Physics 211 Lecture 4, Slide 1

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Physics 211 Lecture 4
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• Today's Concepts
• Newtons Laws
• Acceleration is caused by forces
• Force changes momentum
• Forces always come in pairs
• Good reference frames

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Stuff you asked about
The directions of force, velocity, and
acceleration
Nothing, lets go on to Rotation!!
Third newtons law
i understood it all
Inertial/non-inertial reference frames.
none. This is old material. I did 98 of this in
high school We just didn't call things
derivatives. But I would be interested in an
example where the mass of an object changes.
This actually isn't something that was discussed
in the prelecture, but something that has always
bothered me is the derivation of the centripetal
force on a banked ramp (it's inverse tangent of
v2/r or something along those lines.
centrifugal force
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Acceleration is caused by force.
A bigger mass makes this harder
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Preflight/Act

• Suppose the net force on some object is parallel
  to the x axis. Which of the following statements
  best describes the motion of the object
• Its velocity is parallel to the x axis
• Its acceleration parallel to the x axis
• Both its velocity and its acceleration are
  parallel to the x axis
• Neither its velocity or its acceleration need be
  parallel to the x axis

Vote again
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Preflight/Act

• Suppose the net force on some object is parallel
  to the x axis. Which of the following statements
  best describes the motion of the object
• Its velocity is parallel to the x axis
• Its acceleration parallel to the x axis
• Both its velocity and its acceleration are
  parallel to the x axis
• Neither its velocity or its acceleration need be
  parallel to the x axis

Your preflight results
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Act
A force F is applied to a small block, that
pushes a larger block. The two blocks accelerate
to the right. Compare the NET FORCE on the block
w/ mass M, to the net force on the block with
mass 5M. A) FM lt F5M B) FM F5M C) FM gt F5M
Same acceleration, so larger mass has larger net
force.
5M
a
F
M
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Preflight/Act

• You are driving a car with constant speed around
  a horizontal circular track.
• The net force acting on your car
• Points radically inward toward the center of the
  circular track
• Points radically outward, away from the center of
  the circular track
• Points forward in the same direction your car is
  moving
• Points backward, opposite to the direction your
  car is moving
• Is zero.

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• You are driving a car with constant speed around
  a horizontal circular track.
• The net force acting on your car
• Points radically inward toward the center of the
  circular track
• Points radically outward, away from the center of
  the circular track
• Points forward in the same direction your car is
  moving
• Points backward, opposite to the direction your
  car is moving
• Is zero.

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Preflight/Act

• You are driving a car with constant speed around
  a horizontal circular track.
• The net force acting on your car
• Points radically inward toward the center of the
  circular track
• Points radically outward, away from the center of
  the circular track
• Points forward in the same direction your car is
  moving
• Points backward, opposite to the direction your
  car is moving
• Is zero.

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Aside Centripetal acceleration and force
1) Objects moving in a circle always have a
component of acceleration, called centripetal,
which is toward the center of the circle.
2) Centripetal acceleration must be caused by a
force - Friction, gravity whatever force
keeps it moving in a circle. - This
force is often called the centripetal force
3) There is no new kind of force here.
4) There is no such thing as centrifugal force.
Wineglass demo
They can have also have tangential acceleration
if their speed is not constant
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Tablecloth Dishes
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Preflight/Act

• You are driving a car with constant speed around
  a horizontal circular track.
• The momentum of your car
• Points radically inward toward the center of the
  circular track
• Points radically outward, away from the center of
  the circular track
• Points forward in the same direction your car is
  moving
• Points backward, opposite to the direction your
  car is moving
• Is zero.

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Ice-puck
Fake Forces Coriolis (YouTube)
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This effect can make cannons miss their targetif
you dont takeit into account.
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Forces come in pairs!
Fire-cart
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A small guy and a large football player moving at
the same speed collide head-on. Which person
experiences the larger force during the collision?

• The small guy.
• The football player.
• They experience the same force.

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A small guy and a large football player moving at
the same speed collide head-on. Which person
experiences the larger acceleration during the
collision?

• The small guy.
• The football player.
• The accelerations are the same.

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A small guy moving at a high speed collides with
a stationary large football player. Now, which
person experiences the larger force during the
collision?

• The small guy experiences the larger force.
• The football player experiences the larger force.
• Both experience the same force.