newton's 2nd law
TRANSCRIPT
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Motion & Forces
. Force & Acceleration Newton’s Second Law Gravity Air Resistance Calculations
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A. 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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A. Newton’s Second Law
F = maF: force (N)m: mass (kg)a: accel (m/s2)
1 N = 1 kg ·m/s2
am
Fa
Fm
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B. GravityGravity
force of attraction between any two objects in the universe increases as...
• mass increases• distance decreases
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B. GravityWho 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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B. GravityWeight
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)
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B. GravityWould you weigh more on Earth
or Jupiter?
greater gravity
greater weight
greater mass
Jupiter because...
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B. GravityAccel. due to gravity (g)
In the absence of air resistance, all falling objects have the same acceleration!
On Earth: g = 9.8 m/s2
mWg
elephant
m
Wg
featherAnimation from “Multimedia Physics Studios.”
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C. Air ResistanceAir Resistance
a.k.a. “fluid friction” or “drag” force that air exerts on a moving
object to oppose its motion depends on:
• speed• surface area• shape• density of fluid
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C. Air ResistanceTerminal Velocity
maximum velocity reached by a falling object
reached when… Fgrav = Fair
Fair
Fgrav
no net force no acceleration constant velocity
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C. Air ResistanceTerminal Velocity
increasing speed increasing air resistance until…
Fair = FgravityAnimation from “Multimedia Physics Studios.”
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C. Air ResistanceFalling with air resistance
Fgrav = Fair
Animation from “Multimedia Physics Studios.”
heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity
larger Fgrav
need larger Fair
need higher speed
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D. CalculationsWhat force would be required to
accelerate a 40 kg mass by 4 m/s2?
GIVEN:F = ?m = 40 kga = 4 m/s2
WORK:F = ma
F = (40 kg)(4 m/s2)
F = 160 N
mF
a
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D. CalculationsA 4.0 kg shotput is thrown with 30 N of
force. What is its acceleration?
GIVEN:m = 4.0 kgF = 30 Na = ?
WORK:a = F ÷ m
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
mF
a
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ConcepTest TRUE or FALSE Does mass depend on gravity?
False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.
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D. CalculationsMrs. J. weighs 557 N. What is her
mass?
GIVEN:F(W) = 557 Nm = ?a(g) = 9.8 m/s2
WORK:m = F ÷ a
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg
mF
a