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Physical Science Ch. 3: Forces

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Newton's 2nd Law of Motion If a net force is applied to an object, that object will accelerate in the direction of the force. Force = Mass x Acceleration (N) (kg) (m/s/s)

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F = M x A 72 = 6 x 12 F = M x A 72 = 12 x 6 F = M x A 72 = 18 x 4

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Examples of Newton's 2nd law: -Car engines -A semi (full or empty) -Pushing a kid in a swing

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Practice problem: If a motorcycle has a mass of 212 kg and the engine produces 3816 N of force, at what rate will the motorcycle accelerate?

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Weight is a measure of gravitational pull, which is a force. So if F = M x A, then weight also = M x A More specifically though, the acceleration for weight is due to gravity, so you can automatically put 9.8 m/s/s in the equation. So the equation ends up being: Weight = Mass x 9.8

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Projectiles A projectile is any object which has been shot or thrown through the air, with a 1 time force. Projectiles follow a perfectly curved path through the air (in a vacuum).

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This perfectly curved path is called a parabola.

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Is a bottle rocket a projectile?

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A projectile has both horizontal force and vertical force applied to it. One makes it want to go up and down, the other wants it to go sideways. The combination of the 2 forces creates the curved path.

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Regardless of the shape, the path followed by a projectile is called its trajectory. Golf Ball Trajectory

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In real life, projectiles are effected by air resistance (the force which air exerts on a moving object) and therefore may not follow a perfectly curved path. Like the flight of a golf ball.

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Examples of air resistance: Car aerodynamics Spoilers on racecars Buses Hand surfing

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Falling Objects A falling object will accelerate downward at 9.8 m/s/s (in a vacuum). However, in real life air resistance will gradually slow it down. Terminal velocity is the top speed reached by a falling object. At terminal velocity air resistance is equal to gravitational pull.

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Which will reach terminal velocity first, the feather or the coin? Which will have the higher terminal velocity?

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The world’s record for the highest dropped ball that was caught was 2,400 m, dropped from a hot air balloon. Based on what you know about falling objects, answer the following: What was the downward velocity of the ball when it was caught, if it fell for 16 sec. (disregard air resistance)? In real life, it wouldn’t matter if it fell for 16 sec. or 60 sec. Why?

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An object thrown into the air will deccelerate at -9.8 m/s/s on the way up, and accelerate at +9.8 m/s/s on the way down. This effects only vertical velocity, not horizontal.

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So if a projectile has the same negative acceleration on the way up as positive acceleration on the way down (-9.8 and +9.8 m/s/s), answer the following: If a baseball is hit and is in the air for 4 seconds total, how long is it moving upward? Downward? What would be it’s vertical speed when it hits the ground? What was it’s vertical speed when it was hit if it took 2 sec. to reach the top of it’s trajectory?

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A punted football has a hangtime of 4 sec. What was it's vertical velocity when it hit the ground?

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1. Larry's car has run out of gas. So while pushing it to the gas station, Larry applies a force of 845 N which causes the car to accelerate at a rate of.75 m/s/s. What was the mass of the car? 2. The car being pushed in problem #1 starts to coast down a hill on it's own and reaches a speed of 5.4 m/s. What was the momentum of the car? 3. A person throws a round projectile through the air. The person applies 59 N of force to the object, which causes it to accelerate at 7.5 m/s/s through the air. Was the object thrown most likely a bowling ball, a baseball, or a ping-pong ball? Explain. 4. If a 20 kg object is dropped out of a 10 story window, how much downward force does gravity exert on the object? 5. When the object in problem #4 reaches terminal velocity, what will be the exact force of air resistance experienced by the object? 6. A car with a mass of 1,000 kg strikes a car with a mass of 850 kg. If the first car's rate of acceleration after the collision was 1.5 m/s/s, what was the second car's rate of acceleration after the collision?

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A falling object is said to be in freefall when it is effected only by gravity. Is a paruchutist experiencing freefall?

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Circular Motion According to Newton's 1st Law, an object will move in a straight line unless a net force causes it to do otherwise. Therefore, an object moving in a curved or circular path, like a tetherball, must have a force pulling it in a circle.

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A centripetal force is a force which pulls an object toward the center of a curved or circular path. For example, the friction between the tires and the pavement will pull the car through the curve. What if the road were icy?

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Tell what is producing the curved path in each of the following examples: The Gravitron Motorcycle Cage O' Death Rock and sling Race track curves

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Other examples of centripetal force: A centrifuge Washing machine Fighter jets Hammer throw

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If the mass and/or acceleration of the object exceeds the centripetal force, then the object will not make the curve. Like swinging a 10 lb weight around on a piece of string.

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What will happen to the weight hanger as the mass is swung faster and faster? Why?

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When an object is pulled in a curved or circular path, this is called centripetal acceleration, since it is changing directions.

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A car has a mass of 1,200 kg and is accelerating at a rate of 5 m/s/s through a curve. How much centripetal force will the tires need to provide in order to get the car through the curve? If the tires produce 5,000 N of friction force, what exactly will happen to the car as is goes through the curve?

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Newton's 3rd Law of Motion For every force, there is a force equal in size, but opposite in direction. Ex.: - hitting a wall -rocket propulsion -the kick of a gun -an arrow being shot -hard hit in football -swimming -pogo stick

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F 1 = F 2 or M 1 x A 1 = M 2 x A 2 Practice problem: If Larry kicks a.5 kg ball with 90 N of force, at what rate will the ball accelerate?

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A 12 gauge shotgun fires a slug which has a mass of.15 kg and accelerates at 250 m/s/s. What is the force of the recoil (kick) of the gun when it is fired? Does the shotgun slug experience any centripetal force, and if so, what is producing that force?

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Momentum Momentum is inertia in motion. It is determined by how big an object is and how fast it is moving. p = m x v (kg x m/s) A stationary object has zero momentum. p = m x v

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Practice problem: A car has a mass of 1050 kg and has 5250 kg x m/s of momentum as it drives down the road. What is the velocity of the car?

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Is it possible for a little Volkswagon to have more momentum than a Mack truck? Explain.

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The Law of Conservation of Momentum states that the total amount of momentum of an object or group of objects does not change (unless acted on by a net force).

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Think of it like a class poker day in Physical Science. Everybody brings their money, and some of that money will change hands throughout the course of the hour. But from beginning to end, the total amount of money at the beginning of the hour will be the same as at the end of the hour.

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Examples: –Newton's Cradle –Dog pile –Jar of marbles dropping –Pool break –Car wreck

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Satellite Motion Satellites are objects in space which orbit a larger body. Man-made satellites are used for communications, weather, surveillance, positioning systems, etc.

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1. Kenny drops an object out of a window. If gravity exerts a force of 147 N on the object, what is the mass of the object? 2. A baseball player hits a high fly ball to the outfield. If the ball is in the air for a total of 6 seconds, how fast was the ball traveling downward when the outfielder caught it? 3. In problem #3, how fast was the ball traveling upward when it left the bat? Explain. 4. Two bumper cars collide with each other. The first car has a mass of 124 kg (car and driver), while the second car has a total mass of 148 kg. When the cars collide, the first is knocked backwards with a rate of acceleration of 4.77 m/s/s. At what rate of acceleration was the other car knocked backwards? 5. Which will have more momentum, a softball (.42 kg) thrown at 45 m/s, or a baseball (.35 kg) thrown at 55 m/s? Show work.

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In 1999, Krishna Gopal Shrivestava of India set an unverified world record by pulling a ship with a mass of 244,000 kg (536,800 pounds) with his teeth! His efforts can teach us about force, mass, and acceleration. 1. What would happen if the man were pulling a small rowboat instead of a large ship? 2. When he stops pulling, will the ship stop moving? Explain. 3. What direction is the ship moving? Why?

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There is no up or down in orbit. If not secured, objects simply float here and there. Planes like NASA’s KC-135, the “Vomit Comet,” are able to briefly simulate the conditions of being in orbit. 1.On the plane, how can you tell up from down? 2. If a scale floated by and an astronaut stood on it, what would the scale say?

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It takes more than one firefighter to aim a fire hose. The force of the water shooting out of the nozzle causes a reaction that can be difficult to control. 1.Describe what happens if you step from a canoe or small boat onto a dock. How is this related to the fire hose? 2. Does a garden hose need more than one person to hold it? What’s the difference between a garden hose and a fire hose? 3. What would happen if the firefighters dropped the hose?

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The dart is attached to the hoop by a string fed through an i-screw. So when the dart is thrown at the hoop, the string will slacken and the hoop will drop downward as the dart flies towards it. Where should you aim in order to make the dart go through the middle of the hoop as it is falling?

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Which one do you think moved further, him forward or the canoe backwards, and why? Eric (mass=70 kg) was canoeing at the lake and decided to take a swim. When he dove off the front of the canoe, the canoe moved backward at 4.5 m/s/s. If the canoe had a mass of 55 kg, at what rate did Eric accelerate forward when he dove off the canoe?

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Jim and Andy both threw a baseball through the air. Jim’s baseball landed 180 ft. away, while Andy’s landed 110 ft. away. Which ball was in the air for a longer period of time? What are the forces which cause the baseballs to follow a curved path? How are these forces related?

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A mortar is fired at a target and it takes the shell 10 seconds to hit the target. (assume no effect of air resistance) What was the shells vertical velocity when it left the mortar? If the shell travelled 800 m, what was it’s horizontal velocity?

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Four objects are dropped out of a window: a bowling ball, a feather, an iron weight, and a cat. Which one will reach terminal velocity first?

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A dump truck (4,000 kg) is carrying a load of gravel (6,000 kg) A.How much momentum does it have if it’s traveling 60 mi/hr (26.7 m/s) B. What are 2 ways that the momentum figured in A could be decreased by ½?

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A parachutist jumps from a plane, and due to air resistance she accelerates downward at ½ the normal rate. If her terminal velocity is 78.4 mi/hr, how long will it take for her to reach that speed? What, in general would happen to her terminal velocity if she went head first instead of spread-eagle?

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A sky-diving instructor is taking a tandem jump with one of his students. What effect, if any, would this have on the instructors terminal velocity if he had another person strapped to his back? Hmmmm………

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