PHYSICS - Force Problems Continue

OPENING QUESTION:

Why is it that your weight (mg) NEVER changes in elevator problems (it's TRUE!)

If that is the case (and it is, as long as you are on or near Earth's surface), work with your team to explain WHY?

LEARNING TARGET: I will review elevator problems during today's class.

I will work with my team to design a ramp problem with friction during today's class.

I will complete a side-hanger problem during today's class.

  • Force ("Push" or "Pull")

  • Newton (kgm/s2)

  • Force Diagram (A sketch showing ALL forces acting on an object)

  • Net Force Diagram (A sketch showing only the resulting (net) forces acting on an object)

  • Newton's 1st Law ("An object will continue whatever it is currently doing unless acted on by an unbalanced force"

  • Newton's 3rd Law

    • (Short Version: "Forces ALWAYS come in pairs")

    • (Quick memory aid version: Noun1 Verb Noun2 | Noun2 Verb Noun1)

    • (Formal Version: For every force acting on one object, there is an equal and opposite force acting on a second object

FORMULAE OBJECTUS:

  • f = μₛN: friction force = coefficient of static friction x Normal force
  • F = ma (Newton's 2nd Law)
  • ΣFx = max (Newton's 2nd Law applied to multiple forces acting on an object horizontally) Notice how that works so well with Newton's 1st:
    • If the sum of the forces acting on an object are unbalanced in x, the object will accelerate in x.
    • If the sum of the forces acting on an object in x are balanced the object will continue doing whatever it is currently doing.
    • If the object is at rest then the sum-of-the-forces-in-x MUST be balanced!!!
  • ΣFy = may (Newton's 2nd Law applied to multiple forces acting on an object vertically) Notice how that works so well with Newton's 1st:
    • If the sum of the forces acting on an object are unbalanced in y, the object will accelerate in y.
    • If the sum of the forces acting on an object in y are balanced the object will continue doing whatever it is currently doing.
    • If the object is at rest then the sum-of-the-forces-in-y MUST be balanced!!!

 

CALENDAR: Forces Unit Test:

Newton's 3

Hangers/Ramps/Elevator problems

Friction

Monday Next

WORK O' THE DAY

Sketch a person in an elevator standing on a bathroom scale:

By the by, why do we ALWAYS imagine a person standing on a bathroom scale? What does that scale allow us to visualize? Persons designated by the color orange, please lead the conversation (and the conversations below too)

Now please work with your group to determine why it is that elevators NEVER accelerate downwards great her than 9.81 m/s/s?

Let's say an elevator accelerated downwards at 10. m/s/s. What would happen? Now do the math.

For that matter, why do elevators never accelerate upwards at 9.81 m/s/s?

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A 125 kg object is supported by a wire pulling directly to the left with a force T1 and a wire T2 pulling upward at an angle of 27 degrees to the horizontal. Sketch that situation.

Solve for T1 and T2

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Now please work with your team to design a ramp problem thusly:

1) Provide your colleague with the mass of an object sliding down a ramp, the angle of the ramp, the ramp material and the object material (that means you have to do a wee bit of research online to find the coefficient of static friction μₛ for those materials FIRST)

2) Have your colleagues calculate the acceleration down the ramp. Keep in mind, it is *entirely* possible the object will not accelerate down the ramp, but how will your colleagues make that determination?