Download Canvas-g02 UCM Dyn - Clayton School District

Phyz Q&A: The Dynamics of UCM
THE CENTRIPETAL FORCE BILL
Is an object in uniform circular motion accelerating? Consider:
According to definition, acceleration is due to change in ________
Does the speed of the particle change? ( YES / NO )
Does the direction of motion change? ( YES / NO )
Is an object in uniform circular motion accelerating? (YES / NO)
If the particle is accelerating, what must be true about the forces
acting on it? (Newton I, II)__________________________________
How big is this centripetal force? First, write Newton’s second law
(the relation of force to mass and acceleration) _________________
Next, write the equation describing the centripetal acceleration in
terms of the velocity and radius of a particle in UCM. ____________
Now, write Newton II again, except now substitute “a” with the
expression for centripetal acceleration............................................
∑F =
THIS IS THE EQUATION FOR THE AMOUNT OF FORCE REQUIRED BY ANY OBJECT IN UNIFORM CIRCULAR
MOTION (UCM). BECAUSE THE FORCE ACTS TO ACCELERATE THE OBJECT TOWARD THE CENTER OF THE
CIRCLE, IT IS CALLED “CENTRIPETAL FORCE.”
It is important to note that this equation tells only how much force the object needs in
order to stay in circular motion. The equation gives no clue as to what will provide this
amount of force.
TO MAINTAIN CIRCULAR MOTION...
Think of the equation above, then, as a “force bill.”
PLEASE PAY THIS AMOUNT:
mv2/r
CIRCULAR MOTION UTILITY DISTRICT
The object in circular motion must have a way to pay the force bill, or it cannot stay in
circular motion. This is often referred to as the “no pay, no play” rule of UCM.
CAUTION: Do not say that an object called “centripetal” supplies the force. There is no
such “force due to centripetal.” The word centripetal only indicates the direction of the
force, not the source of the force.
SO HOW DOES THE BILL GET PAID?
What supplies the force to an object in circular motion? What pays the force bill? The
answer to this question varies:
• For Odie, the centripetal force is provided by tension in the string (the tension
transmitted in the string is provided by the hand of the person swinging Odie in circular
motion).
• For the earth in circular motion around the sun, the centripetal force is provided by the
gravitational attraction of the two masses.
• For a charged subatomic particle spiraling in a bubble chamber, the centripetal force is
provided by electromagnetic interaction of the particle and the magnetic field in which it is
immersed. (What did he say?)
The Book of Phyz © Dean Baird. All rights reserved.
10/24/99 11:00 AM
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