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1. Which of the following statements are true of inertia? List all that apply.
Answer: DE a. False  Inertia is not a force. b. False  Inertia is NOT a force. c. False  Inertia is NOT a force. Inertia is simply the tendency of an objects to resist a change in whatever state of motion that it currently has. Put another way, inertia is the tendency of an object to "keep on doing what it is doing." Mass is a measure of an object's inertia. The more mass which an object has, the more that it sluggish towards change. d. True  Bet money on this one. Any object with mass has inertia. (Any object without mass is not an object, but something else like a wave.) e. True  Mass is a measure of an object's inertia. Objects with greater mass have a greater inertia; objects with less mass have less inertia. f. False  The speed of an object has no impact upon the amount of inertia that it has. Inertia has to do with mass alone. g. False  Inertia (or mass) has nothing to do with gravity or lack of gravity. In a location where g is close to 0 m/s/s, an object loses its weight. Yet it still maintains the same amount of inertia as usual. It still has the same tendency to resist changes in its state of motion. h. False  Inertia is NOT the tendency to resist motion, but rather to resist changes in the state of motion. For instance, its the tendency of a moving object to keep moving at a constant velocity (or a stationary object to resist changes from its state of rest). i. False  Once more (refer to g), inertia is unaffected by alterations in the gravitational environment. An alteration in the g value effects the weight of an object but not the mass or inertia of the object. 
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2. Which of the following statements are true of the quantity mass? List all that apply.
Answer: BCFHIJ a. False  Mass is independent of the gravitational environment that an object is in and dependent solely upon the number of atoms in the object and the type of atoms (Carbon: ~12 g/mol; Hydrogen: ~1 g/mol ;Oxygen: ~16 g/mol). Because of this, mass is said to be invariable (unless of course, an object loses some of its atoms)  a constant quantity which is independent of the acceleration of gravity and therefore independent of location. (Weight on the other hand depends upon the gravitational environment.) b. True  Know this one. Kilograms is for mass and Newtons is for force. c. True  This is kind of a simple definition of mass but it does do the job (provided stuff means atoms or material). d. False  See explanation to #2d. e. False  An object has the same mass on Mount Everest as it does at sea level (or near sea level); only the weight of the object would be slightly different in these two locations. f. True  Weight Watcher's participants only use a measurement of their weight as a reflection of how many atoms of flesh that they have burned from their bodies. Their real interest is in losing mass for reasons related to health, appearance, etc. g. False  Pounds is a unit of force commonly used in the British system of measurement. It is not a metric unit and it is not a unit of mass. Kilogram is the standard metric unit of mass and slug is the British unit. h. True  Weight and force of gravity are synomynous terms. You should quickly become comfortable with the terms mass, weight and force of gravity; it will save you many headaches as we continue through the course. i. True  A less massive object has less inertia and as such would offer less resistance to changes in their velocity. For this reason, a less massive object requires less force to bring from a state of motion to a state of rest. j. True  The weight of an object is the mass of the object multiplied by the acceleration of gravity of the object. Mass and weight are mathematically related by the equation: Weight (or F_{grav}) = m•g 
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3. Which of the following statements are true of the quantity weight? List all that apply.
Answer: ABCH and possibly EF a. True  The weight of an object is equal to the force of gravity acting upon the object. It is computed by multiplying the object's mass by the acceleration of gravity (g) at the given location of the object. If the location of the object is changed, say from the Earth to the moon, then the acceleration of gravity is changed and so is the weight. It is in this sense that the weight of an object is dependent upon the acceleration of gravity. b. True  This statement is true in the sense that the weight of an object refers to a force  it is the force of gravity. c. True  The weight of an object depends upon the mass of the object and the acceleration of gravity value for the location where it is at. The acceleration of gravity on the moon is 1/6th the value of g on Earth. As such, the weight of an object on the moon would be 6 times less than that on Earth. d. False  A trip from sea level to the top of Mount Everest would result in only small alterations in the value of g and as such only small alterations in a person's weight. Such a trip might cause a person to lose a pound or two. e. Mostly True  Two objects of the same mass can weigh differently if they are located in different locations. For instance, person A and person B can both have a mass of 60 kg. But if person A is on the Earth, he will weigh ~600 N, whereas person B would weight ~100 N on the moon. f. Kinda True (Mostly False)  Weight is the product of mass and the acceleration of gravity (g). To gain weight, one must either increase their mass or increase the acceleration of gravity for the environment where they are located. So the statement is true if one disregards the word MUST which is found in the statement. g. False  By definition, a freefalling object is an object upon which the only force is gravity. Such an object is accelerating at a rate of 9.8 m/s/s (on Earth) and as such cannot be experiencing a balance of forces. h. True  This statement is the precise definition of weight. Weight is the force of gravity. i. False  This student has determined the mass of the beaker, not the weight. As such, he/she has massed the beaker, not weighed it. 
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4. Which of the following statements are true of an object that experiences balanced forces (or unbalanced forces)? List all that apply.
Answer: None of these are true, though one might make a strong argument for I. a. FALSE  An object which is moving to the right could have unbalanced forces, but only if it is accelerating. The presence of unbalanced forces must always be associated with acceleration, not mere motion. In this case, an object moving to the right could have a balance of forces if it is moving with a constant velocity. b. FALSE  An object would never slow to a stop unless the forces acting upon it were unbalanced. In fact, an object which slows down must have a unbalanced force directed in the direction opposite their motion. c. FALSE  An unbalanced force is only required to accelerate an object. A balance of forces is required to keep an object mogving at a constant velocity. For instance, a car moving to the right at constant velocity encounters as much rightward force as leftward force. d. FALSE  An object which moves in a circle has a changing direction. As such, there is an acceleration and this acceleration requires that there be an unbalanced force present on the object. e. FALSE  Any object that accelerates has a changing velocity. An object that accelerates at a constant rate has a velocity that changes by the same amount each second. For instance, a freefalling object changes its velocity by 9.8 m/s ever second. It is said to have a constant acceleration of 9.8 m/s^{2}. A freefalling object, or any object with an acceleration (whether constant or nonconstant) must be experiencing an unbalanced force. f. FALSE  Consider an object which weighs 1000 N (a 1000 N downward force) which is being pulled on by two people, each exerting 500 N of upward force. Such an object has three forces acting upon it and the three forces together balance each other. g. FALSE  A freefalling object is an object upon which the only force is gravity. As such, there is an unbalanced force acting upon it; this unbalanced force explains its acceleration. h. FALSE  Balanced forces cause stationary objects to stay at rest. However balanced forces would never cause moving objects to stop; an unbalanced force would be required to stop a moving object. i. FALSE  Unbalanced forces do more than cause objects to move; unbalanced forces cause objects to accelerate. Though one could make a strong argument that an object that is accelerating must also be moving (albeit with a changing velocity). In this sense, this statement is true. 
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5. Consider Newton's first law of motion to determine which of the following statements are true? List all that apply.
Answer: AI a. TRUE  Absolutely true. Like all true scientific laws, they govern all objects. In the case of Newton's first law of motion: An object that is nonmoving remains at rest (unless acted upon by an unbalanced force); and a moving object will continue in its motion at a constant velocity (unless acted upon by an unbalanced force). b. FALSE  A football which is moving upwards and rightwards towards its peak, then it has both an upward and a rightward velocity; it does not however have an upward and a rightward force. In fact, if acting as a projectile, it has no horizontal force and maintains a constant horizontal velocity; similarly, it would have a downward force of gravity and a slowing down motion as it rises. If the football were not a projectile, then the horizontal force would be leftward (air resistance opposing its motion) and the vertical force would be gravity and air resistance, both directed downward. c. FALSE  An unbalanced force would accelerate an object. If directed against its motion, then it would actually slow it down rather than keep is motion going. A balance of forces is all that is required to keep an object going at a constant velocity. An unbalanced force directed in the direction of motion would be required to keep an object going with an increasing speed. d. FALSE  If an object is at rest, then there are no unbalanced forces acting upon it. There is a force of gravity and at least one other upward force capable of balancing the force of gravity. e. FALSE  This is dead wrong. It would take a balance of forces to keep an object in motion at constant velocity. An unbalanced force would cause some form of acceleration. f. FALSE  If you answered TRUE, then Galileo and Newton just rolled over in their grave. It is the natural tendency of all objects to maintain their velocity and to resist changes in whatever state of motion that they have. This is the law of inertia. g. FALSE  All objects resist changes in their state of motion. In the absence of unbalanced forces, they maintain their velocity (whether zero or nonzero). The pendulum changes its state of motion due to an ubalanced force  the force of air resistance. h. FALSE  For an object to maintain a constant velocity, 0 Newtons of net force (i.e., a balance of forces) is required. i. TRUE  Unbalanced forces cause stationary objects to accelerate from rest. In the absence of an unbalanced force, a stationary object would remain at rest. 
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6. Which of the following statements are true of the concept of force? List all that apply.
Answer: AFGJ and sort of H. a. TRUE  This is a great definition of force. b. FALSE  According to Newton's third law, "one cannot touch without being touched." The force on Billie is the result of an interaction of Bubba's hands with Billie's body. That force on Billie might cause Billie to go flying, but the reaction force offers resistance to the motion of Bubba's hands and slows them down. In general, forces will always (without exception) come in pairs. c. FALSE  The force of the quarterback on the football is a contact force which can only exist during the interaction (i.e., the contact) between the quarterback's hands and the football. Once thrown, the football continues its horizontal motion due to its own inertia and its vertical motion is effected by the force of gravity. d. FALSE  Be careful if you answered true to this one. If you did, perhaps you believe in the fatal misconception that a rightward force is required to sustain a rightward motion. The sleds motion to the right can be described as a leftward accelerated motion. Such a leftward acceleration demands that there is a leftward force (despite its rightward force). This leftward force slows the rightwardmoving sled down. The hill cannot push on the sled unless the hill is in contact with the sled. e. FALSE  Forces, if unbalanced, can cause objects to accelerate (one form of moving; the other form is moving at a constant velocity). But by no means can one say that forcecs always cause objects to move. For instance, as you sit in your chair, the chair pushes up on your body but your body does not move. f. TRUE  Certainly! As you sit in your chair, the chair pushes up on your body but your body does not accelerate. This upward force (known as the normal force) is balanced by the downward force of gravity. Many objects experience a force yet do not accelerate. g. TRUE  There are two broad categories of forces  contact forces and field forces. Contact forces, by definition, are those which result from the physical contact of two forces. h. TRUE (mostly)  A field force is a force which can acts between two objects even when they are separated by a distance. Field forces have magnitudes which are dependent upon the distance of separation between the two interacting objects. For instance, the force of gravity between the Sun and the earth is a field force whose value depends upon the distance of separation between the center of the Earth and the center of the Sun. In this sense, the force of gravity is a force which acts when two objects are separated in space from each other. Yet field forces can also occur when the two objects are touching each other. In this sense, one can be skeptical of the wording of the statement. i. FALSE  Spring and tension are examples of contact forces. The spring or the rope/cable/wire are in contact with the object upon which it exerts its push or pull. The field forces are electric force, magnetic force, and gravity force. j. TRUE  Forces always have a direction associated with them. As such, force is a vector quantity  a quantity which is fully described by both a magnitude (size, value) and a direction. k. FALSE  Force is measured in Newtons in the metric system and in pounds in the British system. Kilograms is a unit of mass. 
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7. Consider Newton's second law of motion to determine which of the following statements are true? List all that apply.
Answer: ABHJKL a. TRUE  The acceleration is directly related to the net force and the direction of the acceleration is always the same as the direction of the net force. When it comes to force, objects can be thought of as being in the middle of a tugofwar between the individual forces. The force that wins the tugofwar is the force which determines the direction of the acceleration. So if a rightward force wins over a leftward force, the acceleration will be to the right. b. TRUE  An object which is slowing down has an acceleration which is directed opposite the motion of the object. So an object which moves to the right and slows down experiences a leftward acceleration and therefore a leftward net force. c. FALSE  Acceleration involves a change in velocity and velocity is a vector with a magnitude (15 m/s, 22 m/s, etc) and a direction (east, northeast, etc.). Accelerating objects are either changing the magnitude of the velocity by speeding up or slowing down or changing the direction of the velocity by turning. d. FALSE  Vice Versa. The acceleration of an object is inversely dependent upon the mass and directly dependent upon the net force. e. FALSE  Acceleration is directly dependent upon the net force. Whatever alteration is made in the net force, the same alteration must be made in the acceleration. So if the net force is increased by a factor of 2, then the acceleration is increased by a factor of 2 from 8 m/s/s to 16 m/s/s. f. FALSE  Whatever alteration is made in the net force, the same alteration must be made in the acceleration. So if the net force is increased by a factor of 3, then the acceleration is increased by a factor of 3 from 8 m/s/s to 24 m/s/s. g. FALSE  Acceleration is inversely dependent upon the mass. Whatever alteration is made in the mass, the inverse must be made of the acceleration. So if the mass is increased by a factor of 2, then the acceleration is decreased by a factor of 2 from 8 m/s/s to 4 m/s/s. h. TRUE  Acceleration is inversely dependent upon the mass. Whatever alteration is made in the mass, the inverse must be made of the acceleration. So if the mass is increased by a factor of 4, then the acceleration is decreased by a factor of 4 from 8 m/s/s to 2 m/s/s. i. FALSE  Acceleration is inversely dependent upon the mass and directly dependent upon the net force. If the net force is increased by a factor of 2, then the acceleration is increased by a factor of 2. If the mass is decreased by a factor of 2, then the acceleration is increased by a factor of 2. The overall result of the two changes is to increase acceleration by a factor of 4 from 8 m/s/s to 32 m/s/s. j. TRUE  Acceleration is inversely dependent upon the mass and directly dependent upon the net force. If the net force is increased by a factor of 2, then the acceleration is increased by a factor of 2. If the mass is decreased by a factor of 4, then the acceleration is decreased by a factor of 4. The overall result of the two changes is to decrease acceleration by a factor of 2 from 8 m/s/s to 4 m/s/s. k. TRUE  Acceleration is inversely dependent upon the mass and directly dependent upon the net force. If the net force is decreased by a factor of 2, then the acceleration is decreased by a factor of 2. If the mass is decreased by a factor of 4, then the acceleration is decreased by a factor of 4. The overall result of the two changes is to decrease acceleration by a factor of 8 from 8 m/s/s to 1 m/s/s. l. TRUE  Acceleration is inversely dependent upon the mass and directly dependent upon the net force. If the net force is increased by a factor of 4, then the acceleration is increased by a factor of 4. If the mass is increased by a factor of 2, then the acceleration is decreased by a factor of 2. The overall result of the two changes is to increase acceleration by a factor of 2 from 8 m/s/s to 16 m/s/s. m. FALSE  The net force is the product m•a. Acceleration (a) can be calculated as the velocity change per time. The velocity change is +6 m/s (from 0 m/s to 6 m/s), so the acceleration is (+6 m/s) / (3 s) = +2 m/s/s. Therefore the net force is (2 kg)•(+2 m/s/s) = +4 N. The + indicates information about the direction; the 4 N is the magnitude. n. FALSE  The net force is the product m•a. Acceleration (a) can be calculated as the velocity change per time. The velocity change is 15 m/s (from 24 m/s to 9 m/s), so the acceleration is (15 m/s) / (3 s) = 5 m/s/s. Therefore the net force is (10 kg)•(5 m/s/s) = 50 N. The  indicates information about the direction; the 50 N is the magnitude. 
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Last update: 10/11/01