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**IV. Force & Acceleration**

Ch. 3 & 4 Motion & Forces IV. Force & Acceleration Newton’s Second Law Gravity Air Resistance Calculations

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**F = ma 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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**F = ma F m a F m A. Newton’s Second Law F: force (N) m: mass (kg)**

a: accel (m/s2) 1 N = 1 kg ·m/s2

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B. 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? Which exerts more gravity - the Earth or the moon? more mass less distance

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**W = mg B. Gravity Weight the force of gravity on an object MASS WEIGHT**

W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) MASS always the same (kg) WEIGHT depends on gravity (N)

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**B. Gravity Would you weigh more on Earth or Jupiter?**

Jupiter because... greater mass greater gravity greater weight

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**Animation from “Multimedia Physics Studios.”**

B. 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 elephant feather Animation from “Multimedia Physics Studios.”

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**C. Air Resistance Air 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 Resistance Fair Fgrav Terminal Velocity**

maximum velocity reached by a falling object reached when… Fgrav = Fair Fair no net force no acceleration constant velocity Fgrav

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**Animation from “Multimedia Physics Studios.”**

C. Air Resistance Terminal Velocity increasing speed increasing air resistance until… Fair = Fgrav Animation from “Multimedia Physics Studios.”

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**Animation from “Multimedia Physics Studios.”**

C. Air Resistance Falling with air resistance heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity Fgrav = Fair larger Fgrav need larger Fair need higher speed Animation from “Multimedia Physics Studios.”

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**a F m D. Calculations F = ? F = ma m = 40 kg F = (40 kg)(4 m/s2)**

What 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 m F a

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**a F m D. Calculations m = 4.0 kg a = F ÷ m F = 30 N**

A 4.0 kg shotput 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 m F a

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**a F m D. Calculations F(W) = 557 N m = F ÷ a m = ?**

Mrs. J. weighs 557 N. What is her mass? GIVEN: F(W) = 557 N m = ? a(g) = 9.8 m/s2 WORK: m = F ÷ a m = (557 N) ÷ (9.8 m/s2) m = 56.8 kg m F a

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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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