Science w/ Ms. Hendryx 11/8/11. Spinning things!!! Everything that spins/twists/rotates involves circular motion. One way to describe the size of a circle.

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Presentation transcript:

Science w/ Ms. Hendryx 11/8/11

Spinning things!!! Everything that spins/twists/rotates involves circular motion. One way to describe the size of a circle is by its radius, r. So to deal with circular motion, we’re going to throw in the radius of the imaginary circle about which an object is moving (r has units of length, like meters).

1.Torque (rhymes with fork): how much you can twist/be twisted using a given amount of force. torque = F*r (r = radius) I’m changing a tire: if I need 20 N*m of torque to loosen the lug nut with a wrench, how much force to I have to apply if my grip on the wrench is 5 cm from the nut (convert to meters)? What if I hold the wrench 0.5 m from the nut? *Hint: the nut is the center of your circle of motion, and the distance from the nut to your hand is the radius of that circle.

In your book, they say angular momentum (L) is moment of inertia (I, corresponds to mass) times angular velocity (ω). So if you can remember p = mv, you can remember L = I ω. The only difference is that the angle of motion is changing (hence the circle). What is the angular momentum of an ice skater whose moment of inertia is 2.5 kg*m spinning with an angular velocity of 5 m/s?

We know that acceleration causes velocity to change with time, right? Well now the velocity is changing by changing direction, not speed. So if you swing a rock on a string, the acceleration is toward your hand. That means the centripetal force is also toward your hand.

And the velocity doesn’t change as much if the string is longer (the circle is bigger), so the acceleration decreases as radius increases: a cent = v 2 /r. (Think curves in a go-kart—do you feel the tight turns more or less?) vv a a

*Important note: “velocity” is distance per time. “Angular velocity” is angle per time. (Think ceiling fan.)

If a softball pitcher has a 2.5-ft. arm (r), what is the centripetal acceleration of the ball if the pitch is a 60 mile-per-hour fast ball?

A softball pitcher has a 2.5-ft. arm (r) We can still use Newton’s 2 nd Law, so F cent = ma cent. What is the centripetal force the pitcher above is applying to the ball? Assume the ball weighs 1lb, and include units.