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**Chapter 12 Forces and Motion**

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**A force is a push or a pull that acts on an object.**

What Is a Force? A force is a push or a pull that acts on an object. It can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

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Units of Force One newton (N) is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/s2). 1 N = 1 kg•m/s2

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Combining Forces When the forces on an object are balanced, the net force is zero There is no change in the object’s motion. When an unbalanced force acts on an object, the object accelerates.

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**The net force is the overall force acting on an object after all the forces are combined.**

Forces in the same direction add together. Forces in opposite directions subtract from one another.

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Balanced Forces Balanced forces are forces that combine to produce a net force of zero. An unlimited number of individual forces can act on an object to produce a net force of zero.

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Unbalanced Forces An unbalanced force is a force that results when the net force acting on an object is not equal to zero.

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Activity Calculating Tugboat A exerts a force of 4000 N on a barge. Tugboat B exerts a force of 8000 N on the barge in the same direction. What is the combined force? Using arrows draw the combined forces. Then draw the forces involved if the tugboats were pulling in opposite directions.

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ANSWER: In the same direction: 4000N N = 12000N In opposite directions: 4000N – 8000N = 4000N toward the direction of tugboat B

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Friction Friction is a force that opposes the motion of objects that touch as they move past each other. four main types of friction: static friction sliding friction rolling friction fluid friction.

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Static Friction Static friction is the friction force that acts on objects that are not moving. Static friction always acts in the direction opposite to that of the applied force.

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Rolling Friction Rolling friction is the friction force that acts on rolling objects. about 100 to 1000 times less than the force of static or sliding friction.

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Sliding Friction Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface. Sliding friction is less than static friction.

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Fluid Friction Fluid friction opposes the motion of an object through a fluid. increases as the speed of the object moving through the fluid increases. Fluid friction acting on an object moving through the air is known as air resistance.

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**Earth’s gravity acts downward toward the center of Earth.**

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**How do gravity and air resistance affect a falling object?**

Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.

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**Gravity is a force that acts between any two masses.**

an attractive force. can act over large distances.

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Free Fall Free fall – motion of an object when gravity is the only force acting on it All objects near earth accelerate at 9.8 m/s2 in the absence of air resistance

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Terminal Velocity Terminal Velocity – when air resistance balances weight and the object stops accelerating and reaches its maximum velocity 320 km/h (200 mi/h)

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Projectile Motion A thrown ball follows a curved path. Projectile motion is the motion of a falling object (projectile) after it is given an initial forward velocity. Air resistance and gravity are the only forces acting on a projectile.

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REVIEW If an object is at rest, which of the following statements must be true? There are no forces acting on the object. There is no friction acting on the object. The forces acting on the object are unbalanced. The net force acting on the object is zero.

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REVIEW If an object is at rest, which of the following statements must be true? There are no forces acting on the object. There is no friction acting on the object. The forces acting on the object are unbalanced. The net force acting on the object is zero.

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**Which of the following is not a type of friction? static friction **

REVIEW Which of the following is not a type of friction? static friction sliding friction fluid friction pull friction

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**Which of the following is not a type of friction? static friction **

REVIEW Which of the following is not a type of friction? static friction sliding friction fluid friction pull friction

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**REVIEW In which direction does Earth’s gravitational force act?**

opposite the direction of motion downward toward the center of Earth upward away from the center of Earth in the direction of motion

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**REVIEW In which direction does Earth’s gravitational force act?**

opposite the direction of motion downward toward the center of Earth upward away from the center of Earth in the direction of motion

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REVIEW A ball thrown into the air follows a projectile course due to the initial velocity and the force of gravity. effect of air resistance. motion of Earth beneath it. mass of the ball.

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REVIEW A ball thrown into the air follows a projectile course due to the initial velocity and the force of gravity. effect of air resistance. motion of Earth beneath it. mass of the ball.

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REVIEW 5. The SI unit for force is 1 kg•m/s2, also called one kepler. True False

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REVIEW The SI unit for force is 1 kg•m/s2, also called one kepler. False: The SI unit for force is 1 kg•m/s2, also called one Newton.

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**Chapter 12 Forces and Motion**

12.2 Newton’s first and second laws of motion

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Newton Newton built on the work of scientists such as Galileo. Newton first defined mass and force. He then introduced his laws of motion.

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**Newton’s First Law of Motion**

According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.

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Unless acted upon by an unbalanced force, an object at rest remains at rest and an object in motion remains in motion with the same speed and direction. Inertia is the tendency of an object to resist a change in its motion.

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**Newton’s Second Law of Motion**

Newton’s second law of motion: the acceleration of an object is equal to the net force acting on it divided by the object’s mass.

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F = ma Newton’s Second Law of Motion The acceleration of an object is directly proportional to the net force acting on it. also depends upon its mass. Mass is a measure of the inertia of an object.

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2nd Law Example How much force is needed to accelerate a 1400 kg car 2 m/s2?

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2nd Law Example How much force is needed to accelerate a 1400 kg car 2 m/s2? F = 1400kg x 2 m/s2 F = 2800N (Remember: 1 N = 1 kg•m/s2 ) F = ma

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Newton’s Second Law An automobile with a mass of 1000 kilograms accelerates when the traffic light turns green. If the net force on the car is 4000 newtons, what is the car’s acceleration?

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**F = ma 4000 / 1000 = 4 m/s2 Acceleration = 4 m/s2 Newton’s Second Law**

4000N = 1000kg x m/s2 4000 / 1000 = 4 m/s2 Acceleration = 4 m/s2

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**Newton’s Second Law of Motion**

1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

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**F = ma 60 = 40kg x m/s2 60 / 40 = 4.5 m/s2 acceleration = 4.5 m/s2**

Newton’s Second Law of Motion A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? F = ma 60 = 40kg x m/s2 60 / 40 = 4.5 m/s2 acceleration = 4.5 m/s2

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2nd Law Practice How much force is needed to accelerate a 66 kg skier 1 m/s2? What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2? A baseball accelerates down at 9.8 m/s2. If the force is 1.4 N, what is the baseball’s mass?

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2nd Law Practice How much force is needed to accelerate a 66 kg skier 1 m/s2? 66N 2. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/s2? 9800 N 3. A baseball accelerates down at 9.8 m/s2. If the force is 1.4 N, what is the baseball’s mass? 0.143kg

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**Mass and weight are related but are not the same.**

Mass is the measure of the amount of material an object contains. Weight is the force of gravity acting on an object. Weight is the product mass and acceleration due to gravity.

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**W = mg is a different form of Newton’s Second Law, F = ma. **

Weight and Mass W = mg is a different form of Newton’s Second Law, F = ma. The value of g in the formula is 9.8 m/s2.

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Weight and Mass If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2?

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Weight and Mass If an astronaut has a mass of 112 kilograms, what is his weight on Earth where the acceleration due to gravity is 9.8 m/s2? Weight = 112 kg x 9.8 m/s2 Weight = N

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**Weight Calculations A 10 kg mass would weigh __**

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**Weight Calculations A 10 kg mass would weigh _98 N_**

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**REVIEW What is inertia? the force of gravity acting on an object**

forces of friction slowing an object’s motion the mass of an object the tendency of an object to resist change in its motion

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**REVIEW What is inertia? the force of gravity acting on an object**

forces of friction slowing an object’s motion the mass of an object the tendency of an object to resist change in its motion

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REVIEW A 3600-N force causes a car to accelerate at a rate of 4 m/s2. What is the mass of the car? 600 kg 900 kg 14,400 kg 1200 kg

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REVIEW A 3600-N force causes a car to accelerate at a rate of 4 m/s2. What is the mass of the car? 600 kg 900 kg = 3600 / 4 14,400 kg 1200 kg

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REVIEW How would your mass and weight change if you were on the moon’s surface? They wouldn’t change. Your mass would remain constant, and your weight would increase. Your mass and weight would decrease. Your mass would remain constant, and your weight would decrease.

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REVIEW How would your mass and weight change if you were on the moon’s surface? They wouldn’t change. Your mass would remain constant, and your weight would increase. Your mass and weight would decrease. Your mass would remain constant, and your weight would decrease. (Acceleration due to gravity would be less on the moon)

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**Chapter 12 Forces and Motion**

12.3 Newton’s Third Law of Motion and Momentum

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Newton’s Third Law According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

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**Momentum is the product of an object’s mass and its velocity.**

momentum is large if the product of its mass and velocity is large.

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**An object with large momentum is harder to stop than an object with small momentum.**

The momentum for any object at rest is zero.

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**p = mv Momentum Mass is measured in kilograms.**

Velocity is measured in meters per second. Momentum is measured kilogram-meters per second.

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Momentum Unit: kg·m/s Ex: Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley

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**p = mv Momentum Unit: kg·m/s**

Ex: Calculate the momentum of a 6.00 kg bowling ball moving at 10.0 m/s down the alley p = mv p = 6 kg x 10 m/s 60 kg·m/s = momentum

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Momentum Which has more momentum, a kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second?

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Momentum Which has more momentum, a kilogram golf ball with a speed of 60.0 meters per second (2.76 kg·m/s) , or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second (42 kg·m/s)?

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**Momentum Practice 75 kg speed skater moving forward at 16 m/s**

135 kg ostrich running north at 16.2 m/s A 5.0 kg baby on a train moving east at 72 m/s 48.5 kg person on a stopped train

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**Momentum Practice 75kg x 16m/s = 1200 kg·m/s**

1. 75 kg speed skater moving forward at 16 m/s 75kg x 16m/s = 1200 kg·m/s kg ostrich running north at 16.2 m/s 135kg x 16.2m/s = 2187 kg·m/s 3. A 5.0 kg baby on a train moving east at 72 m/s 5kg x 72m/s = 360 kg·m/s kg person on a stopped train 48.5kg x 0m/s = 0 kg·m/s

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**Conservation of Momentum**

A closed system means other objects and forces cannot enter or leave a system. In a closed system, the loss of momentum of one object equals the gain in momentum of another object— momentum is conserved.

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**Objects within a closed system can exert forces on one another. **

Conservation of Momentum Objects within a closed system can exert forces on one another. Law of conservation of momentum: if no net force acts on a system, then the total momentum of the system does not change.

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REVIEW A stationary figure skater pushes off the boards around an ice skating rink and begins gliding backward, away from the boards. Which law explains why the figure skater moves backward? the law of conservation of energy the law of inertia Newton’s second law Newton’s third law

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REVIEW A stationary figure skater pushes off the boards around an ice skating rink and begins gliding backward, away from the boards. Which law explains why the figure skater moves backward? the law of conservation of energy the law of inertia Newton’s second law Newton’s third law

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Chapter 12 12.4 Universal Forces

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**Four universal forces exist throughout the universe.**

electromagnetic strong nuclear weak nuclear gravitational

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**Electromagnetic Forces**

Electric force and magnetic force are the only forces that can both attract and repel.

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**Electromagnetic force is associated with charged particles.**

Electromagnetic Forces Electric and magnetic force are two different aspects of the electromagnetic force. Electromagnetic force is associated with charged particles.

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Electric Forces Electric forces act between charged objects or particles. Objects with opposite charges attract one another. Objects with like charges repel one another.

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Magnetic Forces Magnetic forces act on certain metals, the poles of magnets, and moving charges.

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**Magnets have two poles—north and south.**

Two poles that are opposite attract each other. Two poles that are alike repel each other.

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Nuclear Forces Two forces, the strong nuclear force and the weak nuclear force, act within the nucleus to hold it together.

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Nuclear Forces The strong nuclear force overcomes the electric force of repulsion that acts among the protons in the nucleus. The weak nuclear force is involved in certain types of radioactive processes.

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Gravitational Forces Newton’s law of universal gravitation states that every object in the universe attracts every other object.

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Gravitational force is an attractive force that acts between any two masses depends upon mass and distance.

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**decreases with the square of the distance between the objects. **

Gravitational Forces Gravity Acts Over Long Distances The gravitational force between two objects is proportional to their masses. decreases with the square of the distance between the objects. Gravity is the weakest universal force, but it is the most effective force over long distances.

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**The Earth, Moon, and Tides **

Gravitational Forces The Earth, Moon, and Tides Earth’s gravitational attraction keeps the moon in a nearly circular orbit around Earth. A centripetal force is a center-directed force that continuously changes the direction of an object to make it move in a circle.

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REVIEW What are the only forces that can both attract and repel? electromagnetic forces centripetal forces strong nuclear forces gravitational forces

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REVIEW What are the only forces that can both attract and repel? electromagnetic forces centripetal forces strong nuclear forces gravitational forces

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REVIEW The nucleus of an atom is held together primarily by the strong force and weak force. strong force and gravity. weak force and electromagnetic force. electromagnetic force and strong force.

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REVIEW The nucleus of an atom is held together primarily by the strong force and weak force. strong force and gravity. weak force and electromagnetic force. electromagnetic force and strong force.

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REVIEW Which of the following statements about gravitational forces is false? They are the weakest universal forces. They act between any two objects. They become stronger as the distance between two objects increases. They become weaker as the mass of either two objects decreases.

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REVIEW Which of the following statements about gravitational forces is false? They are the weakest universal forces. They act between any two objects. They become stronger as the distance between two objects increases. They become weaker as the mass of either two objects decreases.

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REVIEW 4. A center-directed force that continuously changes the direction of an object’s motion, making it move in a circle, is called the radial force. True False

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REVIEW 4. A center-directed force that continuously changes the direction of an object’s motion, making it move in a circle, is called the radial force. True False

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