Download Unit 2 Laws of Motion

Unit 2: Laws of Motion
Why are heavier objects
harder to start moving or
stop from moving?
What is force?
What is the relationship
between force and motion?
Sir Isaac Newton

Sir Isaac Newton (1642– 1727)
– English physicist,
astronomer, mathematician
– Inventor of calculus
– Described gravitation, laws
of motion, and proved
heliocentricity
– Known for 3 Laws of
Motion
Newton’s 1st Law
• Newton’s 1st Law of
Motion
– An object at rest will
stay at rest, and an
object in motion will
stay in motion,
unless acted on by
an unbalanced force
– Only forces can
change motion
Newton’s 1st Law
• Inertia
– An object’s resistance to change in its motion
– Related to the mass of the object
• The higher the object’s mass, the higher the
inertia
– Causes objects to move at constant speed
Newton’s 1st Law
• Force (F)
– A push or pull
– Any action with the ability to change motion
• Start, speed up, slow, stop, or change direction
of motion
– No motion occurs without a change in force
Newton’s 1st Law
• Force (F)
– In the US, we use the Pound (lb)
• Weight from a 0.454 kg object
– The SI unit is the Newton (N)
• Force to change speed of 1 kg object 1 m/s in 1
s
• Unit:
1 lb = 4.448 N
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Net Force
Net Force
• Normal Force
• Net Force
– Total of all forces on an object
– Includes directions of forces
– Usually includes gravitational force
– No net force – balanced force
– Force from floor
holding up object
– Counters weight of
object on floor
– Equal to weight of
object
Scalars and Vectors
• Scalar
Acceleration
• Acceleration (a)
– Quantity without direction, only magnitude
– Speed is an example
• Vector
– Rate of change of speed
– Constant speed = no acceleration
• Deceleration
– Quantity with direction and magnitude
– Velocity and acceleration are examples
Acceleration
• Acceleration (a)
– Units are speed divided by time
• Ex: (m/sec)/sec = m/sec2
–Most common and used in Newton
calculations
– May have different units of time
• ex: 6 km/hr/sec
– Rate at which speed decreases
Newton’s 2nd Law
• Newton’s 2nd Law of Motion
– Stronger the net force on an object, the greater its
acceleration
F = ma
• F – Force (N)
• m – mass (kg)
• a – acceleration (m/s2)
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Newton’s 2nd Law
• Newton’s 2nd Law
– Directly propotional
• Force/mass and
force/acceleration
– Indirectly proportional
• Mass/acceleration
Newton’s 2nd Law
• Newton’s 2nd Law
– Acceleration is caused by a net force
• No net force, no acceleration, constant speed
– There must be a force to change speed or
direction of motion
Newton’s 3rd Law
• Newton’s 3rd Law
– “For every action, there is
an equal and opposite
reaction”
– Forces always come in
pairs
• Action force and
reaction force
– Without a reaction force,
an action force cannot be
applied
Gravity
• Free Fall
– Acceleration due only to
gravity
– No other forces are
acting
– ALL objects to accelerate
downward at the same
rate
– Ex: Dropping or throwing
Gravity
• Gravity (g)
– Attractive force between two
objects
• All objects in universe have
some of gravity between
them
– Causes objects to accelerate
• Rate depends on objects
involved
– On Earth:
g = 9.8 m/s2
Gravity
• Upward Launches
– Acceleration is always
g (9.8 m/sec2)
– The ball slows until it
reaches peak height
• Speed is 0 m/s
–Appears to stop
– Accelerates
downward to original
speed
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• Weight (Fg)
– Force of gravity on an
object
– Fg = mg
– Fluctuates
throughout the
universe
Terminal Speed
• Terminal Speed
– As speed increases,
so does air resistance
• Eventually, the force of
air resistance equals the
force of gravity
– Net Force = 0
– Object falls at
constant speed
Graphs – Position vs. Time
• Position vs. Time
– Slope equal to
distance/time
• Speed
– Distance traveled is
position at that time
– Flat lines
• No motion
– Straight, sloped lines
• Constant speed
Air Resistance
• Why does a baseball fall quicker than a piece
of paper?
• Air Resistance
– Force against object’s motion in free fall
– Size and shape of an object contribute to air
resistance
• More surface area – more resistance
Terminal Speed
• Terminal Speed
– Less resistance
• Higher terminal
speed
• Crumpled paper falls
faster than flat
paper
– Parachutes slow down
terminal velocity from
>100mph to safety
Graphs – Position vs. Time
• What happens
when there is a
negative slope?
– Constant
speed, but
towards the
observer
Position vs. Time
35
30
25
Position (m)
Weight
20
15
10
5
0
0
1
2
3
4
5
6
7
Time (s)
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Graphs – Position vs. Time
• Accelerated Motion
– Speed changes over time
• Data will not be a
straight line
– If accelerating
• Slope increases with
time
– If decelerating
• Slope decreases with
time
Graphs – Speed vs. Time
• Speed vs. Time
– Straight, sloped
line
• Constant
acceleration
• Slope equals
acceleration
Graphs – Speed vs. Time
• Speed vs. Time
– To calculate distance for constant speed
• Find the area underneath the line
• A=lxw
Graphs – Speed vs. Time
• Speed vs. Time
– Slope equal to
speed/time
• Acceleration
– Flat lines
• Constant speed
• Speed equal to
slope from
position vs.
time graph
Graphs – Speed vs. Time
• Speed vs. Time
– Sloping up
• Positive
acceleration
– Sloping down
• Deceleration
– Flat line
• No acceleration
Graphs – Speed vs. Time
• Speed vs. Time
– To calculate distance for accelerated motion
– Find the area underneath the triangle
• A=½bxh
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