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Transcript
LIVE INTERACTIVE LEARNING @ YOUR DESKTOP NSTA Web Seminar: Force and Motion: Stop Faking It! http://institute.nsta.org/web_seminars.asp NSTA Web Seminar Force and Motion: Stop Faking It! Bill Robertson December 5, 2006 What’s going to happen when he drops the paper clip and the box of paper clips? They will land at the same time The heavier object—the box—will hit first There’s no way to tell what will happen Objects don’t always hit the ground at the same time. A geologic hammer in Apollo 15’s astronaut David Scott's right hand and a falcon feather in his left hand reached the surface of the moon at the same time. Okay, so if you ignore air resistance, all objects land at the same time. Why? The Earth’s gravity The Earth’s gravity The mass of an exerts the same exerts different object has force on all forces on different nothing to do objects, so they objects, but they with how it is end up with the end up with the affected by same acceleration. same acceleration. gravity. If you ignore air resistance, all objects dropped from the same height land at the same time. It turns out they have the same acceleration. Near the surface of the Earth, that acceleration is represented by the letter g. g = 9.8 meters per second per second = 32 feet per second per second change in velocity Remember that acceleration = time for the change Apply Newton’s second law to a falling object F = ma Force of gravity = (mass of object)(accleration of object) Fgrav = mobjectg If we go to a larger mass, what should happen to the size of the girl in this drawing? If we go to a larger mass, what should happen to the size of the girl in this drawing? She should get smaller She should get larger She should stay the same size As the mass gets larger, meaning the girl gets larger, what happens to the acceleration? Remember that we’re talking about objects falling under the influence of gravity. The acceleration gets larger The acceleration gets smaller The acceleration stays the same If the mass gets larger and the acceleration stays the same, what must happen to the gravitational force in order to keep things in balance? If the mass gets larger and the acceleration stays the same, what must happen to the gravitational force in order to keep things in balance? The gravitational The gravitational The gravitational force must get force must get force must larger smaller remain the same. Objects with different masses experience different gravitational forces. This happens in such a way that all objects have the same acceleration. Objects with different masses experience different gravitational forces. This happens in such a way that all objects have the same acceleration. Why? “When I pull the wagon, the ball rolls to the back of the wagon. And when I’m pulling it along, and I suddenly stop, the ball rolls to the front of the wagon. Why is that?” ‘That, nobody knows,’ he said. ‘The general principle is that things which are moving tend to keep on moving, and things which are standing still tend to stand still, unless you push them hard. This tendency is called inertia, but nobody knows why it’s true.’ “Now, that’s a deep understanding.” -- R.P. Feynmann A definition of weight Weight is defined as the force the Earth’s gravity exerts on an object. What’s the difference between mass and weight? Mass and When you head Mass is a measure weight differ by to the moon, of inertia, and is the a conversion your weight quantity that goes factor, such as changes but on the right side of 2.2 pounds per your mass F=ma. Weight is a kilogram. remains the force, and it goes on same. the left side of F=ma. F = ma weight mass For an object near the surface of the Earth: F = ma The acceleration is g, so Weight = mg Away from the surface of the Earth, we need a different expression for gravitational force. A general expression that applies to the gravitational force between any two objects is: Fgrav m1m2 G 2 r G = a very tiny number m1= mass of one object m2= mass of other object r = distance between centers of spherical objects. Fgrav m1m2 G 2 r This is not an expression of F=ma! Let’s write F=ma for an object near the Earth’s surface Fgravity = ma G mEarth mobject rEarth 2 mobject aobject G mEarth mobject rEarth 2 mobject aobject mEarth G a object 2 rEarth The acceleration of the object does not depend on the mass of the object When is an object weightless? When is an object weightless? When it’s in When it’s in orbit around the free-fall toward Earth the Earth Objects are never weightless Weight is the force that the Earth exerts on an object. Fgrav m1m2 G 2 r This force is never equal to zero. When in free-fall, you feel weightless Orbiting is free-fall 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 Judith Lopes, Administrative Assistant Susan Hurstcalderone, Volunteer Chat Moderator LIVE INTERACTIVE LEARNING @ YOUR DESKTOP