Relative Motion

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Relative Motion

• You are swimming across a 50m wide river in which
  the current moves at 1 m/s with respect to the
  shore. Your swimming speed is 2 m/s with respect
  to the water. You swim across in such a way that
  your path is a straight perpendicular line across
  the river.
• How many seconds does it take you to get across?

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What Causes Acceleration?

• Group 1 Go on the internet and find newtons
  laws. Physicsclassroom.com might be good. Be
  prepare to discuss them.
• Group 2 Devise an experiment to test what is
  important in horizontal acceleration. You have a
  bowling ball, and tennis ball as equipment.
  (don't drop the bowling ball) List the things
  that effect the acceleration as you push a ball.
• Group 3 Discuss moving the contents of your
  house what would you least like to move and why?
• Group 4 Push against the wall. Tell me all the
  things that are pushing and being pushed. (Their
  are more than you think.)

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The Laws of Motion

• Isaac Newton (1642 - 1727) published Principia
  Mathematica in 1687. In this work, he proposed
  three laws of motion
• Law 1 An object at rest tends to stay at rest
  and object in motion tends to stay in motion.
• Law 2 For any object, FNET ??F ma
• Law 3 Forces occur in pairs FA ,B - FB ,A
  (For every action there is an equal
  and opposite reaction.)

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Force

• We have an idea of what a force is from everyday
  life.
• Physicist must be precise.
• A force is that which causes a body to
  accelerate.
• (See Newtons Second Law)
• A Force is a push or a pull.
• A Force has magnitude direction (vector).
• Adding forces is like adding vectors

FNET ma
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Force

• We will consider two kinds of forces
• Contact force (physical contact between objects)
  
• This is the most familiar kind.
• Kicking a ball
• I push on the desk.
• The ground pushes on the chair...
• Field Forces (Non-Contact) (action through
  empty space)
• Moon and Earth
• Gravity
• Electricity
• On a microscopic level, all forces are
  non-contact (Hun????)

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Contact forces

• Objects in contact exert forces.
• Convention Fa,b means the force acting on a due
  to b.
• So Fhead,thumb means the force on the head due
  to the thumb.

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An Example
Consider the forces on an object undergoing
projectile motion
EARTH
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Action at a distance

• Gravity

Well come back to this later. Just remember you
dont have to touch to push
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Mass

• We have an idea of what mass is from everyday
  life.
• Physicist must be precise.
• mass (for this class) is a quantity that
  specifies how much inertia an object has.
• (See Newtons First Law)
• Mass is an inherent property of an object.
• Mass and weight are different quantities.
• weight is a force.

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Newtons First Law

• An object subject to no external forces moves
  with a constant velocity if viewed from an
  inertial reference frame.
• If no forces act, there is no acceleration.
• The above statement can be thought of as the
  definition of inertial reference frames.
• An IRF is a reference frame that is not
  accelerating (or rotating) with respect to the
  fixed stars.
• If one IRF exists, infinitely many exist since
  they are related by any arbitrary constant
  velocity vector!

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Newtons Second Law

• The acceleration of an object is directly
  proportional to the net force acting upon it. The
  constant of proportionality is the mass.

• Units
• The units of force are kg m/s2 Newtons (N)
• The English unit of force is Pounds (lbs)
• Ex weight is a force Wmg

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Mass vs. Weight

• An astronaut on Earth kicks a bowling ball and
  hurts his foot. A year later, the same astronaut
  kicks a bowling ball on the moon with the same
  force.
• His foot hurts...

Ouch!
(a) more (b) less (c) the same
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Mass vs. Weight

• THE SAME!!!!
• If I stand on earth and hit myself in the head
  with a hammer it hurts. If I do the same thing
  in space I think you can imagine that it would
  still hurt.

Ouch!
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Mass vs. Weight
Wow! Thats light

• However the weights of the bowling ball and the
  astronaut are less
• Thus it would be easier for the astronaut to pick
  up the bowling ball on the Moon than on the
  Earth.

W m gMoon gMoon lt gEarth
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Newtons Second Law...

• Components of F ma
• FX maX
• FY maY
• Suppose we know m and FX , we can solve for aX
  and apply the things we learned about kinematics
  over the last few weeks

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Example Pushing a Box on Ice.

• A skater is pushing a heavy box (mass m 100 kg)
  across a sheet of ice (horizontal
  frictionless). He applies a force of 50N in the
  i direction. If the box starts at rest, what is
  its speed v after being pushed a distance d10m ?

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Force and acceleration

• A force F acting on a mass m1 results in an
  acceleration a1.The same force acting on a
  different mass m2 results in an acceleration a2
  2a1.

• If m1 and m2 are glued together and the same
  force F acts on this combination, what is the
  resulting acceleration?

(a) 2/3 a1 (b) 3/2 a1 (c) 3/4
a1
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Newtons Second Law
A constant force is exerted on a cart that is
initially at rest on an air table. The force acts
for a short period of time and gives the cart a
certain final speed.
Force
Cart
Air Track
For a second shot, I can apply a force only half
as large (Im getting tired). To reach the same
final speed, for how long must I apply the force ?
A) 4 x as long B) 2 x as long C) Same time D)
1/2 as long E) 1/4 x as long
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Talk about Quadratic and linear relationships
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Newtons 3rd Law

• For every action there is an equal and opposite
  reaction

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Newtons Third Law
A fly gets smushed onto the windshield of a
speeding bus.
The force exerted by the bus on the fly is, A)
greater than B) the same as C) less than that
exerted by the fly on the bus.