THE FORCES IN THE X AND Y DIRECTIONS IS ZERO IN STATIC EQUILIBRIUM!

  11. Example 1: • A block of mass m rests on a flat table. N W

  12. Example 2: • A block of mass m rests on an inclined plane. N fs W

  13. Example 3: • A block of mass m is suspended from a surface by a massless cable. T W

  14. Example 4: • A block of mass m is suspended from a surface by two massless cables. T1 T2 W

  15. You Try! • Draw force diagrams for each of the 8 static equilibrium stations around the room. • Include a sketch of the situation AND a force diagram.

  16. Hooke’s Law • If we try to extend a spring, a force pulls the spring back to its original length. • If we try to compress a spring, a force pushes the spring back to its original length. • The force in the spring, the tension, has a simple relationship to the amount by which the spring is extended or compressed… let’s try to figure that out.

  17. Research Question: What is the relationship between the amount a spring is displaced from its equilibrium and the tension in the spring? Design & conduct an experiment to explore this question. Don’t forget your hypothesis!

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