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LIVE INTERACTIVE LEARNING @ YOUR DESKTOP NSTA Web Seminar: Force and Motion: Stop Faking It! Thursday, January 25, 2007 7:00 p.m. to 8:00 p.m. Eastern time NSTA Web Seminar Force and Motion: Stop Faking It! Bill Robertson January 25, 2007 Circular Motion—Which of the following are true? Objects The force that moving in a causes objects circle at a to move in a constant circle is called speed are not the centripetal accelerating force The force that causes objects to move in a circle is called the centrifugal force Centrifugal force doesn’t really exist Is an object moving in a circle at constant speed accelerating? Velocity is a quantity that includes both the magnitude (the speed) and the direction. Any change in speed and/or direction is a change in velocity. Newton’s second law ΣF = ma ΣF: represents the net force acting on an object. m: represents the mass of an object, which is a numerical measure of its inertia. a: represents the acceleration of the object. So, there must be a net force acting on something moving in a circle at constant speed. Let’s investigate that force So, circular motion at a constant speed requires a force that acts toward the center of the circle. You might be aware that we call that force the centripetal (center seeking) force. Centripetal force is… A force that you A fictitious force that Just a name applied have to add to your doesn’t really exist to the net force that diagram because already exists the object is moving in a circle The centripetal force is a name given to forces that are already present, that happen to cause something to move in a circle. In this case, the friction between Einstein and the record is the force causing Einstein to move in a circle. Therefore, friction is the centripetal force. A centrifugal (center fleeing) force is a force that pushes you away from the center of the circle in which you’re moving. Is the centrifugal force a real force? Yes No Now look at things from Einstein’s point of view When you view things from a rotating frame of reference, a centrifugal force appears. This force is very real to someone in that frame of reference. Another example As viewed in the rotating frame of reference Centrifugal force does not exist if you are not in a rotating frame of reference. Centrifugal force does exist in a rotating frame of reference. You can choose a frame of reference in which the force of gravity does not exist True False In a freely-falling frame of reference (think falling elevator), there is no force of gravity. So, centrifugal force is every bit as real as the force of gravity. Know of any other forces that come into being in a rotating frame of reference? In a rotating frame of reference, any moving object experiences a sideways force known as the coriolis force. Think hurricanes. Newton’s second law ΣF = ma ΣF: represents the net force acting on an object. m: represents the mass of an object, which is a numerical measure of its inertia. a: represents the acceleration of the object. image The rotational analog to F=ma is = Iα • is the net torque acting on the system • I is the moment of inertia of the system • α is the rotational acceleration of the system Push on a door and try to make it swing. First try pushing near the hinge and then try pushing as far from the hinge as possible. In which case is it easiest to make the door swing? Near the hinge Farthest from the hinge Torque includes not just the applied force, but the distance the force is from the point of rotation. Realizing that the net torque acting on an object that’s not rotating is zero gives us the “law of the lever.” National Science Teachers Association Gerry Wheeler, Executive Director Frank Owens, Associate Executive Director Conferences and Programs Al Byers, Assistant Executive Director e-Learning NSTA Web Seminars Flavio Mendez, Program Manager Jeff Layman, Technical Coordinator Susan Hurstcalderone, Volunteer Chat Moderator LIVE INTERACTIVE LEARNING @ YOUR DESKTOP