“ if i have seen farther than others, it is because i have stood on the shoulders of giants.”...
TRANSCRIPT
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“If I have seen farther than others, it is because I have stood on the shoulders of giants.”
Sir Isaac Newton (1642 – 1727) Physicist
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Forces and the Laws of Motion
4-1 Changes in Motion Force – a push or pull applied to an object Unbalanced forces cause objects to accelerate.Cause objects to start moving, stop moving or change direction. SI Unit of Force is the Newton –
1 N = the amount of force acting on a 1 kg mass, produces an acceleration of 1m/s2
1 N = 1kg x m/s2
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Forces can act through contact or without physical contact
Contact force – force through touch (contact)Field Force – electric and magnetic fields, gravity
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FORCES COME IN PAIRS When a force is applied to an object, then the object
applies a counterforce. Force is a Vector – magnitude and direction. Force Diagrams used to show all forces on object. Isolate the object – only show forces acting on it.
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FN – normal force
(always perpendicular to the surface the
object is touching)
Fg - force of gravity
(weight of object – always directed
straight down)
Fa – applied force
(forced applied to object by person or another
object)
Ff - force of friction
(usually opposes motion of object)
Free Body Diagrams – FBD’s
Review: Free Body Diagrams
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4-2 Newton’s First Law
“An object at rest remains at rest and an object in motion remains in motion unless acted upon by an outside force”
Sometimes called “Law of Inertia” – the mass of an object (not it weight) causes it to resist changes in motion.Inertia – “laziness”
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Newton’s 1st Law
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1
Mass is a measure of inertia – easier to accelerate an object with less mass.
Objects are in equilibrium when they are either at rest or moving at constant velocity - The net external forces are zero.
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• Mass and Weight are different! Mass never changes • Weight depends on the gravitational pull on 2 objects.
• Weight = mass of an object times the force of gravity (g)Formula: W = m x g
Weight is a force so the units of weight are in Newton's (N)
Fg (force due to gravity) = m x g
http://www.exploratorium.edu/ronh/weight/
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Astronauts on the orbiting space station are “weightless” because...
a. there is no gravity in space and they do not weigh anything. b. space is a vacuum and there is no gravity in a vacuum.c. space is a vacuum and there is no air resistance in a vacuum.d. the astronauts are far from Earth's surface at a location where gravitation has a minimal affect.
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Astronauts on the orbiting space station are “weightless” because...
a. there is no gravity in space and they do not weigh anything. b. space is a vacuum and there is no gravity in a vacuum.c. space is a vacuum and there is no air resistance in a vacuum.
d. the astronauts are far from Earth's surface at a location where gravitation has a minimal affect.
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Find your mass in kilograms using the conversion: ___lbs = 1 kg
2.2 lbs
Then Determine how much you weigh in Newton's.
W = mass x gravity (m x g) = ______________ N
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Newton’s 2nd Law
A net Force on an object will cause it to accelerate. Formula:
F = m x a
Units of Force: = kg x m/s2 = Newton (N)
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Use equation for weight (force of gravity):
or
Rearrange and get
mgFg maFg
m
Fa g
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Falling and Air ResistanceA. Air resistance decreases the net forces
acting on a falling object1. When air resistance equals downward force on falling object (force of gravity– also called weight) then net force is zero and no further acceleration occurs.
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terminal speed– when acceleration terminates
we call this maximum speed terminal velocity
Fg
Fdrag
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mass
Fg = mg
FII
F ϴ
The Fg (force in Newton’s due to gravity) can be shown
as two useful vectors FII and F
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F perpendicular = FN (normal)
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Together these 2 forces replace the force of gravity
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mass
Fg = mgFII = m x g sin
F = m x g cos
ϴ
ϴ
Using the law of similar triangles and your trigonometric functions
we can solve for FII and F Sin = opp / hyp = FII / m x g
Cos = adj / hyp = F / m x g
Hypotenuse = Fg = m x g
FII = m x g sin - parallel to the plane (x axis)F = m x g cos - the normal force that acts perpendicular to the plane (y axis)
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The net force is the vector sum of all the forces.
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• Static friction = friction between surfaces - Force required to get something moving
• Each surface has its own “coefficient of friction”• Coefficient of Static Friction ratio of the
static friction to the normal force (perpendicular force)
μs = Fs / F
2 Types of Friction: Static and Kinetic
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Kinetic Friction (μk) = friction between moving surfaces.
Examples: Sliding, rolling, fluid friction (air or liquid) Coefficient of Kinetic Friction (μk) = ratio of kinetic friction
to the normal force (perpendicular force)
μk = Fk / F
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Coefficient of Friction – it’s a Decimal – percent of the weight required to move an object
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Example:How much force does it take to move a 100 N object that has a μk
= .33
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Example:How much force does it take to move a 100 N object that has a μk
= .33
Given: Weight: 100 N (FN) = Fµk = .33
Answer: .33 x 100N = 33 N
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Newton’s 3rd Law
For every action there is an equal but opposite reaction
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http://www.nbclearn.com/nhl/cuecard/56615
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