Transcript Web Seminar—Force and Motion

NSTA Web Seminar
Force and Motion
Bill Robertson
February 1, 2006
Newton’s third law
Circular motion
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
Which best describes Newton’s
third law?
• If one object is causing another to accelerate, then
the first object is pushing harder on the second
object than the second object is pushing on the
first object.
• If you push something hard enough, it will fall over.
• Every force exerted on an object results in some
kind of reaction.
Which best describes Newton’s
third law?
• If one object is causing another to accelerate, then
the first object is pushing harder on the second
object than the second object is pushing on the
first object.
• If you push something hard enough, it will fall over.
• Every force exerted on an object results in some
kind of reaction.
Newton’s third law
• Action and reaction
• Whenever object A exerts a force
on object B, object B exerts an
equal and opposite force back on
object A.
Balloon clouds in back
Balloon clouds at bottom
1 donkey
4 donkeys
A
B
C
D
Split donkey carts
A
B
C
D
How does a propeller plane work?
Single Airplane
How does a propeller plane work?
Single Airplane
How does a rocket work?
Rocket Pushes Gas
How does a jet plane work?
You catch a baseball. Which of the following
are true about the forces exerted?
• You exert a larger force on the baseball
than the baseball exerts on you.
• The baseball exerts a larger force on you
than you exert on the baseball.
• The force you exert on the baseball is
equal to the force the baseball exerts on
you.
You catch a baseball. Which of the following
are true about the forces exerted?
• You exert a larger force on the baseball
than the baseball exerts on you.
• The baseball exerts a larger force on you
than you exert on the baseball.
• The force you exert on the baseball is
equal to the force the baseball exerts on
you.
For the baseball:
F = ma
For you:
F = ma
Animation of carts on air track
Circular Motion—Which of the
following are true?
• Objects moving in a circle at a constant speed
are not accelerating
• The force that causes objects to move in a circle
is called the centripetal force.
• The force that causes objects to move in a circle
is called the centrifugal force.
• Centrifugal force doesn’t really exist.
Circular Motion—Which of the
following are true?
• Objects moving in a circle at a constant speed
are not accelerating
• The force that causes objects to move in a circle
is called the centripetal force.
• The force that causes objects to move in a circle
is called the centrifugal force.
• Centrifugal force doesn’t really exist.
Suppose you want to figure out all the forces acting
on something that's moving in a circle, such as a
race car rounding a curve at constant speed.
Should you do the following? Figure out all the
forces of contact on the car, such as friction, and
then add the centripetal force to these forces.
YES
NO
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.
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
Is the centrifugal force a real force?
YES
NO
Know of any other forces that come
into being in a rotating frame of
reference?
Upcoming NSTA Web Seminars:
• Doing Good Science II
February 8, 2006
• Watershed Dynamics II
February 16, 2006
• Investigating Safely II
February 22, 2006
6:30 PM Eastern Time