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**Combining Forces - The Nature of Force**

The combination of all forces acting on an object is called the net force.

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**Unbalanced Forces - The Nature of Force**

Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.

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**Balanced Forces - The Nature of Force**

Balanced forces acting on an object do not change the object’s motion.

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**Asking Questions - The Nature of Force**

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. Question Answer What is a force? A force is a push or pull. What happens when forces combine? Forces combine to produce a net force.

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**Gravity - Friction and Gravity**

Two factors affect the gravitational attraction between objects: mass and distance.

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**Gravity - Friction and Gravity**

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

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- Friction and Gravity

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**Free Fall - Friction and Gravity**

Use the graph to answer the following questions.

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**Free Fall - Friction and Gravity Interpreting Graphs:**

What variable is on the horizontal axis? The vertical axis? Time is on the horizontal axis, and speed is on the vertical axis.

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**Free Fall - Friction and Gravity Calculating:**

Calculate the slope of the graph. What does the slope tell you about the object’s motion? The slope is 9.8. The speed increases by 9.8 m/s each second.

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**Air Resistance - Friction and Gravity**

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

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**Comparing and Contrasting**

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

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**Calculating Force F = ma F = 55 kg x 2 m/s2 = 110 N**

- Newton’s First and Second Laws Calculating Force A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. 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/s2 F = ma F = 55 kg x 2 m/s2 = 110 N

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**Calculating Force Practice Problem - Newton’s First and Second Laws**

What is the net force on a 1,000-kg object accelerating at 3 m/s2? 3,000 N (1,000 kg X 3 m/s2)

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Newton’s First Law If no forces are exerted on an object, the object continues in its original state of motion Newton’s Second Law The acceleration of an object is proportional to the force acting on it and inversely proportional to the mass of the object F = ma What if a=0 ? Newton’s Third Law If two objects interact they exert equal forces on each other in opposite directions

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**Newton’s First and Second Laws**

Newton’s First Law of Motion Inertia Inertia Depends on Mass The Second Law of Motion Changes in Force and Mass

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**Calculating Momentum - Newton’s Third Law**

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

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**Momentum = Mass X Velocity Perform the calculation. **

- Newton’s Third Law Calculating Momentum Momentum = Mass X Velocity Perform the calculation. Smaller sledgehammer = 3.0 kg X 1.5 m/s = 4.5 kg•m/s Larger sledgehammer = 4.0 kg X 0.9 m/s = 3.6 kg•m/s

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**Calculating Momentum Practice Problem - Newton’s Third Law**

A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is 0.14 kg. Which has the greater momentum? Golf ball: kg X 16 m/s = 0.72 kg•m/s Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s The baseball has greater momentum.

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**Conservation of Momentum**

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

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**What Is a Satellite? - Rockets and Satellites**

A projectile follows a curved path because the horizontal and vertical motions combine.

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**What Is a Satellite? - Rockets and Satellites**

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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**What Is a Satellite? - Rockets and Satellites**

Depending on their uses, artificial satellites orbit at different heights.

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**Graphic Organizer Type of Friction Occurs When Example Static Sliding**

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

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