merry go round prob

[piper210_355]

I need help determining whether or not a penny will stay on spinning disk (neglecting air resistance) given these variables:

 

mass of penny: 2.5g

distance from center: 10.0cm

static friction: 0.19

angular velocity: 33.3rpm

speed: 45.0rpm

 

I'm not sure what equation(s) i should use so if you could tell me that would be a great help.

[Bignose]

Have you drawn a free body diagram so you can figure what the situation looks like and what force or forces could be acting in this situation?

 

If you haven't, it is probably a very good habit to get into because 1) it always helps to have a picture to see exactly what is and isn't happening and 2) if you drawn out the diagram, most teachers give at least some partial credit even if you don't do anything else from that point. Not necessarily a lot, but something like 20% is better than 0%.

[swansont]

Spinning disk means circular motion. What equations do you have that apply to circular motion?

[piper210_355]

i looked online and some of the equations that i found are:

Force=mass*centripital acceleration

centripital acceleration= velocity squared/radius

[Bignose]

is this for homework, or is this just for your own personal improvement? Because it seems awfully homework-like (just like the other thread you started), and if so, your text or your class notes should have the equations you need. You shouldn't have to look online.

[tyrannt]

Ok, so wait, the static friction you reported. Is that the coefficient of static friction or the overall force of static friction?

 

If it is the coeffient, you need to find the static frictional force which is

Static Fricion (less than or equal to) coefficient * normal force.

 

Then you would need to find out how much centripital force is being applied to the penny.

Centripital force = (mass)*(velocity^2)/radius

 

If the centripital force is greater then the static frictional force, the penny will slide. It is important to note that the forces act in opposite directions.

[piper210_355]

yeah that is the coefficient of static friction.

 

ok so i read on a website that static friction=mass*coefficient of friction

which would be

F(k)= 2.5*.19

F(k)=.475

 

How can I find out what the normal force is?

And how do I know what the radius is when the problem only tells me that the penny is 10cm from the center? Do I just use 10 as the diameter?

Edited March 8, 2009 by piper210_355

[tyrannt]

Not quite. there are two types of friction. static and kinetic. kinetic is usually lesser in magnitude than static.

 

Static friction <= coefficient * (mass * gravity)

the mass * gravity is refered to as the normal force, which is the force which the object applies on the ground.

The less than or equal to means that this force will change in order to rival whatever force is trying to move it until the moving force becomes greater than the maximum value. At this point the object will begin to move.

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Merged post follows:

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think about this. the radius is the distance from the center right? therefore, the problem does tell us the radius by stating the penny's distance from the center of the circle.