http://institute.nsta.org/web_seminars.asp nsta web seminar: force and motion: stop faking it! live...
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http://institute.nsta.org/web_seminars.asp
NSTA Web Seminar:
Force and Motion: Stop Faking It!
LIVE INTERACTIVE LEARNING @ YOUR DESKTOP
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Bill RobertsonDecember 5, 2006
NSTA Web SeminarForce and Motion: Stop Faking It!
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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
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Objects don’t always hit the ground at the same time.
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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.
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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.
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change in velocity
time for the change
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 =
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Apply Newton’s second law to a falling object
F = ma
Force of gravity = (mass of object)(accleration of object)
Fgrav = mobjectg
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If we go to a larger mass, what should happen to the size of the girl in this
drawing?
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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
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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
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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?
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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?
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Objects with different masses experience different
gravitational forces. This happens in such a way that all
objects have the same acceleration.
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Objects with different masses experience different gravitational
forces. This happens in such a way that all objects have the same
acceleration.
Why?
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“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
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A definition of weight
Weight is defined as the force the Earth’s gravity exerts on an object.
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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.
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F = maweight mass
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For an object near the surface of the Earth:
F = ma
The acceleration is g, so
Weight = mg
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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
m1= mass of one object
m2= mass of other object
r = distance between centers of spherical objects.
1 22grav
mmF G
r
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This is not an expression of F=ma!
1 22grav
mmF G
r
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Let’s write F=ma for an object near the Earth’s surface
Fgravity = ma
2
Earth objectobject object
Earth
m mG m a
r
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Earth objectm mG
2 objectEarth
mr
objecta
2Earth
objectEarth
mG ar
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The acceleration of the object does not depend
on the mass of the object
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When is an object weightless?
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When is an object weightless?
When it’s in orbit around the
Earth
When it’s in free-fall toward
the Earth
Objects are never
weightless
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Weight is the force that the Earth exerts on an object.
This force is never equal to zero.
1 22grav
mmF G
r
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When in free-fall, you feel weightless
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Orbiting is free-fall
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