Regents%20Physics

1
Regents Physics

• Agenda
• Introduction to Forces
• Intro to Newtons three Laws of Motion
• HW Read p. 117-125

2
Whats a Force?

• Weve learned that acceleration is the change in
  an objects velocity..
• And
• Velocity is the change in an objects position..
• By what causes the change in acceleration?

A Force!
3
Whats a Force?

• Forces can be described as a push or a pull that
  is applied to an object by something else..
• Forces are vectors magnitude and direction
• The ability to understand how forces affect us is
  crucial to success in many fields
• Ex building of homes and bridges

4
Kinds of Forces

• Contact force acts on an object only by
  touching it
• Ex book on table, friction
• Long-range force is exerted without contact
• Ex magnetic force, force of gravity

5
Forces have agents..

• Each force has a specific, identifiable,
  immediate cause called the agent
• The agent can be animate such as a person
• or
• inanimate such as a desk, floor or magnet

Whats the agent for the pull of gravity?
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Solving Force Problems - intro

• First step is to draw a pictorial model, called a
  free body diagram, and identify the contact and
  long range forces
• Draw the vectors
• Example

F desk on book


Book on a table
F book on desk
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Solving Force Problems - intro

• Examples

Ball on a rope
Skydiver

F rope on ball
F air on diver
F ball on rope
F gravity on diver
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Practice Problems

• Draw a free body diagram for each of the
  following
• Book held in your hand
• Book pushed across the desk by your hand
• Book pulled across the desk by a string
• Book on a desk with you hand pushing down
• Ball just after the string that was holding it
  broke

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The manSir Isaac Newton

• 300 hundred years ago an apple fell on his
  headand he wondered why?
• Explained the way forces influence motion
• Summed it up in three famous laws

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2nd Law of Motion

• The force exerted on an object is equal to the
  objects mass times its acceleration, or F ma
• Expressed in newtons kg x m/s2
• Example Mr. O
• Mass 95 kg
• Acceleration gravity 9.80 m/s2

F ma (95 kg)(9.80 m/s2) 931N
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2nd law continued

• F ma can be rearranged to be
• we say that the force exerted on an object is
  proportional to its acceleration (since the
  objects mass doesnt change)
• The larger the force..the greater the
  acceleration
• We have a linear relationship!

F m
a
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2nd law continued

• Multiple forces can combine and act on a system
• They could act in the same direction or in
  different directions
• Because forces are vectors, the total force on an
  object is the vector sum of all forces on the
  object
• This vector sum is called the net force

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Finding a net force

• Two horizontal forces act, 225 N and 165 N, are
  exerted in the same direction on a crate (assume
  no friction). Find the net horizontal force on
  the crate.
• Step 1 draw a free body diagram

Fnet Facting on the crate
F 225 N
Fnet 225N 165N 390N
F 165 N
worksheet
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Regents Physics

• Agenda
• Newtons first law of motion - Intro
• Newtons second law practice probs
• HW Chap Problems
• RC 1,3,5,6,8,9
• AC 10, 15, 16
• Probs 20, 22, 25, 27, 30, 32, 36, 38, 40

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1st Law of Motion

• An object that is at rest will remain at rest or
  an object that is moving will continue to move in
  a straight line with a constant speed, if and
  only if the net force acting on that object is
  zero.
• Also called the Law of Inertial Mass
• What does that mean to us?

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1st Law of Motion

• What is inertia?

Inertia is the tendency of an object to resist
change Anything that has mass has inertia! We
resist what tries to change us! Examples Sittin
g still / moving at a constant velocity Object in
space Turning in a car
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1st Law of Motion

• What is equilibrium?

If an object is at rest or it is moving at a
constant velocity, the net force is zero in both
cases
We are happy just chillin..
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Some Types of Forces

• See table 6.2 pg. 123

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Common Misconceptions

• When we throw a ball, the force from our hand
  stays on it
• A force is needed to keep an object moving
• Inertia, itself, is a force
• Air does not exert a force

20
Calculations with Newtons 2nd Law

• Using F ma
• Mass and weight..whats the difference?
• Ex a bathroom scale
• Draw a freebody diagram for this and label the
  forces

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A system of solving..

• Read the problem and visualize!
• Choose a coordinate system
• Write your known and unknowns
• Use f ma to link acceleration and net force
• Rearrange, plug in numbers and solve
• Check your answer to see if its reasonable

22
Practice Problem

• On Earth, a scale shows that you weigh 585 N
• a) What is your mass?
• B) What would the scale read on the moon? (g
  1.60 m/s2)

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Regents Physics

• Agenda
• More on Newtons Second Law
• Review Intro to Forces Worksheet
• Drawing Free-body Diagrams Worksheet
• HW More Advanced Newtons Second Law Problems

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Practice Problem - elevator

• You still weight 585 N. Find the reading on a
  scale in an elevator when
• a) the elevator moves up at a constant speed
• b) it slows at 2.0 m/s2, while moving upward
• c) It speeds up while moving 2 m/s2 downward
• d) it moves downward at a constant speed
• e) it slows to a stop at a constant magnitude of
  acceleration

End
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Working with friction...

• Friction is a force that generally opposes the
  direction of motion
• Different surfaces have different amounts of
  friction that affect an objects movement
• There are types of friction
• Kinetic Friction uk
• Static Friction us

Ffr Fnu
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Solving Problems with Friction

• Find the net force on an object that has a mass
  of 20.0 kg, acceleration of 2.3 m/s2 on an
  surface with a friction coefficient of 0.21

Known FnFgmg(20.0kg)(9.8 m/s2) 196 N Fa
ma (20.0kg)(2.3 m/s2) 46N Ffr Fnuk
(196N)(0.21) 41.2 N
Fn
Ffr
Fa
Fnet Fa - Ffr Fnet Fa - Fnuk Fnet
46N - 41.2N 5N
Fg
Worksheet
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3rd Law of Motion

• Every force has an equal and opposite force,
• or
• FA on B -FB on A

Fyou on wall Fwall on you