1. http://institute.nsta.org/web_seminars.asp NSTA Web Seminar: Force and Motion: Stop Faking It! LIVE INTERACTIVE LEARNING @ YOUR DESKTOP

2. Bill Robertson December 5, 2006 NSTA Web Seminar Force and Motion: Stop Faking It!

4. 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

5. Objects don’t always hit the ground at the same time.

6. 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.

7. Okay, so if you ignore air resistance, all objects land at the same time. Why? The Earth’s gravity exerts the same force on all objects, so they end up with the same acceleration. The Earth’s gravity exerts different forces on different objects, but they end up with the same acceleration. The mass of an object has nothing to do with how it is affected by gravity.

8. 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 Remember that acceleration =

9. Apply Newton’s second law to a falling object F = ma Force of gravity = (mass of object)(accleration of object) F grav = m object g

10. If we go to a larger mass, what should happen to the size of the girl in this drawing?

11. She should get smaller She should get larger She should stay the same size

12. 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

13. 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?

14. The gravitational force must get larger The gravitational force must get smaller The gravitational force must remain 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?

16. Objects with different masses experience different gravitational forces. This happens in such a way that all objects have the same acceleration.

17. Objects with different masses experience different gravitational forces. This happens in such a way that all objects have the same acceleration. Why?

18. “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

19. A definition of weight Weight is defined as the force the Earth’s gravity exerts on an object.

20. What’s the difference between mass and weight? Mass and weight differ by a conversion factor, such as 2.2 pounds per kilogram. When you head to the moon, your weight changes but your mass remains the same. Mass is a measure of inertia, and is the quantity that goes on the right side of F=ma. Weight is a force, and it goes on the left side of F=ma.

21. F = ma weightmass

22. For an object near the surface of the Earth: F = ma The acceleration is g, so Weight = mg

25. 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: G = a very tiny number m 1 = mass of one object m 2 = mass of other object r = distance between centers of spherical objects.

26. This is not an expression of F=ma!

27. Let’s write F=ma for an object near the Earth’s surface F gravity = ma

29. The acceleration of the object does not depend on the mass of the object

30. When is an object weightless?

31. When it’s in orbit around the Earth When it’s in free-fall toward the Earth Objects are never weightless

32. Weight is the force that the Earth exerts on an object. This force is never equal to zero.

33. When in free-fall, you feel weightless

34. Orbiting is free-fall

35. National Science Teachers Association Gerry Wheeler, Executive Director Frank Owens, Associate Executive Director Conferences and Programs Al Byers, Assistant Executive Director e-Learning LIVE INTERACTIVE LEARNING @ YOUR DESKTOP NSTA Web Seminars Flavio Mendez, Program Manager Jeff Layman, Technical Coordinator Judith Lopes, Administrative Assistant Susan Hurstcalderone, Volunteer Chat Moderator