Download CHANGES IN MOTION - Van Buren Public Schools

Newton’s 1st Law
Types of Forces
Inertia
Newton’s 2nd & 3rd Laws
Friction
Newton’s Laws
 3 simple laws that apply to describing motion
(kinematics) and to understanding the causes of
motion (dynamics)
 Foundation of Physics
Force
 Push or pull on an object
 Forces can cause
acceleration
 Causes objects to speed
up, slow down, or change
direction
 Symbol = F
 Units = Newtons (N)
Types of Forces
 Contact Force
 When an object from the
external world touches a
system and exerts a force on it
 Ex: String pulling a book
 Field Force
 Force that affects the system
without touching it



Gravity
Magnetism
Electric
Types of Forces - Normal
 Normal Force (FN)
 Contact force
 Exerted by a surface on an object
 Perpendicular to the surface
 Points away from the surface
Types of Forces - Weight
 Weight (Fg)
 Force due to gravitational attraction between two
objects
 Generally between the earth and an object
 Points straight down to center of the earth
Types of Forces - Friction
 Friction (Ff )
 Contact force
 Acts to oppose sliding motion between surfaces
 Parallel to surface
 Opposite the direction of sliding
Types of Force - Spring
 Spring Force (Fsp)
 Contact force that is restoring
 Push or pull of spring exerts force on object
 Points opposite to the displacement of the object
Types of Forces - Tension
 Tension (FT)
 Contact force
 Pull exerted by string, rope, cable, when attached to a
body and pulled taut
 Force points away from an object and parallel to the
string
Force as a Vector
 Magnitude
 Strength of the force
 Direction
 In which direction
the object is being
pushed or pulled
Drawing Force Diagrams
 Helps analyze the
situation
 Identify all of the
forces acting on an
object
How to Draw a FBD
 Treat the object as a single point
 Point particle
 Identify all of the external forces acting on an object
 Choose the length of the force vectors
 Choose a coordinate system
Combining Forces
 2 people pushing against a table
 In opposite direction
 Each with the same force of 100 N
 Table will not move
Combining Forces
 2 people pushing against a table
 In same direction
 Each with the same force of 100 N
 Table will move
Combining Forces
 2 people pushing against a table
 In opposite direction
 One with a force of 100 N
 One with a force of 200 N
 Table will move
Net Force
 Fnet
 Sum of all of the vector forces in all
directions
 An unbalanced force (net force) causes
acceleration (change in velocity)
Newton’s First Law of Motion
 An object at rest remains at rest as long as no net
force acts on it (Ex: Sitting on a skateboard)
 An object moving with constant velocity will
continue at the same speed and in the same
direction as long as no net force acts on it (Ex:
Friction)
 Also called the Law of Inertia
Inertia
 Inertia: The tendency of an
object to resist being moved
(accelerated) if stationary.
 Also resistance to direction
or speed changes if moving
 Mass is a measure of inertia
 The greater the mass, the
more the inertia
Newton’s 2nd & 3rd Laws
Friction
Newton’s Second Law
 F = ma
 Can be applied in
coordinate directions
 Fx = max
 Fy = may
Newton’s Third Law
 Newton realized that a
force is exerted on an
object when that object
interacts with another
object
 Ex: When you dress up
like a ghost, stand on a
skateboard and push
against a wall, the wall
exerts a force back on
your hand
Newton’s Third Law
 Forces always exist in pairs!
 The forces two objects exert
on each other when
interacting are called an
action/reaction pair.
 For every action, there is an
equal and opposite reaction!
The Big Question
 How can the horse pull
the cart if the cart is
pulling back on the
horse with an equal,
but opposite force?
 Aren’t these “balanced
forces” resulting in no
acceleration?
The Big Answer
 Newton’s Second Law applies to the net force
on a SINGLE object!
 The motion of the cart is affected only by the
forces acting on the cart!
Break It Down Now
Friction
 Force that opposes motion
 Due to the presence of microscopic hills and valleys on a
surface that restrict motion
 We need friction to walk!
 The force that allows us to accelerate when we walk is
the force of friction between our shoes and the ground.
 Two Types
 Kinetic Friction 𝑓𝑘
 Static Friction 𝑓𝑠
Kinetic Friction
 Symbol = 𝑓𝐾
 Units = N
 This type of friction is
encountered when two
surfaces slide against each
other with relative speed
 Acts opposite the direction of
sliding motion
Common Coefficients
Equation for Kinetic Friction
𝑓𝑘 = 𝜇𝑘 𝐹𝑁
 𝑓𝑘 = Force of kinetic friction
 𝜇𝑘 = Coefficient of friction
 𝐹𝑁 = Normal Force
Coefficient of Friction
 k
 Pronounced (mew sub K)
 Dimensionless constant (no units)
 Reflects the roughness/smoothness of a surface
 Ranges between 0 and 1
 The smaller the k the smaller the friction!
Static Friction
 Symbol = 𝑓𝑠
 Unit = N
 Force that keeps two
surfaces from moving
relative to one another
 Typically stronger than
kinetic friction
 Acts in response to a force
trying to cause a stationary
object to move
Equation for Static Friction
𝑓𝑠 = 𝜇𝑠 𝐹𝑁
 𝑓𝑠 = Force of static friction
 𝜇𝑠 = Coefficient of friction
 𝐹𝑁 = Normal Force
Normal Force
Gravity
Apparent Weight
Friction in General
 The force of friction
opposes motion
 Depends on mass
 Does not depend on area
of contact (surface area)
between surfaces
 Affected by surface
material
 This is accounted for by
the coefficient of
friction
Fk = kFN
Mass
 The amount of matter in an object
 Measured in kg
 Never changes!
 Not due to gravity, it is an intrinsic property of an object
What is Weight?
 An object’s weight is how
hard gravity is pulling on it
 Scales give the
measurement of the pull
of the Earth’s gravity
 Weight can change
depending on gravity
 In outer space
 In free fall
 On the Moon
 a = 1.6 m/s2
Mass vs. Weight
 On Earth, mass and weight are usually used
synonymously
 Gravity is nearly the same all over on Earth
 They can be different!!
 Elevation differences
 Mass is constant, but weight can vary
Weight in Physics
 Symbols
 W
 Fg
 Unit = N
 Weight is a force, so it is
also a vector
 Has a magnitude and
direction
Inside the “Vomit Comet”
 Mass is a scalar
http://www.youtube.com/watch?
 Only has magnitude
v=2V9h42yspbo
Example Idea
 Falling apple
 What is the only force
acting on it?

Fg
 F = ma
 a = -g
 Fg = mg
Normal Force
 Soup sitting on a countertop
 Fg is gravity pushing down on
soup
 FN is upward force exerted by
counter
 KEY: When is there no vertical
acceleration?
 Fg = mg = FN
 The forces are balanced
 What if the object is
accelerating in the vertical
direction
 FN = m (a + g)
Apparent Weight
 Normal Force
 What the scale actually
measures when
accelerating
 The feeling of being light
in an elevator or heavy
 Not our true mass
Scales
 Scales measure “weight”,
but aren’t in N
 Scales measure a
downward force
 Could be something
other than gravity
 Jump on a scale
Case 1: No acceleration of elevator
In this case, the action and reaction
force pair between the person and the
scale is just the weight.
The person pushes down on the scale
with a force of – 𝑊 = 𝑚𝑔 (negative
direction) and the scale pushes back
up against the man with a Normal
Force of 𝐹𝑁 = +𝑊 = +𝑚𝑔.
Case 2: Going Up & Speeding Up
𝐹𝑛𝑒𝑡 = 𝑚𝑎 = −𝑚𝑔 + 𝐹𝑁
 We want to know FN
because that is the number
we read off the scale
 𝐹𝑁 = 𝑚𝑔 + 𝑚𝑎, which is
GREATER than the true
weight.
Case 3: Going up & slowing down
𝐹𝑛𝑒𝑡 = −𝑚𝑎 = −𝑚𝑔 + 𝐹𝑁
 We want to know FN
because that is the number
we read off the scale
 𝐹𝑁 = 𝑚𝑔 − 𝑚𝑎, which is
LESS than the true weight.
Case 4: Going down and slowing down
𝐹𝑛𝑒𝑡 = 𝑚𝑎 = −𝑚𝑔 + 𝐹𝑁
 We want to know FN
because that is the number
we read off the scale
 𝐹𝑁 = 𝑚𝑔 + 𝑚𝑎, which is
GREATER than the true
weight.
* NOTE: This is the same
equation as we got in case 2.
Case 5: Going down & speeding up
𝐹𝑛𝑒𝑡 = −𝑚𝑎 = −𝑚𝑔 + 𝐹𝑁
 We want to know FN
because that is the number
we read off the scale
 𝐹𝑁 = 𝑚𝑔 − 𝑚𝑎, which is
LESS than the true weight.
* NOTE: This is the same
equation as we got in case
3.
Case 6: Freefall
 Note that this is a special
case of downward
acceleration, which was
discussed in Case 3 and
Case 5. Just as in Cases 3
and 5, the apparent
weight (which is zero
when 𝑎 = −𝑔) is less
than the true weight.
Pictorial summary of
apparent weight