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Answer all questions on a piece of loose leaf and turn in at the end of the period for homework credit. Unit 4 Review

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Mass vs Weight Mass- Amount of matter in an object – Units- kg – Unchanged by gravitational field Weight- Force of pull on an object caused by gravity – Units- N – Depends on gravitational field strength

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Weight (F g ) Always directed towards Earth (down) Force of Gravity F g = mg g = 9.8m/s 2 or 9.8N/kg

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FNFN FgFg For an object resting on a level surface F N = -F g

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1) What is the weight of a 25kg object? 2) What is the normal force acting on this object if it rests on a level, horizontal surface? 3) If an object weighs 250N on Earth, what is the mass of the object?

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Net Force F Net Sum or “Net” of all forces acting on an object F Net = F 1 + F 2 + F 3 … Not in reference table

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Equilibrium A state where F Net is 0N and an object has constant velocity A force that causes equilibrium is called an equilibrant An equilibrant is equal in magnitude to, but opposite in direction of a resultant (or F Net )

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Newton’s First Law “An object at rest will stay at rest and an object in motion will stay in motion unless acted on by an unbalanced force.” Inertia- The resistance of an object to a change in motion Inertia is directly proportional to mass

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35N 10N 4) 5) An 80N wooden block is sliding down a wooden incline at constant velocity. The incline is angled at 25°. What is the net force acting on the block?

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Newton’s Second Law The acceleration of an object is directly proportional to the net force acting on it and indirectly proportional to it’s mass. Think of it this way: Bigger forces cause bigger accelerations Bigger masses cause smaller accelerations

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Newton’s Second Law

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6) What is the acceleration of this object? 10 N5 N m = 1kg

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Newton’s Third Law “For every action there is an equal and opposite reaction”

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7) A football player kicks a football with a force of 15N. If the football is accelerated at a rate of 13 m/s 2 at an angle of 30°, what force does the football apply to the player’s foot?

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FgFg F Net = a = + 2 m/s 2 m = 70kg Fg=FN=Fg=FN= F N (What the scale reads) 8) Find what the scale reads:

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What do F II and F ˔ mean? FNFN FgFg F II F˔F˔ θ θ

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What do F II and F ˔ mean? FNFN FgFg F II F˔F˔ θ

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Calculating F II and F ˔ Sin(θ) = F II / F g F II = F g sin(θ) Cos(θ) = F ˔ / F g F ˔ = F g cos(θ)

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