Ch. 12 Motion & Forces
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Transcript of Ch. 12 Motion & Forces
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Ch. 12Motion & Forces
I. Newton’s Laws of Motion “If I have seen far, it is because I have stood
on the shoulders of giants.”- Sir Isaac Newton (referring to Galileo)
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Newton’s First Law
• Newton’s First Law of Motion– An object at rest will remain at rest and an
object in motion will continue moving at a constant velocity unless acted upon by a net force.
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Newton’s Second Law
• Newton’s Second Law of Motion– The acceleration of an object is directly
proportional to the net force acting on it and inversely proportional to its mass.
F = ma
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Newton’s Third Law
• Newton’s Third Law of Motion– When one object exerts a force on a second
object, the second object exerts an equal but opposite force on the first.
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III. Defining Force Force
Newton’s First Law Friction
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A. Force• Force
– a push or pull that one body exerts on another– What forces are being
exerted on the football?
Fkick
Fgrav
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A. Force• Balanced Forces
– forces acting on an object that are opposite in direction and equal in size
–no change in velocity
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A. Force• Net Force
–unbalanced forces that are not opposite and equal
–velocity changes (object accelerates)
Ffriction
W
Fpull
Fnet
NN
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Calculating Net Force from Images• Net force is the sum of forces acting on an object
– If forces are acting in the same direction they combine (add together)
– If forces are acting in opposite directions they reduce (subtract)
• If net force is zero the motion of the object will NOT change
• If net force is not zero the object will accelerate (this may be positive or negative acceleration).
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What is the net force on the objects below?
Opposite = subtract8N – 3N = 5 N
Same = addition5N + 2N = 7 N
2 N + 5N – 8N = 1 N 7N + 2N – 3N = 6N
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B. Newton’s First Law• Newton’s First Law of Motion
–“Law of Inertia”• Inertia
–tendency of an object to resist any change in its motion
–increases as mass increases
• Newton’s First Law of Motion– An object at rest will remain at rest and an
object in motion will continue moving at a constant velocity unless acted upon by a net force.
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ConcepTest 1
TRUE or FALSE?
The object shown in the diagram must be at rest since there is no net force acting on it.
FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration.
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Concep Test 2
1. Before and after the collision, there is a rightward force pushing you into the door.
2. Starting at the time of collision, the door exerts a leftward force on you.
3. both of the above
2. Starting at the time of collision, the door exerts a leftward force on you.
You are a passenger in a car and not wearing your seat belt.
Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door.
Which is the correct analysis of the situation? ...
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C. Newton’s Second Law• Newton’s Second Law of Motion
–The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
F = ma
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Newton’s Second Law
F = maF: force (N)m: mass (kg)a: accel (m/s2)
1 N = 1 kg ·m/s2
mF
a
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CalculationsWhat force would be required to
accelerate a 40 kg mass by 4 m/s2?
GIVEN:
F = ?
m = 40 kg
a = 4 m/s2
WORK:
F = ma
F = (40 kg)(4 m/s2)
F = 160 N
mF
a
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CalculationsA 4.0 kg shot-put is thrown with 30 N of
force. What is its acceleration?
GIVEN:
m = 4.0 kg
F = 30 N
a = ?
WORK:
a = F ÷ m
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
mF
a
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D. Gravity• Gravity
– force of attraction between any two objects in the universe
– increases as...• mass increases• distance decreases
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B. Gravity• Who experiences more gravity - the
astronaut or the politician?
less distance
more mass
Which exerts more gravity - the Earth or the moon?
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D. Gravity• Would you weigh more on Earth or
Jupiter?
greater gravity
greater weight
greater mass
Jupiter because...
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Free Fall• When gravity is the
only force acting on an object
• Represented by the letter g
• Near Earth’s surface g = 9.8 m/s2
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B. Gravity• Weight
– the force of gravity on an object
MASSalways the same
(kg)
WEIGHTdepends on gravity
(N)
W = mgW: weight (N)m: mass (kg)g: acceleration due
to gravity (m/s2)
W: weight (N)m: mass (kg)g: acceleration due
to gravity (m/s2)
W: weight (N)m: mass (kg)g: acceleration due
to gravity (m/s2)
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D. Gravity• Accel. due to gravity (g)
In the absence of air resistance, all falling objects have the same acceleration!
On Earth: g = 9.8 m/s2
mW
g
elephant
m
Wg
feather
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CalculationsMrs. J. weighs 557 N. What is her
mass?
GIVEN:
W = 557 N
m = ?
g = 9.8 m/s2
WORK:
m = W ÷ g
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
mW
g
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ConcepTest• Is the following statement true or false?
– An astronaut has less mass on the moon since the moon exerts a weaker gravitational force.
False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.
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E. Newton’s 3rd Law• For every action
force, there is an equal and opposite reaction force.
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3rd law: Action / Reaction• The action force and the
reaction force occur to different objects so the force is not balanced.
• Not all action and reaction forces produce motion.
• Unbalanced forces equal changes in motion
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Newton’s third law• Kicking a soccer ball
– Action force = your foot hits the ball– Reaction force = the ball pushes against your foot.
• Ball moves b/c action force is larger than reaction force
• Leaning against a wall– Action force = you pushing against a wall– Reaction force = wall pushing against you
• Nothing moves b/c the action force equals the reaction force.
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F. Friction• Friction is a force that opposes the
motion of objects that touch as they move past each other.
• Friction acts at the surface where object are in contact
• Four main types of friction: static friction, sliding friction, rolling friction and fluid friction.
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C. Friction
• Friction– is the force that opposes motion
between 2 surfaces–depends on the:
• types of surfaces• force between the surfaces
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C. Friction• Friction is greater...
–between rough surfaces–when there’s a greater
force between the surfaces (e.g. more weight)
• Pros and Cons?
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Static Friction• Static friction is the friction
force that acts on objects that are not moving.
• Static friction always acts in the direction opposite to that of the applied force
• Prevents objects from sliding.
• Example: pushing a dresser that does NOT move
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Sliding Friction• Sliding Friction is a force
that opposes the direction of motion of an object as it slides over a surface.
• Occurs when there is enough force to overcome the static friction
• There will be a net force in the direction of motion
• Pushing a desk that slides against the floor.
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Rolling Friction• Rolling friction is the
force that acts on rolling objects.
• Rolling friction is about 100 to 1000 times less than the force of static or sliding friction
• Skate boarding down a hill
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Fluid Friction• Fluid friction opposes the
motion of an object through a fluid.
• Fluid examples: air, water, quick sand, and cake batter
• Air Resistance is the fluid friction action on an object moving through the air.
What are the forces acting on a falling leaf?
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Air Resistance• Terminal Velocity
–maximum velocity reached by a falling object
– reached when… Fgrav = Fair
Fair
Fgrav
no net force no acceleration constant velocity still falling
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Projectile Motion
• Projectile motion the motion of a falling object after it is given an initial forward velocity.
• this is a curved path• Objects with different mass fall at
the same rate.• The combination of an initial
forward velocity and the downward vertical force of gravity causes the ball to follow a curved path.
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Momentum • Momentum is the product of an object’s mass
and its velocity, an object with large momentum is hard to stop
• momentum for any object at rest is zero• Law of conservation of momentum if not net
forces acts on a system, then the total momentum of the system does not change
• In a closed system the loss of momentum of one object equals the gain in momentum of another object--- momentum is conserved
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