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Thread: need formulas

  1. #1 need formulas 
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    Join Date
    Oct 2008
    i was wondering if any one knows a formula that would figure out how much power it would take for an electromagnet to push a certain amount of weight?


    (this is hard to explain)
    i am also looking for a formula to figure out the speed of a point on a fulcrum...
    shit.. its hard to explain so maybe this makes more since:

    say you tied a rock to a 2 foot string and then tied the other end of the string to a motor that spun at 10 miles an hour and did not "reel in the string" so the string stays at 2ft, how much faster would the rock be moving than the motor.
    ^i need a formula for that.

    i hope that makes since.

    the reason i need it is because i wanted to figure out how fast the motor would have to spin, and how long the "string" would have to be to make the "rock" travel at near light speed. even though its supposedly impossible, i still am interested. and i hope that this post makes since.

    another attempt to elaborate:

    length of string = L
    speed of motor = M
    speed of rock = R

    so if the length of the string is ____ and the speed of the motor is ____, then the speed of the rock is ____.

    i hope that makes at least some since.

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  3. #2  
    Forum Radioactive Isotope MagiMaster's Avatar
    Join Date
    Jul 2006
    I don't know about the electromagnet, but the other is easy.

    First, consider this: How do you spin a mile? The answer is, you don't. Motors can't spin 10 miles an hour. They can spin 10 revolutions a second though. Then all you need to do is to consider the circumference of the circle the rock would follow and how calculate how fast it'd have to be going to cover that circle that often.

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  4. #3  
    Forum Bachelors Degree Demen Tolden's Avatar
    Join Date
    Sep 2007
    St. Paul, Minnesota, USA
    Power is work applied over a time interval. What you are asking for your first question is similar to asking how fast you are traveling if you travel 50 miles. What you should be asking is how much force is required for an electromagnet to push a certain amount weight. The answer to this question is enough to overcome static friction, or the frictional force of a surface that resists any force applied to a stationary object. The formula for static friction is:

    where is the force of the static friction, is the coefficient of static friction, and is the normal force that the surface applies to the object. If no upward or downward force is applied to the object besides gravity, the normal force is equal to the mass of the object multiplied by the gravitational pull of earth, or in other words:

    can be thought of as the ability of a particular surface to grab the surface of another object. For example, ice has a low frictional coefficient while sand paper has a high frictional coefficient.

    As another note, when the object begins to move, the static frictional force no longer applies, and the kinetic friction force is what fights against horizontal forces. Typically kinetic friction is much less than static friction, or in other words, when an object is moving, the frictional force on the object is less. The formula for kinetic friction is:

    Its basically the same formula except you just need to know the coefficient of kinetic friction instead of static friction.

    For your second question, ignoring reletivistic effects, you just have to consider that speed is a ratio of distance over time. The outer object is traveling a larger distance over the same amount of time as the inner object. The formulas for the distance of the outer:

    for the inner:

    where x is the distance of the inner object to the center of the system and L is the length of the string from the inner object to the outer object.
    Divide these by the time interval to get their speed.

    However you are asking about reletivistic speeds, so I see that a major problem is that as an object increases its speed, it increases its mass. An object with any mass that travels the speed of light will have an infinite mass.
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