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FORCE Chapter 10 Text

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Force A push or a pull in a certain direction SI Unit = Newton (N)

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Combining Forces The combination of all forces acting on an object is the Net Force.

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When forces act in the SAME DIRECTION = ADDITION

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When forces act in the OPPOSITE DIRECTION = SUBTRACTION

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Balanced Forces result in NO MOVEMENT

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= 0

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Unbalanced forces cause a change in the object’s motion.

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=

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=

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Friction and Gravity Friction – the force that two surfaces exert on each other when they rub

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Types of Friction 1. Static Friction 2. Sliding Friction 3. Rolling Friction 4. Fluid Friction

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Static Friction Friction that acts on objects that are NOT moving

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Sliding Friction Occurs when two surfaces slide over each other

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Rolling Friction Occurs when an object rolls across a surface

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Fluid Friction Occurs when solid objects moves through fluids such as water, oil, or air

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Friction depends on two factors: 1. How hard the surfaces push together

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2. The type of surfaces involved

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Gravity A force that pulls objects toward each other

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2 Factors Affecting Gravity 1. Distance 2. Mass

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If Distance Increases the force of gravity Decreases

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Mass The amount of matter in an object

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If mass increases, the force of gravity increases

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Mass vs. Weight Mass is Constant because of the amount of matter does not change

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The force of gravity on a person or object at the surface of a planet is known as weight.

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Weight varies with the strength of the gravitational force, mass does not.

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Gravity and Motion Free fall – when the only force acting on a falling object is gravity

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Acceleration due to gravity is 9.8 m/s/s

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All objects in free fall accelerate at the SAME rate regardless of their masses

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Why don’t objects fall at the same rate?

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Air Resistance Falling objects with a greater surface area experience more air resistance.

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Terminal Velocity Reached when the force of air resistance equals the weight of the object

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Newton’s Laws of Motion

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Newton’s 1 st Law an object at rest will remain at rest, an object in motion will stay in motion UNLESS acted upon by another force.

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Inertia Resistance to a change in motion Newton’s 1 st Law

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Inertia depends on mass, the greater the mass the greater the inertia

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Newton’s 2 nd Law Force = mass x acc.

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Acceleration =Net Force Mass

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End Unit for Acceleration kgm/s/s Kgm/s 2 Newton

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Textbook samples

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Calculating: Calculate the slope of the graph. What does the slope tell you about the object’s motion?

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The slope is 9.8. The speed increases by 9.8 m/s each second.

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– What will the speed of the object be at 6 seconds? 58.8 m/s

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A speedboat pulls a 55- kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the force that causes this acceleration.

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F net = m X a = 55 kg X 2.0 m/s 2 F = 110 kg m/s 2 F = 110 N

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What is the net force on a 1,000-kg object accelerating at 3 m/s 2 ?

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Force = m x acc. (1,000 kg X 3 m/s 2 ) 3,000 N

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What net force is needed to accelerate a 25-kg cart at 14 m/s 2 ?

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Force = mass x acc. (25 kg X 14 m/s 2 ) 350 N

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Newton’s 3 rd Law For every action there is an equal and opposite reaction

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Momentum Momentum = Mass x Velocity End Unit = kg m/s

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Conservation of Momentum The total momentum of any group of objects remains the same, or is conserved, unless outside forces act on the objects

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Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s?

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Momentum = Mass X Velocity Smaller sledgehammer = 3.0 km X 1.5 m/s = 4.5 kgm/s Larger sledgehammer = 4.0 km X 0.9 m/s = 3.6 kgm/s

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A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which has the greater momentum?

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Golf ball: 0.045 kg X 16 m/s = 0.72 kgm/s Baseball: 0.14 kg X 7 m/s = 0.98 kgm/s The baseball has greater momentum.

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What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s?

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(0.018 kg X 15 m/s = 0.27 kgm/s

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Energy Chapter 13

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Energy The ability to do work

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2 Kinds of Energy 1. Kinetic Energy 2. Potential Energy

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Kinetic Energy Energy of MOTION

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Kinetic energy increases as mass and velocity increases

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Potential Energy Stored Energy

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Gravitational potential energy increases as weight and height increase.

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