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Static Equilibrium

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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

- 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!