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Does the scale read: Ch 5-4 N, N, or 3. Zero?

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**Does the scale read: 100 N, 2. 200 N, or 3. Zero? Ch 5-4 Answer: 1**

Although the net force on the whole system is zero (as evidenced by no acceleration), the scale reading is 100 N, the tension in the string. Note that the string tension is 100 N in all the positions shown. 100 N, N, or 3. Zero?

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**3. Neither. The force is the same.**

Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greater force exerted on the rope is by 1. Arnold. 2. Suzie. 3. Neither. The force is the same. Ch 5-5

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**1. Arnold. 2. Suzie. 3. Neither. The force is the same.**

Arnold Strongman and Suzie Small pull on opposite ends of a rope in a tug of war. The greater force exerted on the rope is by 1. Arnold. 2. Suzie. 3. Neither. The force is the same. Ch 5-5 Answer: 3 Arnold can pull no harder on the rope than Suzie. Rope tension is the same all along the rope, including the ends. Just as a wheel on ice can exert no more force on the ice than the ice exerts on the wheel, and just as one cannot punch an empty paper bag with any more force than the bag can exert on the puncher, Arnold can exert no more force on his end of the rope than Suzie exerts on her end. Arnold can push harder on the ground than Suzie can, so even though the pulls on the rope are the same, Arnold will likely win the tug of war!

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Two identical rubber bands connect masses A and B to a string over a frictionless pulley of negligible mass. The amount of stretch is greater in the band that connects 1. A. 2. B. 3. Both the same. Ch 5-7 Thanks to Pablo Robinson.

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Two identical rubber bands connect masses A and B to a string over a frictionless pulley of negligible mass. The amount of stretch is greater in the band that connects 1. A. 2. B. 3. Both the same. Ch 5-7 Thanks to Pablo Robinson. Answer: 3 The tension that stretches the rubber bands is the same as the tension in the string—same at both ends, in accord with Newton’s third law.

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Suppose a cannon is propped against a massive tree to reduce recoil when it fires. Then the range of the cannonball will be 1. increased. 2. decreased. 3. unchanged. Ch 5-8 Thanks to David Vasquez.

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**1. increased. 2. decreased. 3. unchanged.**

Suppose a cannon is propped against a massive tree to reduce recoil when it fires. Then the range of the cannonball will be 1. increased. 2. decreased. 3. unchanged. Ch 5-8 Thanks to David Vasquez. Answer: 1. Its range is increased. To understand why, think energy conservation. Most of the potential energy of the gunpowder is converted into kinetic energy when the gunpowder fires. That’s both kinetic energy of the cannonball and kinetic energy of the recoiling cannon. Because the tree reduces recoil, the cannonball gets a greater share of kinetic energy—hence its increased range.

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**Which boat takes the shortest path to the opposite shore?**

Motorboats cross a river pointing in the three directions shown. The boats all have the same speed relative to the water, and all experience the same water flow. Which boat takes the shortest path to the opposite shore? Ch 5-10 1. a b c

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**Which boat takes the shortest path to the opposite shore?**

Motorboats cross a river pointing in the three directions shown. The boats all have the same speed relative to the water, and all experience the same water flow. Which boat takes the shortest path to the opposite shore? Ch 5-10 Answer: 1 The shortest path to the opposite shore occurs for Boat a, which is indicated by the resultant velocity vector. The boat moves directly across the river, perpendicular to the flow. 1. a b c

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**Which boat reaches the opposite shore first?**

Motorboats cross a river pointing in the three directions shown. The boats all have the same speed relative to the water, and all experience the same water flow. Which boat reaches the opposite shore first? Ch 5-10 1. a b c

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**Which boat reaches the opposite shore first?**

Motorboats cross a river pointing in the three directions shown. The boats all have the same speed relative to the water, and all experience the same water flow. Which boat reaches the opposite shore first? Ch 5-10 Answer: 2 Time-wise, the opposite shore is reached first by Boat b, since the velocity provided by the motor is directly across the river. 1. a b c

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**Which boat provides the fastest ride?**

Motorboats cross a river pointing in the three directions shown. The boats all have the same speed relative to the water, and all experience the same water flow. Which boat provides the fastest ride? Ch 5-10 1. a b c

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**Which boat provides the fastest ride?**

Motorboats cross a river pointing in the three directions shown. The boats all have the same speed relative to the water, and all experience the same water flow. Which boat provides the fastest ride? Ch 5-10 Answer: 3 The fastest ride is on Boat c, as seen by the greatest resultant velocity vector. 1. a b c

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**3. 50/50 chance of either side breaking**

Nellie Newton hangs motionless by one hand from a clothesline as shown—which is on the verge of breaking. Which side of the line is most likely to break? 1. Left side 2. Right side 3. 50/50 chance of either side breaking 5-11

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**1. Left side 2. Right side 3. 50/50 chance of either side breaking**

Nellie Newton hangs motionless by one hand from a clothesline as shown—which is on the verge of breaking. Which side of the line is most likely to break? 1. Left side 2. Right side 3. 50/50 chance of either side breaking 5-11 Answer: 2 Nellie hangs motionless, which means all the forces acting on her equal zero: The force due to gravity acting downward, her weight, is shown by the bold vector in A. Equilibrium dictates an equal force upward, supplied by the ropes, indicated by the dashed vector. This dashed vector has to be the resultant of tensions in the left and right sides of the rope. Their relative sizes are found by constructing a parallelogram, with the dashed vector as its diagonal (B). Aha, the relative magnitudes of these tensions are shown in C. The right side is under greater tension, and therefore is the most likely to break.

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**She holds the book stationary against the wall as shown**

She holds the book stationary against the wall as shown. Friction on the book by the wall acts 1. upward. 2. downward. 3. can’t say. Ch 5-12 Thanks to Arnold Arons.

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**1. upward. 2. downward. 3. can’t say.**

She holds the book stationary against the wall as shown. Friction on the book by the wall acts 1. upward. 2. downward. 3. can’t say. Ch 5-12 Thanks to Arnold Arons. Answer: 3 Why? If she barely pushes the book so that the vertical component of her push is less than the book’s weight, then friction acts upward to keep the book stationary. If she pushes so that the vertical component of her push equals the book’s weight, then there’s zero wall friction on the book. If she pushes harder so that the vertical component of her push exceeds the book’s weight, then friction acts downward. So unless we know how the vertical component of her push compares with the weight of the book, we can’t specify the direction of friction between the book and the wall.

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