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Unit 5: Forces & Newton’s Laws Page:Contents: 29weight / force 30Newton’s First Law / Newton’s Second Law 31Example: How to calculate the friction force 32 Newton’s Third Law / free body diagram 33centripetal force / Law of Universal Gravitation 34Lab SUMUPS SP1. Students will analyze the relationships between force, mass, gravity, and the motion of objects. b. Compare and contrast scalar and vector quantities. d. Measure and calculate the magnitude of frictional forces and Newton’s three Laws of Motion. e. Measure and calculate the magnitude of gravitational forces. g. Measure and calculate centripetal force. h. Determine the conditions required to maintain a body in a state of static equilibrium.

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Newton’s Laws Inertia Fnet=MA F=-F <>

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Inertia Newton’s First Law:

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A force is… a push or a pull. Friction, Drag, Gravity, and Weight are forces…

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Acceleration = Change in speed … or… Direction

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What causes a change in speed or direction?

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Net Force The difference is 20 N, to the left. The car will speed up.

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Inertia = Mass Bigger masses have more… Smaller masses have less inertia.

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Inertia = Resistance to… changes in motion. net forces. changing velocity. changing direction.

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Which can accelerate easily? Small masses change motion easily.

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It is easy to accelerate small masses.

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Inertia = resistance to changes in motion. Large masses resist net forces and do not accelerate much. A large mass resists changing its velocity.

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What if we put rockets on each creature? Large masses resist changing motion. Small masses change motion easily.

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Is a force needed to keep an object in motion? No! An object in motion stays in motion at constant velocity forever! (Unless a new net force acts…)

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If no net force… Fnet = MA 0=MA No Acceleration Constant Speed!

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Inertia Would this experiment work well using foam plates and plastic cutlery?

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An object at rest…

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stays at rest unless acted on… by a net force.

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Rest = Rest

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Which is harder to move? Inertia = resistance to changes in motion. The gorilla has more inertia.

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Inertia also says an object in motion…

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…will stay in motion in a straight line, unless…

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…acted on by a net force.

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Motion = motion.

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Which is harder to stop? The larger mass has more inertia. It has more resistance to changes in motion!

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Which would be harder to accelerate?

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It is hard to accelerate large masses.

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F net =MA Newton’s 2nd Law:

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Net Force The difference is 50 N, to the left. The bike will slow down.

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Net Force?

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Acceleration = Change in speed … or… Direction

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F net = MA Net Force = Mass x Acceleration

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F net = MA 100 N = 10 x A 100 N = 2 x A

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100 N = 10 x 10 100 N = 2 x A F net = MA

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100 N = 10 x 10 100 N = 2 x 50 F net = MA

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Small masses are… easy to accelerate. F net = MA

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Which car wins the race assuming drag, friction and Fnet are the same for both?

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F = M x A 10 = 10 x 1 10 = 5 x 2 10 = 1 x 10 For a given Net Force, Mass and Acceleration are inversely proportional. As mass decreases, acceleration increases.

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Fnet = MA It is hard to change the speed and/or direction of… Large Masses

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F net =MA

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Action Reaction Newton’s 3rd Law:

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Pushes and Pulls are Equal!

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How many force pairs? (Assume no air) Push pair Weight pair Friction Pair

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Pushes and Pulls are Equal!

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Newton's 3rd Law states that forces always come in equal and opposite pairs.

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Two cars crash. What is true?

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They hit with the same force. Neither hits harder.

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You exert force on the weights and…

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The weights exert the same force on you.

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Both forces are…

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Equal!

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The forces are… Crash site = Action/Reaction Opposite and Equal!

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Acceleration = Change in speed … or… Direction

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Which object will have a greater acceleration? (Fnet=MA)

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Weight pair Friction pair Push pair Weight pair Net Force= 8 N Why Objects Move: If the push is 10 N and the Fridge Friction is 2 N …

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A man pushes his daughter!

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Newton’s 3rd Law: Action = girl rolls left. Reaction = dad rolls right.

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Compare the forces!

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Newton’s 3rd Law: The forces are equal and in opposite directions!

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Who will have a greater change in speed after pushing?

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Skaters! Small mass Large change in speed!

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Skaters! A big mass means… A small acceleration

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Planes and Newton’s Laws!

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Equal and opposite forces Newton’s 3rd Law:

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Compare the forces. Equal.

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ACTION: Air is pushed down. REACTION: Wing is pushed up.

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If a plane is pushed up with 100 Newtons of force… Then… ____ is pushed down with ____tons of force.

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If a 100-Newton plane is pushed up… Then… air is pushed down with 100 Newtons of force.

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Equal and opposite forces Newton’s 3rd Law:

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Water is pushed back and…

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the boat is pushed forward.

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How does thrust work?

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The propeller pushes the air back and…

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The air pushes the propeller (and plane) forward!

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The force on the air is equal to the the air on the propeller.

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Action Reaction Newton’s 3rd Law:

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The air or the wing? Which accelerates more?

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Since the air has less mass, it speeds up more.

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Mass x Acceleration Air 2nd Law: F=ma

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Wing Mass x Acceleration 2nd Law: F net =ma

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