Teeter Totter Balance: Why Does the Lighter Person's End Go Down?

[physicsgurl12]

Homework Statement

suppose that a heavy person and a light person are balanced on a teeter totter made of plank wood. each person moves toward the fulcrum 25cm. why will the lighter persons end go down??

Homework Equations

not needed

The Attempt at a Solution

something about forces and gravity??

 

[phinds]

Do you know the math to write the balancing equation for the two sides of the seesaw?

 

[physicsgurl12]

Do you know the math to write the balancing equation for the two sides of the seesaw?

math? i don't think so.

 

[phinds]

OK, I'm a bit puzzled as to how you can be working on such a simple problem and not know the math. What grade are you in?

 

[physicsgurl12]

OK, I'm a bit puzzled as to how you can be working on such a simple problem and not know the math. What grade are you in?

im a senior. i don't understand why we need math. were not finding a number.

 

[phinds]

im a senior. i don't understand why we need math. were not finding a number.

No, you're not trying to get a number, you're trying to get an ANSWER, and the quantitative asnwer is a number and if you don't understand how to get the quantitative answer, you don't understand the answer.

Do you have no idea how a fulcrum works mathematically?

 

[physicsgurl12]

um not really. its like the piviot point. that's about all i know

 

[phinds]

If you had a seesaw with a 100 pound kid sitting out 10 feet from the center (the fulcrum), how much effort do you think it would be to keep him in the air if you could put your hand on the (strong, but weightless) seesaw a mile out on the other end?

How about 100 feet out?

How about 1 foot out?

What does thinking about this tell you?

 

[physicsgurl12]

that the amount of force depends on the distance

 

[phinds]

good. Can you quantify that?

 

[physicsgurl12]

umm i can try.
m1*d1=m2*d2

 

[phinds]

excellent.

SO ... if we have, for example, a 100lb kid 10 feet out and a 200 lb man 5 feet out, then they balance, according to your equation (which is correct).

Now do the math if they both move in 1 foot.

 

[physicsgurl12]

900 =600

 

[phinds]

well, actually they are NOT equal, are they? So what does it mean in terms of who goes up and who goes down?

What do those numbers represent?

[by the way, check your math]

 

[physicsgurl12]

sorry 800 haha. those number represent the force? It means the fat kid would go down?

 

[phinds]

sorry 800 haha. those number represent the force? It means the fat kid would go down?

Yes it represents force. Which side is exerting the most force? Does the most force cause the side that has it to go up or does the OTHER side go up?