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Newton

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Newton s Laws Isaac Newton born: Dec 25, 1642 – PowerPoint PPT presentation

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Title: Newton


1
Newtons Laws
  • Isaac Newton born Dec 25, 1642

2
Brief History The Greeks thought there were two
types of motion, natural and violent. They also
thought that a force was required to keep objects
in constant motion. Today we know they were
incorrect. They had no concept of friction. The
Greeks basically thought that objects wanted to
return their "natural" resting place in the
universe. If you tried to move an object against
this natural motion it was called violent motion.
Thus, there were two types of motion, natural and
violent.
3
Brief History Isaac Newton 1665 Newton was an
amazing individual. He did more for physics than
almost every other scientist before of after him.
Newton's ideas on motion were so unique that they
started revolutions in Europe. The laws of motion
actually gave the common person a new view on how
the world worked. His laws of motion can be
summarized as follows
4
1st. Law of motion The Law of Inertia (Actually,
this is not Newton's idea, it is a restatement
of Galileo's work on inertia.) The Law of
Inertia states Objects at rest tend to stay at
rest while objects in motion tend to stay in
motion in a straight line (constant velocity)
unless acted on by an outside force. Simply put
Things keep doing what they are already doing.
5
Newtons 1st Law
  • Every object continues in its state of rest, or
    uniform motion in a straight line,

unless it is acted upon by an outside force
6
Inertia
  • This tendency to continue in a given state,
  • (1st law) is called Inertia.
  • The more mass an object has, the more inertia it
    has.

7
Inertia
I have more inertia
  • The tendency of an object to stay at rest or in
    motion.
  • Objects with greater mass have greater inertia

but we fall at the same rate
8
Inertia keeps astronauts in motion
9
Seat belts protect against inertia
10
Inertia Question
  • Answer
  • B
  • As it emerges, it continues to follow the path it
    was on. Inertia.

11
Inertia Question 2
  • Answer
  • B
  • At first, the ball continues to move
    horizontally, but gravity soon acts on it
    creating the usual parabolic trajectory.

12
Force Unit Newton
  • We are familiar with the English unit for force,
    the pound, lb.
  • However, the typical metric unit for force is the
    Newton, N.
  • In Earths gravity 1 kg 9.8N

13
Mass vs. Weight
  • Mass The amount of matter in an object.
  • Units kg, g
  • Weight The force upon an object due to gravity.
  • Units lbs, Newtons

Dont confuse mass weight!
14
Two Questions
  • If you travel deep into space, does your mass
    change?

No
If you travel deep into space, does your weight
change?
Yes
15
Newtons 2nd Law
  • Fnet ma
  • The acceleration of an object is directly
    proportional to the net force, and inversely
    proportional to the mass.
  • Notice that a Newton, N, is equivalent to a
    kgm/s2.

16
  • This is common sense! A larger mass is more
    difficult to accelerate than a smaller one!

Also, a larger force accelerates a mass more than
a smaller one!
http//www.espace-sciences.org/science/images/imag
es-maj/Perso/manipulations/acceleration/index.htm
17
Example
  • If a 10kg block rests on a friction-less surface,
    how much will it accelerate if a 50N force is
    applied to it?

Fnet ma a Fnet/m a 50N/10kg a 5m/s2
Given Fnet 50N m 10kg Unknown a?
18
Net Force
  • The net force is simply the resultant of all the
    forces acting on an object.

Consider your paycheck if youve ever held a part
time job...
19
Net Force
20
Net Force
21
Net Force
22
Newtons 2nd Law?
  • F net ma
  • Whats a net force?
  • The resultant of all the forces acting on an
    object.

23
Equilibrium
  • When all the forces on an object balance out, or
    cancel out, the object has a net force of 0.

This condition is known as equilibrium.
24
Static Equilibrium
  • When an object is in equilibrium, and not moving,
    this is called static equilibrium.

25
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26
Dynamic Equilibrium
  • An object can be moving and still be in
    equilibrium.
  • It could be moving at a constant velocity. There
    would be no net force or acceleration on it.
    This is called dynamic equilibrium.

27
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28
4 Forces of the Universe
  • Nuclear Strong forces holding the nucleus of an
    atom together (gluon)
  • Weak Forces as a result of some atoms that
    decay the nucleus ( radioactive )
  • Electromagnetic a force based on electricity,
    magnetism and light properties
  • Gravitational The weakest force caused by the 2
    factors, MASS and DISTANCE

29
Mechanical Forces
  • Friction the contact force that acts to oppose
    sliding motion between surfaces
  • Normal the contact force exerted perpendicular
    to the surface contact
  • Tension the pull exerted by a cable when
    attached to a body
  • Compression the push exerted to the object
  • Weight force of attraction due to distance from
    the center of the earth and mass of the object
  • Units are Measured in NEWTONS ( N )

30
Sample Force Problem 1
  • Using Newtons Second law of Motion ( the
    acceleration of a body is directly proportional
    to the net force on it and inversely proportional
    to the mass, F ma).
  • What net force is
  • required to accelerate
  • a 1500 kg car
  • at 3.00 m/s2?

31
Solution Problem 1
  • F m a
  • F 1500 kg x 3.00 m/s2
  • F 4500 kg-m/s2
  • or
  • F 4500 N

Given m 1500 Kg a 3.00 m/s2 Unknown F ?
32
Static Balance (free body) Problem 2
  • Forces of 10 N and 15N are arranged according to
    diagram. What is the net force on this object.
    What is the rate of acceleration of the 12 Kg
    object?

10
15
12 Kg
33
Solution 2
  • Given
  • Force 10 N and 15 N in opposite directions
  • Mass 12 Kg
  • Find
  • Net Force
  • Acceleration

Net Force F1 - F2 15N 10N 5 N
F ma 5N 12 Kg (a) 5N
a 12 Kg 0.417 m/s2 a
34
Friction
  • Whenever surfaces come in contact friction
    results.
  • It results from microscopic irregularities in the
    surfaces in contact.

35
Direction of Friction
  • Friction always opposes motion.

If the crate moves at a constant velocity, the
net force on it must be zero. Thus, there must
be 75N of friction countering the 75N push.
36
Types of Friction
  • Static Friction The friction between two
    surfaces when they are stationary.
  • Sliding (kinetic) Friction The friction between
    two moving surfaces.
  • Static friction is generally always higher than
    kinetic friction.

37
P. 6 Workbook Problems
  • 1. 20 lb force applied to a 64 lb object for 5 s

FN Weight
Fnet
Weight mg (F ma)
  • Weight 64N therefore W mg solve for m ?
  • m W/g ? m 64 lb/ 32
    ft/s2 2 slugs

b. Fnet ma therefore a Fnet / m ? a 20
lb / 2 slugs
a 10 ft/s2
38
FN Weight
Fnet
Weight mg (F ma)
  • Kinematics problem now
  • G vi 0 ft/s a 10 ft/s2 t 5 s
  • U vf ?
  • E - vf vi at ? vf 0 ft/s (10
    ft/s2)(5 s) 50 ft/s

39
FN Weight
Fnet
Weight mg (F ma)
d. Kinematics problem now G vi 0 ft/s
a 10 ft/s2 t 5 s U d ? E - d
vit 1/2at2 ? d 0 ft/s (5 ft/s2)(5 s)2
125 ft
40
P. 6 Workbook Problems
  • 2. A force applied to a 50 kg object to
    accelerate-20 m/s2

FN Weight
Fnet
Weight mg (F ma)
a. Fnet ma ? Fnet (50 kg)(20 m/s2) 1000 N

b. Weight mg ? W (50 kg)(9.8 m/s2) 490 N
  • G vi 0 m/s a 20 m/s2 t 12 s U
    vf ?
  • E - vf vi at ? vf 0 m/s (20
    m/s2)(12 s) 240 m/s

d. G vi 0 m/s a 20 m/s2 t 12 s U d
? E - d vit 1/2at2 ? d (10
ft/s2)(12 s)2 1440 m
41
P. 9 Workbook Problems
  • A force of 400N to a 100 kg object to move _at_ cv

FN Weight
Ff
Fapp
Weight mg (F ma)
a. Weight mg ? W (100 kg)(9.8 m/s2) 980 N
b. Since the box is moving _at_ constant velocity -
therefore Fapp Ff ? 400 N µ FN ? µ 400 N/
980N .41
  • Fnet Fapp - Ff ? 600 N 400 N 200 N- now
  • Fnet ma ? a Fnet / m ?(200N)/(100 kg)
    2 m/s2

42
Free Fall Revisited
  • If air resistance is negligible, which hits the
    ground first, a heavy cannon ball, or a lighter
    pinball?
  • They should tie! Both accelerate at the same
    rate.

43
  • But why does it work that way?
  • The cannon ball has more mass, thus it has more
    weight, and gravity pulls on it more...

44
  • While the heavier object may have more weight, it
    also has more mass to move (more inertia).
  • Since the acceleration any object feels is F/m,
    the two factors cancel out, yielding the same
    acceleration.

45
Realistic Free Fall
  • In reality, things dont fall in a vacuum. There
    is significant air resistance, or drag.
  • The more air an object must plow through, the
    more resistance.
  • This diminishes an objects acceleration.

46
  • Weight and drag oppose each other.
  • Friction always opposes motion.
  • Subtract to get Fnet.

47
Less than free fall...
  • As a skydiver jumps, at first, she accelerates at
    nearly 9.8 m/s2.

As she goes faster, she must push more and more
air out of the way. This reduces her
acceleration.
48
Terminal Velocity
  • Eventually, her weight is completely balanced by
    air resistance and she doesnt accelerate
    anymore.
  • This top speed that is reached is called
  • terminal velocity.
  • Once this speed is reached, a skydiver would
    continue the rest of the way at this top speed.

49
  • For a skydiver, terminal velocity is about 200
    km/hr, although this can vary with weight, and
    area.

Terminal velocity for a feather is only a few
cm/s.
50
Newtons 3rd Law
  • For every force, there is an equal and opposite
    force.
  • For every action, there is an equal and opposite
    reaction.

51
Examples of Newtons 3rd Law
52
  • In all these cases, there is a pair of equal
    forces opposing each other.
  • Forces dont appear alone.

53
http//www.teachertube.com/view_video.php?viewkey
f5b8c02c0e46513b98f9
54
Gravity
The attractive force between all objects in
the universe
55
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56
F M1 M2 d2
  • More mass means greater gravity
  • More distance means less gravity

My gravitational attraction is small no matter
what the distance!
Massive objects have lots of gravity
57
The earth exerts a force on the student and the
student exerts a force on the earth because of
GRAVITY!
58
Gravity warps the fabric of space-time
59
Gravity
60
gravity pulls it in inertia keeps it going
What forces are acting on the moon?
Inertia
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