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Forces and Newton

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Forces and Newton s Laws of Motion – PowerPoint PPT presentation

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Title: Forces and Newton


1
  • Forces and Newtons Laws of Motion

2
Isaac Newton (1642-1727)
3
  • Force
  • A push or a pull.
  • Force is not a thing in itself,
  • but rather an interaction between two objects.

4
Newtons First Law
  • The Law of Inertia
  • A body remains at rest or moves in a straight
    line at a constant speed unless acted upon by a
    net force.
  • Objects do not accelerate unless a net force is
    applied.

5
Net Force
  • Determined by combining ALL forces acting on an
    object.
  • Zero net force zero acceleration
  • If there is a net force, there will be an
    acceleration.

6
Balanced Forces (zero net)
7
Unbalanced Forces (non-zero net)
8
Mass
  • the amount of matter in an object
  • a constant property
  • a measure of the inertia
  • measured in kilograms (kg)

9
Weight
  • the force upon an object due to gravity
  • weight mass ? accel. due to gravity
  • Fw mg
  • measured in Newtons (N)

10
  • The weight of a 10 kg brick is...
  • A) 98 N
  • B) 10 kg
  • C) 9.8 kg
  • D) 10 N
  • E) 98 kg

11
Weight
Location
Mass
Earth
18.4 kg
180 N
Moon
18.4 kg
30 N
Space
18.4 kg
0 N
12
Mass and Weight
  • On the Moon, the force of gravity is only 1/6 as
    strong as on the Earth.
  • In space you are practically weightless but your
    mass remains unchanged.
  • Your mass does not depend on where your are.
  • e.g. Earth, Moon, or space

13
Equilibrium
  • Balance
  • In regards to motion an object has constant
    velocity if it is in equilibrium.
  • NO acceleration.
  • Types
  • Static Equilibrium
  • Dynamic Equilibrium

14
Static Equilibrium
  • Velocity is zero
  • Examples

Scales pushing up
Weighing yourself on a set of scales
Weight down
Car parked on an incline
Normal
Friction

Weight down
15
  • Dynamic Equilibrium
  • Velocity is nonzero and constant
  • Examples
  • Driving at constant velocity

Normal up
Friction
Force from road
Air resistance
Weight down
Terminal velocity when parachuting

Weight down
16
Newtons Second Law
  • The accel. of an object is directly proportional
    to the net force acting on the object, and
    inversely proportional to the mass of the object.

17
Newtons Second law of Motion mathematically
Net Force (mass)(accel) Fnet ma
18
When the acceleration is g we have Free Fall
m
2m
F
2F
19
NEWTON'S 2nd LAW OF MOTION
a
F
F
a
F
a
F
a
F
a
F
a

20
Terminal Velocity
Net Force
Acceleration g
Velocity 0 but motion is about to begin
mg
F
Acceleration lt g
v increasing downward
mg
F
Acceleration ltlt g
v still increasing downward just not as rapidly
as before
mg
F
Acceleration 0
Terminal velocity

mg
21
Falling with Air Resistance
  • Air resistance increases with speed and increased
    cross-sectional area.

22
Terminal Velocity
  • When falling the force of air resistance becomes
    large enough to balance the force of gravity.
  • At this instant in time, there is no net force
    the object stops accelerating (se D below)
    terminal velocity has been reached.

23
  • Example
  • Light and heavy parachutists


24
Friction
  • The force that opposes the motion between two
    surfaces that are in contact.
  • Friction is the "evil monster" of all motion.
    Regardless of which direction something moves in,
    friction pulls it the other way.
  • Move something left, friction pulls right. Move
    something up, friction pulls down. It appears as
    if nature has given us friction to stop us from
    moving anything.
  • Friction is actually a force that appears when
    there is relative motion between two objects.
  • Although two objects might look smooth,
    microscopically, they're very rough and jagged.

25
Static (starting) Friction
  • The force that opposes the start of the motion.
  • Static means stationary ( not moving).

26
Kinetic (sliding) Friction
  • The force between surfaces in relative motion
  • For the same object, why is the force of kinetic
    friction less than the force of starting friction?

27
Pressure
  • The amount of force applied over a
    given area.
  • Pressure Force ? Area
  • P FA ? A
  • Measured in pascals (Pa)

28
Your Force vs Your Pressure
  • How could you determine the amount of pressure
    you exert on the ground while standing?
  • How would this change if you stood on on only one
    foot?

How do the pressures compare? All the acrobats
weigh the same.
29
Atmospheric Pressure
  • Air pressure is the force exerted on you by the
    weight of tiny particles of air
  • Although air molecules are invisible, they still
    have mass and take up space.

30
How much pressure are you under?
  • Earth's atmosphere is pressing against each
    square inch of you with a force of 1 kilogram per
    square centimeter (14.7 pounds per square inch).
  • The force on 1,000 square centimeters (a little
    larger than a square foot) is about a ton!

31
Newtons Third Law
  • Action-Reaction
  • When one object exerts a force on another object,
    the second object exerts a force of equal
    strength in the opposite direction on the first
    object.

32
Newtons Third Law of Motion
For every action, there is always an equal
(magnitude) and opposite (direction) reaction.
By action or reaction, we mean a force.
Action/reaction forces do not act on the same
object.
33
Reaction road pushes on tire
Action tire pushes on road

34
Reaction gases push on rocket
Action rocket pushes on gases

35
Action earth pulls on kid
Reaction kid pulls on earth

36
  • Horse pulls on cartcart pulls on horse. How
    does the cart ever move forward?
  • Use Newtons 2nd and 3rd laws of motion to
    explain.

37
Identify at least six pairs of action-reaction
force pairs in the following diagram
38
Tug-a-war
  • If Fido and Rover play tug-a war, how do their
    pulls compare?
  • If each dog pulls with 50 N of force, what is the
    tension force in the middle of the rope (between
    the dogs)?
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