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# Static Equilibrium - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about ' Static Equilibrium' - lecea

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### Static Equilibrium

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

Static = not moving

• 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.

• 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)

• 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).

• 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

• 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.

• 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.

• 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

• 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)

• 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!

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

N

W

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

N

fs

W

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

T

W

Example 4:

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

T1

T2

W

• Draw force diagrams for each of the 8 static equilibrium stations around the room.

• Include a sketch of the situation AND a force diagram.

• 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.

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!