Transcript Insert Figure 4.1 from Force and Motion book

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Force and Motion: Stop Faking It!
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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
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