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Forces The Nature of Force Friction and Gravity Newton’s First and Second Laws Newton’s Third Law Rockets and Satellites Table of Contents

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Forces Force A force is simply a push or a pull. Isaac Newton accurately described the force of all moving objects in 1687 when he published his thesis. We still use these laws as the basis of all physics to this day.

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Forces Units of Force When describing the strength of a force we use the SI unit of Newtons (N) The newton is the SI unit for force; it is equal to the amount of net force required to accelerate a mass of one kilogram at a rate of one meter per second squared. F = ma: or multiplying mass (kg) by acceleration (m/s 2 ) Therefore:1 N = 1 kg·m/s 2

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Forces Real World Examples of the Newton 1 N is the force of Earth's gravity on a mass of about 102 g or roughly the mass of a small apple On Earth's surface, a mass of 1 kg exerts a force of approximately 9.8 N [down] (or 1.0 kilogram-force; 1 kgf=9.80665 N by definition) The force of Earth's gravity on a human being with a mass of 70 kg is approximately 686 N It is common to see forces expressed in kilonewtons or kN (1 kN = 1,000 N)

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Forces - The Nature of Force Combining Forces The net force determines whether an object moves and also the direction it will move! The combination of all forces acting on an object is called the net force.

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Forces - The Nature of Force Unbalanced Forces Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.

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Forces - The Nature of Force Balanced Forces Balanced forces acting on an object do not change the object’s motion.

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Forces Asking Questions Before you read, preview the red headings. In a graphic organizer like the one below, ask a what or how question for each heading. As you read, write answers to your questions. What is a force? A force is a push or pull. What happens when forces combine? Forces combine to produce a net force. Question Answer - The Nature of Force

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Forces End of Section: The Nature of Force

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Forces Friction Friction is the force that is exerted on two surfaces as they rub against one another. The strength of the friction force depends on two things: 1.How hard the surfaces are pushed together 2.What material the surface is made of

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Forces Types of Friction 1.Static friction acts on objects that are at rest 2.Sliding friction occurs when two solid surfaces slide past one another 3.Rolling friction occurs when an object rolls across a surface 4.Fluid friction occurs when a solid object moves through a fluid

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Forces What type of friction is occuring?

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Forces What type of friction is occurring?

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Forces Compare and Contrast

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Forces What type of friction is occurring?

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Forces What type of friction is occurring?

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Forces How were the Egyptian Pyramids Built?

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Forces Gravity Gravity is a force that pulls objects toward one another Gravity keeps objects in place unless they are acted on by an outside force! Sir Isaac Newton accurately described this in his law of universal gravitation Gravity acts on all objects regardless of their size and or location in the universe This means that all objects are attracted to one another but there are some rules involved in how much attraction occurs This is mostly due to differences in mass What is the difference between mass and weight? Mass is the amount of matter in an object. Weight is the measure of gravitational force on an object.

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Forces - Friction and Gravity Gravity Two factors affect the gravitational attraction between objects: mass and distance.

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Forces - Friction and Gravity Gravity The force of gravity on a person or object at the surface of a planet is known as weight. Can you find anything wrong with this picture?

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Forces Gravity and Motion An object is said to be in free fall when gravity is the only force acting on it. This results in an unbalanced force and causes the object to accelerate in a downward motion at 9.8m/s This acceleration is not dependent on mass! Air resistance is fluid friction that acts in the opposite direction to motion Air resistance is not the same for all objects! Terminal velocity occurs when the force of air resistance equals the weight of the object. *Therefore the object will continue to fall with a constant velocity but with no acceleration!

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Forces Free Fall Use the graph to answer the following questions. - Friction and Gravity

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Forces Free Fall Time is on the horizontal axis, and speed is on the vertical axis. Interpreting Graphs: What variable is on the horizontal axis? The vertical axis? - Friction and Gravity

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Forces Free Fall The slope is 9.8. The speed increases by 9.8 m/s each second. Calculating: Calculate the slope of the graph. What does the slope tell you about the object’s motion? - Friction and Gravity

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Forces Free Fall 58.8 m/s Predicting: What will the speed of the object be at 6 seconds? - Friction and Gravity

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Forces Free Fall The speed values would not change. Drawing Conclusions: Suppose another object of the same size but with a greater mass was dropped instead. How would the speed values change? - Friction and Gravity

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Forces - Friction and Gravity Air Resistance Falling objects with a greater surface area experience more air resistance!

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Forces Projectile Motion A projectile is any object projected into space by the exertion of a force. Horizontal velocity does not affect how fast an object will fall! Therefore, both the red and yellow ball will hit the ground at the same time On our planet, all objects are accelerated by the force of gravity (9.8m/s) towards the center of the earth.

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Forces What is going on in this diagram?

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Forces FrictionGravity Comparing and Contrasting As you read, compare and contrast friction and gravity by completing a table like the one below. Effect on motion Opposes motion Pulls objects toward one another Depends on Types of surfaces involved, how hard the surfaces push together Mass and distance Measured inNewtons - Friction and Gravity

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Forces End of Section: Friction and Gravity

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Forces Newton’s First Law of Motion An object at rest will remain at rest & An object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force *Gravity and friction are both unbalanced forces that work together to change an object’s motion

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Forces Inertia Newton’s first law is also called the Law of Inertia Inertia refers to an object’s resistance to change in motion Inertia depends on mass! The greater the mass of an object, the greater its inertia and the greater the force required to change the object’s motion

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Forces Inertia

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Forces Inertia

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Forces Newton’s Second Law of Motion Second law: The acceleration (a) of an object is directly proportional to the net force (F) and inversely proportional to the mass (m) F = maorAcceleration = Net Force/Mass Recall acceleration is measured in meters per second per second & Mass is measured in Kilograms *Force is measured in Newtons (N) or Kg x m/s 2

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Forces Describe the following picture

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Forces Compare and Contrast

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Forces How is this picture different?

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Forces Changes in Force and Mass F = maorAcceleration = Net Force/Mass You can increase the acceleration of an object by changing the force or mass of the object If you increase the force you will increase the acceleration If you decrease the force you will decrease the acceleration *Force and acceleration are directly proportional! If you increase the mass then acceleration will decrease if the force remains constant If you decrease the mass then acceleration will increase if the force remains constant *Acceleration and mass are inversely proportional!

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Forces Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the net force that causes this acceleration. Read and Understand What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s 2 - Newton’s First and Second Laws

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Forces Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the net force that causes this acceleration. Plan and Solve What quantity are you trying to calculate? The net force (F net ) = __ What formula contains the given quantities and the unknown quantity? a = F net /m or F net = m X a Perform the calculation. F net = m X a = 55 kg X 2.0 m/s 2 F = 110 kg m/s 2 F = 110 N - Newton’s First and Second Laws

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Forces Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s 2. Calculate the net force that causes this acceleration. Look Back and Check Does your answer make sense? A net force of 110 N is required to accelerate the water-skier. This may not seem like enough force, but it does not include the force of the speedboat's pull that overcomes friction. - Newton’s First and Second Laws

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Forces Calculating Force Practice Problem What is the net force on a 1,000-kg object accelerating at 3 m/s 2 ? 3,000 N (1,000 kg X 3 m/s 2 ) - Newton’s First and Second Laws

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Forces Calculating Force Practice Problem What net force is needed to accelerate a 25-kg cart at 14 m/s 2 ? 350 N (25 kg X 14 m/s 2 ) - Newton’s First and Second Laws

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Forces Outlining As you read, make an outline about Newton’s first and second laws. Use the red headings for the main topics and the blue headings for the subtopics. Newton’s First and Second Laws I.Newton’s First Law of Motion A.Inertia B.Inertia Depends on Mass II.The Second Law of Motion A.Changes in Force and Mass - Newton’s First and Second Laws

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Forces End of Section: Newton’s First and Second Laws

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Forces Calculating Momentum 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? Read and Understand What information have you been given? Mass of smaller sledgehammer = 3.0 kg Velocity of smaller sledgehammer = 1.5 m/s Mass of larger sledgehammer = 4.0 kg Velocity of larger sledgehammer = 0.9 m/s - Newton’s Third Law

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Forces Calculating Momentum 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? Plan and Solve What quantities are you trying to calculate? The momentum of each sledgehammer = __ What formula contains the given quantities and the unknown quantity? Momentum = Mass X Velocity Perform the calculation. Smaller sledgehammer = 3.0 km X 1.5 m/s = 4.5 kgm/s Smaller sledgehammer = 4.0 km X 0.9 m/s = 3.6 kgm/s - Newton’s Third Law

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Forces Calculating Momentum 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? Look Back and Check Does your answer make sense? The 3.0-kg hammer has more momentum than the 4.0-kg one. This answer makes sense because the 3.0-kg hammer is swung at a greater velocity. - Newton’s Third Law

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Forces Calculating Momentum Practice Problem 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? 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. - Newton’s Third Law

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Forces Calculating Momentum Practice Problem What is the momentum of a bird with a mass of 0.018 kg flying at 15 m/s? 0.27 kgm/s (0.018 kg X 15 m/s = 0.27 kgm/s) - Newton’s Third Law

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Forces - Conservation of Momentum In the absence of friction, momentum is conserved when two train cars collide. - Newton’s Third Law

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Forces Momentum Activity Click the Active Art button to open a browser window and access Active Art about momentum. - Newton’s Third Law

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Forces Previewing Visuals Before you read, preview Figure 18. Then write two questions that you have about the diagram in a graphic organizer like the one below. As you read, answer your questions. Conservation of Momentum Q. What happens when two moving objects collide? A. In the absence of friction, the total momentum is the same before and after the collision. Q. What is the momentum of an object? A. Its mass multiplied by its velocity - Newton’s Third Law

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Forces End of Section: Newton’s Third Law

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Forces - Rockets and Satellites What Is a Satellite? A projectile follows a curved path because the horizontal and vertical motions combine.

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Forces - Rockets and Satellites What Is a Satellite? The faster a projectile is thrown, the father it travels before it hits the ground. A projectile with enough velocity moves in a circular orbit.

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Forces - Rockets and Satellites What Is a Satellite? Depending on their uses, artificial satellites orbit at different heights.

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Forces Main Idea Detail Identifying Main Ideas As you read the section “What Is a Satellite?” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea. A satellite stays in orbit due to… its inertiaEarth’s gravityEarth’s shape - Rockets and Satellites

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Forces End of Section: Rockets and Satellites

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Forces Graphic Organizer Friction between an unmoving book and desk Type of Friction Static Occurs WhenExample Sliding Rolling Fluid An object is not moving Two solid surfaces slide over each other Rubber pads on a bicycle’s brakes An object rolls across a surface Ball bearings in skateboard wheels A solid object moves through a fluid Air resistance

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Forces End of Section: Graphic Organizer

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