Transcript Motion

Forces & Motion
The concepts of forces and
motion
Motion
• Motion is described as when
an object is changing its
distance from another object.
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*Motion is usually very
easy to see. (Ex. Walking
toward the door).
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** Sometimes it is difficult
to see. (Ex. The Earth in
motion or small or fast
moving objects).
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The movement of an
object depends on your
reference point (place to
compare movement from). You
must assume that a reference
point is not moving.
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To fully understand
motion, you must also
understand distance &
distance measurements.
Speed
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Speed is a rate. Speed is the
distance traveled over a unit of
time.
Speed = Distance / Time or S =
D/T
*Speed is usually
measured in meters per second.
(m/s or m/sec.)
Ex. Problem
If the Flash (the fastest
man alive) can travel 1000 km in
two seconds, what is his
speed? What is his speed in
meters an hour?
Solution: S = D/T so
S = 1000 km / 2 sec. = 500
km/sec.
(T)
B. Use unit multipliers to solve:
500 km/sec. X 1000 m/km X
3600 sec./hr = 1,800,000,000
m/hr
Speed (cont.)
• If an object moves the
same direction & speed
for a while it is said to be
moving at a constant
speed.
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*Most objects do not
travel at a constant
speed, but change
speeds as they move.
Because of this you must
often calculate an
average speed & not a
specific speed.
• Avg. Speed = Total
distance/total time
Velocity
• Velocity is similar to
speed except velocity
gives you the
direction the object is
moving in.
– Ex. Speed = 5 m/s
– Velocity = 5 m/s
West
Acceleration
• Acceleration is the rate at
which velocity changes. This
means that acceleration can
mean changing speed
(speeding up or slowing down)
or changing direction.
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*Slowing down is often
called negative acceleration or
deceleration.
• To calculate acceleration you
must use this formula:
• Acceleration = Final Velocity
– Initial Velocity / Time
• Ac = Fv – Iv / T
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* Acceleration is usually
measured in meters per
squared seconds (m/s2 )
Acceleration (cont.)
• Sample Problem
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A roller coaster
rolling down a slope picks
up speed. It begins down
the slope at 4 m/s & ends
up at 22 m/s at the bottom
of the slope 3 seconds
later. What is its
acceleration?
• Solution:
• Ac = Fv – Iv / T
• Ac = 22 m/s – 4 m/s / 3s
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(Fv)
(Iv)
(T)
• Ac = 18 m/s / 3s
• Ac = 6 m/s2
Motion Graphs
Acceleration
Graphs
Motion Graphs (cont.)
Speed Graph
Forces
• A force is simply a
push or pull.
• Like velocity &
acceleration a force is
given with the
direction it acts in.
• Force diagrams page
627 & other
examples.
Force diagrams
Forces (cont.)
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Unbalanced Forces
Unbalanced forces are when 2
or more forces act in different
directions & at different strengths.
Unbalanced forces cause or change
motion. (Fig F-2)(previous slide)
When 2 different forces act in the
same direction they act like a stronger
force in that direction. (Ex. 2 people
pushing on the same car can move
it better than one)
When 2 forces push in opposite
directions but one force is greater than
the other the object will move in the
direction of the greater force but
slower because some of the force is
cancelled. It is like subtracting or
adding a positive & negative number.
When you calculate an unbalanced
force the sum of all forces acting on an
object is called a Net Force.
Balanced Forces
Balanced forces occur when the
sum of all forces acting on an object =
zero.
*Balanced forces do not cause a
change in motion.
Friction
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Friction is the force that opposes
motion or that is caused when 2
objects are rubbed together.
*Friction depends on 2 factors.
– Surface types
– How hard the objects are rubbed
together.
*Friction is useful. It keeps us from
sliding around & is the principle force
in automobile brakes.
*Types of Friction:
– Sliding Friction – when solid
surfaces slide over each other
– Rolling Friction – when rounded
objects roll over surfaces. Less
force is need to overcome rolling
friction than sliding friction. This is
why ball bearings are often added
between rotating metal parts like
in skateboards & bikes.
– Fluid Friction – friction that
occurs when an object moves
between a liquid or a gas. Usually
less than sliding friction. This is
why cars use brake fluid.
Newton’s Laws
• A review of Newton’s
laws
Newton’s Laws of Motion
• Newton’s first 3 laws
are called the laws of
motion
Newton’s 1st Law
• Newton’s First Law is
also called the Law of
Inertia
• Inertia is defined as the
tendency of an object to
resist a change in its
motion.
• It states “Every object in a
state of uniform motion
tends to remain in that
state of motion unless an
external force is applied
to it.”
Newton’s 2nd Law
• This law defines force
in terms of mass and
acceleration.
• It says “The force of
an object (in Newton’s
or kgm/s2) is equal to
its mass (in kg)
multiplied by its
acceleration (in m/s2).
• F=ma
Newton’s 2nd Law (cont.)
• Sample Problem:
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A 52 kg wrestler is
body slammed. He is
accelerated to 2 m/s2
toward the mat. What is
the net force he is being
slammed with?
• Solution:
• F=ma
• F=52 kg X 2 m/s2
• F = 104 N
Newton’s 3rd Law
• It says: “For every
action there is an
equal and opposite
reaction.”
• It explains how
brakes work and why
you get hurt when you
fall.
Newton’s Law of Universal
Gravitation
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Also known as the law of gravity.
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where:
– F is the magnitude of the
(attractive) gravitational force
between the two point masses
– G is the gravitational constant
– m1 is the mass of the first point
mass
– m2 is the mass of the second
point mass
– r is the distance between the
two point masses
– Every object exerts a pull of
gravity on every other object.
– Depends on mass of objects
& the distance apart.
– The closer the objects the
stronger the pull. The larger the
object the stronger the pull.
Summary
Freefall & Projectile Motion
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Free Fall
Free Fall is the condition where
gravity is the only force that is working
on an object. The acceleration of
gravity on an object is 9.8 m/s2.
Projectile Motion
Projectile Motion is motion of an
object when it is thrown.
*In a vacuum (a condition where there
is no air resistance), a dropped ball &
a projectile will hit the ground at the
same time.
Air Resistance
A type of fluid friction caused by
objects falling through the air.
*It is an upward force.
*It is not the same for all
objects. The greater the surface area,
the greater the air resistance. This is
why an acorn hits the ground before a
leaf.
*Terminal Velocity- the
greatest velocity an object can reach.
**Review differences between mass &
weight.
Momentum
• Momentum is Mass X
Velocity.
– *Measured in kg
m/s (Note
difference between
momentum &
force!)
– *Has a direction.
Law of Conservation of Momentum
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*Law of Conservation of Momentum –
the total momentum of a group of objects
that interact remains the same unless an
outside force acts on the objects.
– 2 moving objects – if 2 objects
moving at different velocities collide
in an elastic collision then the slower
object will gain momentum & the
faster object will lose momentum but
total momentum will be the same.
– 1 moving & 1 stationary object – if a
moving object collides in an elastic
collision with a non moving object the
non moving object will gain
momentum & move but the moving
object will stop. Total momentum will
be the same.
– Connected objects – if 2 objects ( 2
moving or 1 non moving) collide &
stick together ( Non elastic collision),
they will move together as a single
heavier unit at a slower velocity but
overall momentum will be the same.
*ALL the above examples assume that
friction is absent. In most real
situations friction will play a role.
Centripetal Force
• Centripetal means,
“center seeking” & is any
force that causes an
object to move in a circle.
• This is the force that
holds satellites in orbit
around the earth. It also
holds the moon & planets
in orbits.
• Centripetal force is a
product of gravity &
inertia.
• Satellites continuously fall
toward earth (gravity), but
the earth is a sphere &
this causes it to fall
around the earth by its
own inertia.