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Motion & Force

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Aristotle ~375 BC

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All motion on the Earth is linear.

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All motion in the heavens is perfectly circular.

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Aristotle’s falling bodies – heavy fall faster.

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Motion continues so long as there is an applied force (push or pull) to an object. Removing the force stops the object.

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Everyone accepted Aristotle’s view until Galileo. That's about 2000 years of false knowledge.

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Galileo ~ 1600 Began Experimentation and observation. Collected empirical data.

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Galileo - all objects fall to the ground at the same speed.

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Galileo’s Experiment – objects attain same height.

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What is right side made flat?

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Objects will move in straight line at constant velocity forever with no friction (or other force) to stop them!

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Sir Isaac Newton ~1660 Quantified Galileo’s Ideas Developed Laws of Motion

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Newton’s 3 Laws of Motion 1. Law of inertia – deals with objects with zero net force. 2. Law of acceleration – deals with accelerating objects which have a net force. 3. Action/Reaction or Force pairs. Deals with equal & opposite forces exerted on & by pairs of objects.

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What is a force? Push or pull on object Force unit = Newton N.

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1 N = kg m/s 2. The amount of force needed to accelerate a 1 kg object 1 m/s 2.

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Newton’s 1 st Law: Inertia An object with no net (extra) force continues at rest or in straight line motion with the constant speed unless acted upon by an unbalanced force. Unbalanced force = F net. More F in 1 direction. If forces are balanced, they are equal & opposite.

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If all forces balanced, F net = 0, acceleration =? a = 0 Object at rest or moving at constant velocity!

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Inertia Inertia = the resistance an object has to a change in its state of motion – acceleration. If it’s moving, inertia makes it want to keep moving. If it’s at rest, inertia makes it want to stay at rest.

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The truck stops from the force applied by the car. The ladder does not have the same applied force. It keeps going. Why? Inertia ! The inertia of the ladder makes it keep its state of motion.

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Inertia depends on: Mass only High InertiaLow Inertia

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1. You are in floating space with no gravity or friction of any kind. Your (ex) best friend throws a 10-kg bowling ball at 1 m/s and a 4-g penny moving at 500 m/s at you. Which one has more inertia? 1. The bowling ball. 2. The penny. 3. Neither – since there is no gravity or friction. 4. Impossible to tell from information given.

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Objects can have force on them but won’t necessarily move. Force & Motion

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Net Force (unbalanced) = more force in one direction. Acceleration to right.

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An unbalanced/net force causes the object to accelerate in the direction of net force. Observe the cartoon. What is the F net ? What is the Acceleration? What is the Direction?

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Inertia Resists acceleration or change. Stopping, starting, and changing direction. The more mass an object has, the harder it is to change its motion. It is directly proportional to mass. Two times the mass = 2x the effort for v. Inertia is NOT a force!! It is a property of mass.

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Net Force F net - Unbalanced forces cause acceleration in the same direction as the unbalanced force. Equilibrium – The sum ( F) of all forces = 0. They cancel in all directions. Left F = Right F, Up F = down F. West F = East F. Acceleration = 0.

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2. A car is speeding on a straight flat highway at 100 miles/hour. The engine is pushing the car forward with a force of 25,000 N. What can you say about the sum of the resistive (backward) forces on the car? 1. The resistive forces = 0 2. The resistive forces = 25,000 N 3. The resistive forces < 25,000 N. 4. Cannot tell from the information supplied.

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Acceleration To change an object’s velocity, a, there must be an unbalanced or net force (F net ) acting on the object. This is true for a change in speed or a change in direction. The more inertia, the more F net needed for acceleration.

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3. This car is moving at a constant 30 mi/hr. What can you say with certainty? 1. It has no forces acting on it. 2. The forces on it are balanced. 3. It is accelerating forward. 4. Not enough information to tell.

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Demo Accelerometer

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2 outcomes of Balanced Forces Objects stay at rest Objects at constant velocity = no acceleration

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Balanced Forces = Equilibrium Forces of equal magnitude and in opposite directions, balance each other. They are said to be at equilibrium. F = 0

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The forces add to zero there is no net force on the person.

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In the absence of an unbalanced force to stop it, inertia will keep an object moving in a straight line forever! The block is in equilibrium.

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Unit of Force = Newton (N) 1 N = 1 kg m s 2. Force is a vector quantity same direction as acceleration Force units:

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Hwk read Holt text pg 124 – 131 do 151 #1 – 6. Write full sentences on separate sheet.

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Newton’s 1st Law space station 6 min Newton’s 3 laws with hockey 6 minutes. 4e38f75d9142/newtons-three-laws-motion Film Clips

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