Download Forces & the Laws of Motion

QOD

What is force?
Forces
& the
Laws of Motion
Physics I
Mr. Padilla
Aristotle (~400 B.C.)

Divided motion into 2
types

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Natural
Violent
Natural – Straight up
or straight down.
Objects seek their
“natural” resting place

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Clouds – Up
Rocks – Down
Heavens – Perfect
Circles around Earth
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Violent – imposed
motion
A result of forces that
pushed or pulled. Had
an external cause.
These ideas held for
about 2000 years.
Copernicus (1473-1534)
Nicholas Copernicus was the first to
reason that the Earth & planets moved
around the sun.
 Heavily persecuted for his ideas and
worked in secret to hide them.
 His work was published for the first time
on the day he died.

Galileo


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Believed Copernicus
and so was placed
under house arrest
Force – any push or
pull
Friction – any force
that acts between
materials as they
move past each other


Only when friction is
present is a force
needed to keep an
object in motion
Discredited Aristotle’s
ideas of motion
Changes in Motion
Forces can act through contact or at a
distance
 Contact forces – result from the physical
contact between 2 objects.

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Ex: Jordan pushes Kevin
Field forces – does not involve physical
contact.
Ex: Gravity, electric fields
 Field forces were developed to explain how
an object can exert force on another without
touching.

Newton

Newton’s 1st Law of Motion

Every object continues in a state of rest, or of
motion in a straight line at a constant speed,
unless it is compelled to change that state by
forces exerted upon it.
An object in motion stays in motion, an
object at rest stays at rest.
 AKA law of inertia

Inertia in Action
Inertia: the reluctance of any body to a
change in the state of motion.
 Book on a table
 Air hockey table
 Table cloth & dishes
 Where would they go?

Ball thrown in outer space
 Car without a driving force
 Planets without the sun’s gravity

Mass vs. Weight

The inertia of an
object depends on its
mass

How much material is
in the object

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The more mass =
more inertia
Mass is a measure of
the inertia of an
object.

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Mass is not weight.
Weight – force of
gravity of an object
Weight can change in
different environments:
mass, and therefore
inertia, remain
constant.
Mass & Weight are
proportional.
Weight

Weight can be calculated by taking an
objects mass times gravity

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The SI unit for weight is


W = mg
kg (x) m/s2 = kgm/s2 or Newtons (N)
A mass of 1kg has a weight of 9.8N, or
2.2lbs
Mass Example
Felicia, the ballet dancer, has a mass of
45.0 kg. a) What is Felicia’s weight on
Earth? b) What is Felicia’s mass on
Jupiter, where acceleration due to gravity
is 25.0 m/s2? c) What is Felicia’s weight on
Jupiter?
Net force

Force is a vector quantity.

Has magnitude and direction.
Net force – When considering Newton’s 1st
Law, you must think of net forces not just
forces.
 Net Force – combination of all forces
acting on an object.
 All individual forces can be added and
treated as a single (net) force

FBD

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A free body diagram
can be drawn to
represent the forces
acting on an object.
A FBD shows vectors
for forces acting on a
single object
Equilibrium

What forces act on a book at rest on a
table?

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If the net force acting on an object is zero,
it is in equilibrium.

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At least 2 – weight and a support (normal)
force
No change in state of motion.
Normal force – always acts at a right angle
to the surface.
QOD

Determine your weight, in Newtons, on the
other planets in our solar system.
Mercury 3.59 m/s2
 Venus 8.87 m/s2
 Mars 3.77 m/s2
 Jupiter 25.0 m/s2
 Saturn 11.08 m/s2
 Uranus 10.67 m/s2
 Neptune 14.07 m/s2
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Acceleration

So far: a = ΔV/t

This is the definition of a
The cause of acceleration is force.
 Applying a force (or net force) to an object
causes it to accelerate

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More force causes more acceleration (direct)
Mass resists the acceleration

more mass causes less acceleration
(inversely)
2nd Law
Acceleration produced by a net force on
an object is directly proportional to the
magnitude of the net force, is in the same
direction as the net force, and is inversely
proportional to the mass of the object.
 Or

a ~ (net force)/mass
 Better yet… Fnet = ma

a = F/m
Examples
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A force of 30.N is
applied to a 12kg
mass, how fast does
it accelerate?
How much force is
needed to accelerate
a 700.kg vehicle at a
rate of 4.0m/s2?

F = ma
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30.N = 12kg(a)
a = 30N/12kg
a = 2.5 m/s2
F = ma
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F = (700.kg)(4.0m/s2)
F = 2800N
More Great Examples
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Ben and Sam push on
opposite sides of Chris.
Ben pushes with 400.N of
force, Sam with 200.N.
What is the net force on
Chris?
If Chris has a mass of
75kg, what will his
acceleration be?
A constant force exerted
on an object, will result in
what kind of motion?

Fnet = ΣF acting on Chris
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F = ma
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Fnet = 400.N – 200.N
Fnet = 200.N
200N = (75kg)a
a = 200N/75kg
a = 2.7 m/s2
Constant force produces
constant acceleration.
Free Fallin’
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In a vacuum all things fall at the same rate due
to gravity.
In reality, air resistance provides friction and
causes some things to slow down.
All objects reach a point where the force of
gravity pulling them down is equal to the force of
air resistance pushing back on them.
This is called terminal velocity.

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Humans = 150-200 km/h in free fall
15-20 km/h with a parachute.
Forces
Forces are not a thing by themselves, but
part of an interaction.
 Forces always occur in pairs.
 One force is called the action force, the
other is called the reaction force.

Newton’s Third Law
 Whenever
one object exerts a force
on a second object, the second object
exerts an equal and opposite force on
the first object.
 To every action, there is an equal and
opposite reaction.
Action/Reaction Pairs
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When a hammer hits
a nail, which is the
action and which is
the reaction force?
Identify the
action/reaction forces
when you walk.
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Action: Object A
exerts force on
object B
Reaction: Object B
exerts force on
object A
Different Masses
Forces objects exert on each other are
always equal in strength and in opposite
directions.
 Newton’s second law (F=ma) still applies.
 If the masses are greatly different, then
the object with the smaller mass will
undergo greater acceleration.
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A gun with a mass
of 20kg shoots out a
.04kg bullet. The
gun kicks back at
.2m/s2, at what rate
does the bullet
accelerate?
F=F
ma = ma
(20kg)(.2m/s2) =
(.04kg)a
4N = (.04kg)a
100m/s2 = a
Examples

Calculate the kick of a
50kg cannon that
shoots a 3kg
cannonball out with
an acceleration of
12m/s2.
Action - Reaction
A car accelerates along a road. Strictly
speaking, what is the force that moves the
car?
 We know that Earth pulls on the moon.
Does the moon also pull on the Earth? If
so, which pull is stronger?
 Can you identify the action and reaction
forces of an object falling in the vacuum of
outer space?
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Cancellation
Do action/reaction forces cancel each
other out?
 Create a system to determine net force on
an object.
 Ex: Boy on a skateboard, pulling against a
girl standing on the ground.
 Draw systems, identify action reaction
forces.

Friction
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Friction is a force that
affects motion.
It acts when materials are
in contact with one
another and always
opposes motion.
Friction is due to the
irregularities in the two
surfaces.
Constant speed = no net
force so friction = driving
force
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Different surfaces have
different amounts of
friction between them (μ)
Friction is not restricted to
solids , it also occurs in
fluids
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This can be felt even at
slow speeds
Air resistance is friction
acting on something
moving through air.
Friction

Two types of Friction
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Static friction (Fs)
 Restrictive
 Fs

force when object is not in motion
= -Fa
Kinetic friction (Fk)
 Restrictive
force when object is in motion
 Net external force acting on an object is equal to
the difference between applied force and force of
kinetic friction

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F - Fk
Kinetic friction is less than static friction.
Friction

The force of friction is proportional to the
normal force.

Easier to push a chair across the floor at a
constant speed than to push a heavy desk at
the same speed.
Coefficient of Friction is a Ratio
of Forces
Defined as the ratio between the normal
force and the force of friction between two
surfaces.
 Coefficient of Kinetic Friction
μk = Fk
Fn
 Coefficient of Static Friction
μs = Fs,max
Fn

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If the value of μ and the normal force on
the object are known, then the magnitude
of the force of friction can be calculated
directly.
Ff = μFn
Coefficients of Friction
Material
steel on steel
Aluminum on steel
Rubber on dry concrete
Rubber on wet concrete
Wood on wood
Glass on glass
Waxed wood on wet snow
Waxed wood on dry snow
Metal on metal (lubricated)
Ice on ice
Teflon on teflon
Synovial joints in humans
μs
0.74
0.61
1.0
----0.4
0.9
0.14
----0.15
0.1
0.04
0.01
μk
0.57
0.47
0.8
0.5
0.2
0.4
0.1
0.04
0.06
0.03
0.04
0.003
Air Resistance is a form on friction

Whenever an object moves through a fluid
medium (air or water), that fluid provides a
resistance to the object’s motion.
Car at low speed = FR is roughly proportional
to the car’s speed in the opposite direction
 Car at higher speed = FR is roughly
proportional to the square of the car’s speed
in the opposite direction

 When
the magnitude of FR equals the magnitude
of the force moving the car forward, the net force is
zero and the car moves at a constant speed.
Examples
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A 24 kg crate initially at rest on a horizontal
floor requires 75 N horizontal force to set it
in motion. Find the coefficient of static
friction between the crate and the floor.
A student moves a box of books by
attaching a rope to the box and pulling with
a force of 90.0 N at an angle of 30.0o. The
box of books has a mass of 20.0 kg, and
the coefficient of kinetic friction between the
bottom of the box and the sidewalk is 0.50.
Find the acceleration of the box.
Ch. 4 Mixed Review Answers
45. 55 N to the right
47. 13 N down the incline
49. a. 2.00 N
b. 6.04 N
51. a. 0
b. 33.9 N
53. 50. m
55. a. 1.78 m/s2
b. 0.367
c. 9.36 N
d. 2.67 m/s
57. 510 N
59. 32.2 N
61. 1.41o
63. a. 20. m/s2 to the rt.
b. F1 = 40. N
F2 = 60. N
F3 = 80. N
c. m2 by m1 = 140 N
m1 by m2 = -140 N
m3 by m2 = 80. N
m2 by m3 = -80. N
65. a. between the bottom block
and the table.
b. 0.47