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Chapter 3 Forces & Newtons Laws

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3.1- Forces Forces –What is a Force? –Force – –Measured in Newtons –Changing Motion- Contact ForcesAction-at-a-Distance Forces Frictional ForceGravitational Force Tension ForceElectrical Force Normal ForceMagnetic Force Air Resistance Force Applied Force Spring Force

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3.1- Forces Forces –Net force – Rules for Adding Forces 1.Add forces in the same direction. 2.Subtract forces in opposite directions 3.Forces not in the same directions\ or in opposite directions cannot be directly added together.

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Balanced forces – Unbalanced forces Forces

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Friction –Friction- –What causes friction? Static friction Sliding friction Rolling friction

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3.1- Forces Gravity –Gravity- –Law of Universal Gravitation Force increases as mass increases or objects move closer Gravity & You The Range of Gravity The Gravitational Field

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3.1- Forces Gravity –Weight – –Weight (N) = mass(kg) x gravitational strength (N/kg) Fg =mg

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Lets Practice 1.If your mass is 70 kg on Earth, what is your weight? 2.A boy weighs 400 N. What is his mass? 3.An astronaut has a mass of 100 kg and a weight of 370 N on Mars. What is the gravitational strength of Mars?

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3.1- Forces Gravity –Weight and Mass –Weight on Earth –Finding other Planets

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3.2- Newtons Laws of Motion Newtons 1 st Law of Motion Newtons 1 st Law – Break it Down – Things want to keep doing what they are doing Inertia – Inertia and Mass Mass corresponds to an objects inertia More massive objects have more inertia than less massive objects

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3.2- Newtons Laws of Motion Newtons 2 nd Law of Motion Newtons 2 nd Law Break It Down – F=ma Acceleration is caused by net force Acceleration is directly proportional to net force Push an object, object accelerates Push an object with twice as much force, then it accelerates twice as much Direction of acceleration is always the direction of the net force

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3.2- Newtons Laws of Motion Newtons 2 nd Law of Motion Mass and acceleration More massive objects are harder to accelerate Acceleration is inversely proportional to mass Push a brick, brick accelerates Push 2 bricks with same amount of force, accelerates ½ as much 3 bricks accelerates 1/3 as much

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3.2- Newtons Laws of Motion Newtons 2 nd Law of Motion Mass and acceleration More massive objects are harder to accelerate Acceleration is inversely proportional to mass Push a brick, brick accelerates Push 2 bricks with same amount of force, accelerates ½ as much 3 bricks accelerates 1/3 as much

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Lets Practice!!! 1.With what force will a car hit a tree if the car has a mass of 3,000 kg and it is accelerating at a rate of 2 m/s 2 ? 2.A 10 kg bowling ball would require what force to accelerate it at 4 m/s 2 ? 3.If a helicopters mass is 4,500 kg and the net force on it is 18,000 N upward, what is its acceleration?

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3.2- Newtons Laws of Motion Newtons 3 rd Law of Motion Newtons 3 rd Law Break It Down - forces always occur in pairs

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A Simple Rule Helps to Identify Action and Reaction Action: Object A exerts a force on Object B Reaction: Object B exerts a force on Object A

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Identifying Force Pairs Identify the following Force Pairs Action: Enclosed air particles push balloon wall outwards. Reaction: ? Action :Baseball pushes glove leftwards Reaction:? Action: Shuttle pushes exhaust gases down Reaction: ?

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3.2- Newtons Laws of Motion Action and reaction forces do not cancel each other out Forces only cancel when they act on the same body Action-Reaction forces work on different objects

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3.3- Using Newtons Laws What happens in a crash? –Newtons 1 st Law and Safety Belts –Air Bags in Cars

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3.3- Using Newtons Laws Newtons 2 nd Law and Gravitational Acceleration –Air Resistance –Depends on size and shape –Depends on speed of object –More speed, more air resistance –Terminal velocity -

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3.3- Using Newtons Laws Newtons 2 nd Law and Gravitational Acceleration –Free Fall – Weightlessness What does weightless mean?

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3.3- Using Newtons Laws Centripetal Forces –Centripetal force- –Force acting toward the center of circle

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3.3- Using Newtons Laws Force and Momentum –P = mv –Conservation of Momentum Law of Conservation of momentum Momentum conserved

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