Static equilibrium
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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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Static equilibrium

Static Equilibrium

Still a constant velocity…but this time, it’s zero.

Static = not moving


Static equilibrium1

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

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)


Weight w

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).


Tension

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


Normal reaction contact forces

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.


Static equilibrium

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.


Upthrust

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


Frictional force

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)


Force diagrams

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!


Example 1

Example 1:

  • A block of mass m rests on a flat table.

N

W


Example 2

Example 2:

  • A block of mass m rests on an inclined plane.

N

fs

W


Example 3

Example 3:

  • A block of mass m is suspended from a surface by a massless cable.

T

W


Example 4

Example 4:

  • A block of mass m is suspended from a surface by two massless cables.

T1

T2

W


You try

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.


Hooke s law

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.


Research question

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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