Newton's Law of Motion

True/False To gain weight, one must put on more mass.

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.

Big Bubba has a mass of 100 kg on the earth. What is Big Bubba's mass on the moon where the force of gravity is approximately 1/6-th that of Earth's? ________ Explain or show your work.

100 kg- Mass is a quantity which is independent of the location of the object. So if Big Bubba has a mass of 100 kg on Earth, then he also has a mass of 100 kg on the moon. Only the weight would change as Big Bubba is moved from the Earth to the moon. He weighs ~1000 N on Earth and 1/6-th this value (~167 N) on the moon.

The amount of net force required to keep a 5-kg object moving rightward with a constant velocity of 2 m/s is ____. a. 0 N b. 0.4 N c. 2 N d. 2.5 N e. 5 N

A-Net force is always m•a. In this case, the velocity is constant so the acceleration is zero and the net force is zero. Constant velocity motion can always be associated with a zero net force.

During which time interval(s), if any, is there a net force acting upon the object? List all that apply.

ACEG - If an object has a net force upon it, then it is accelerating. Acceleration is represented by a sloped line on a velocity-time plot.

During which time interval(s), if any, are the forces acting upon the object balanced? List all that apply.

BDFH - If the forces are balanced, then an object is moving with a constant velocity. This is represented by a horizontal line on a velocity-time plot.

True/False If a football is moving upwards and rightwards towards the peak of its trajectory, then there are both rightwards and upwards forces acting upon it.

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.

True/False A free-falling object experiences a balance of forces.

False - A free-falling 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.

True/False A person could reduce their weight significantly by taking an airplane ride to the top of Mount Everest.

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.

True/False Bubba approaches Billie and gives him a swift shove. Timid little Billie keeps his hands in his pocket during this interaction. Subsequently, while Bubba places a force upon Billie, Billie does not place a force upon Bubba.

False - According to Newton's third law, one cannot push on an object without being pushed back. 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.

True/False A pendulum bob is set into its usual back-and-forth periodic motion. After some time (perhaps 10 minutes), the pendulum bob comes to a rest position. This is best explained by the idea of inertia - all objects eventually resist motion.

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 unbalanced force - the force of air resistance.

True/False An object would have more mass on Mount Everest than the same object in the middle of Lake Michigan.

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.

True/False If a person is moving to the right, then the forces acting upon it are NOT balanced.

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.

True/False If an object is moving with a constant speed in a circle, then the forces acting upon the object are balanced.

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.

True/False A balance of forces is demonstrated by an object which is slowing to a stop.

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.

True/False It would take an unbalanced force to keep an object in motion.

False - An unbalanced force is only required to accelerate an object. A balance of forces is required to keep an object moving at a constant velocity. For instance, a car moving to the right at constant velocity encounters as much rightward force as leftward force.

True/False It would take an unbalanced force to keep an object in motion.

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.

True/False If an object is accelerating at a constant rate of acceleration, then the forces acting upon the object are balanced.

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 free-falling object changes its velocity by -9.8 m/s ever second. It is said to have a constant acceleration of -9.8 m/s2. A free-falling object, or any object with an acceleration (whether constant or non-constant) must be experiencing an unbalanced force.

True/False Balanced forces cause stationary objects to remain at rest and moving objects to come to rest.

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.

True/False A sled slides down the hill and reaches the bottom where it gradually slows to a stop. Once on the level ground, the force of the hill persists upon the sled to allow it to continue its forward motion.

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 rightward-moving sled down. The hill cannot push on the sled unless the hill is in contact with the sled.

True/False The weight of an object can be measured in kilograms.

False - By definition, a free-falling 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.

True/False It is NOT possible for just three forces to be acting upon an object and they still balance each other.

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.

True/False If a 3-kg rock is thrown at a speed of 2 m/s in a gravity-free environment (presuming one could be found), then an unbalanced force of 6 N would be required to keep the rock moving at a constant speed.

False - For an object to maintain a constant velocity, 0 Newtons of net force (i.e., a balance of forces) is required.

True/False Force can be measured in kilograms or Newtons depending upon the system of measurement (metric or otherwise).

False - Force is measured in Newtons in the metric system and in pounds in the British system. Kilograms is a unit of mass.

True/False Forces always cause objects to move

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 forces 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.

True/False If an object is at rest, then there are no forces acting upon the object.

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.

True/False It is the natural tendency of all objects to eventually come to a rest position.

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.

True/False An object would not have any inertia in a gravity-free environment (if there is such a place).

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.

True/False Inertia is a force which keeps stationary objects at rest and moving objects in motion at constant velocity.

False - Inertia is NOT a force.

True/False Inertia is a force which brings all objects to a rest position.

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.

True/False Inertia is the tendency of all objects to resist motion and ultimately stop.

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).

True/False Inertia is a force.

False - Inertia is not a force.

True/False The mass of an object is dependent upon the value of the acceleration of gravity.

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.)

True/False In a gravity-free environment (should there be one), a person with a lot of inertia would have the same ability to make a turn as a person with a small amount of inertia.

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.

True/False The mass of an object can be measured in pounds.

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.

True/False Spring and tension forces are examples of field forces.

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.

True/False A quarterback throws a football down field. Once thrown, the force from the quarterback persists upon the ball to cause it to continue on its upward trajectory towards its peak.

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.

True/False Fast-moving objects have more inertia than slow-moving objects.

False - The speed of an object has no impact upon the amount of inertia that it has. Inertia has to do with mass alone.

True/False It would take an unbalanced force to keep an object in motion at a constant velocity.

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.

True/False When a chemistry student places a beaker on a balance and determines it to be 84.3 grams, they have weighed the beaker.

False - This student has determined the mass of the beaker, not the weight. As such, he/she has massed the beaker, not weighed it.

True/False Unbalanced forces cause objects to move.

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.

True/False An object which is moving rightward has a rightward force acting upon it.

False An object which is accelerating rightward must have a rightward force and a rightward net force acting upon it. But an object which is merely moving rightward does not necessarily have a rightward force upon it. A car that is moving rightward and skidding to a stop would not have a rightward force acting upon it.

True/False The mass of an object is variable and dependent upon its location.

False-Know this one. Kilograms is for mass and Newtons is for force.

True/False For an object resting upon a non-accelerating surface, the normal force is equal to the weight of the object.

False-Quite surprisingly to many, the normal force is not necessarily always equal to the weight of an object. Suppose that a person weighs 800 N and sits at rest upon a table. Then suppose another person comes along and pushes downwards upon the persons shoulders, applying a downward force of 200 N. With the additional downward force of 200 N acting upon the person, the total upward force must be 1000 N. The normal force supplies the upward force to support both the force of gravity and the applied force acting upon the person. Its value is equal to 1000 N which is not the same as the force of gravity of the person.

True/False Two objects of the same mass can weigh differently.

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.

During which time interval(s), if any, are there no forces acting upon the object? List all that apply.

None - If an object is on a surface, one can be guaranteed of at least two forces - gravity and normal force.

True/False A field force results from the action of two objects which are positioned some distance away.

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.

True/False If all other variables are equal, then it would require less exerted force to stop a less massive object than to stop a more massive object.

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.

True/False Newton's first law of motion is applicable to both moving and nonmoving objects.

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).

True/False All objects have inertia.

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.)

True/False An object can experience two or more forces and not accelerate.

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.

True/False A force is a vector quantity; there is always a direction associated with it.

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.

True/False The standard metric unit of mass is the kilogram.

True - Know this one. Kilograms is for mass and Newtons is for force.

True/False A more massive object has more inertia than a less massive object.

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.

True/False The weight of an object would be less on the Moon than on the Earth.

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 1/6-th 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.

True/False The weight of an object is dependent upon the value of the acceleration of gravity.

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.

True/False The mass of an object is mathematically related to the weight of the object.

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 Fgrav) = m•g

True/False A contact force results from the physical contact between two objects.

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.

True/False A force is a push or pull exerted upon an object which results from the interaction of that object with its environment.

True - This is a great definition of force.

True/False Mass depends on how much stuff is present in an object.

True - This is kind of a simple definition of mass but it does do the job (provided stuff means atoms or material).

True/False The weight of an object is equal to the force of gravity acting upon the object.

True - This statement is the precise definition of weight. Weight is the force of gravity.

True/False Weight refers to a force experienced by an object.

True - This statement is true in the sense that the weight of an object refers to a force - it is the force of gravity.

True/False It would take an unbalanced force to cause an object to accelerate from rest.

True - Unbalanced forces cause stationary objects to accelerate from rest. In the absence of an unbalanced force, a stationary object would remain at rest.

True/False People in Weight Watcher's are really concerned about their mass (they're mass watchers).

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.

True/False If all other variables are equal, then an object with a greater mass would have a more difficult time accelerating.

True - Weight and force of gravity are synonymous 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.

All the forces acting upon an object are balanced. (of an object that is at equilibrium?)

always true

The object has an acceleration of zero. (of an object that is at equilibrium?)

always true

The sum of all the forces is 0 N. (of an object that is at equilibrium?)

always true

There is no change in the object's velocity. (of an object that is at equilibrium?)

always true

The object is accelerating. (of an object that is at equilibrium?)

never true

The object is in free-fall. (of an object that is at equilibrium?)

never true

The object is moving in a circle at constant speed. (of an object that is at equilibrium?)

never true

There is a net force acting upon the object. (of an object that is at equilibrium?)

never true

All the forces acting upon the object are equal. (of an object that is at equilibrium?)

not always true

The object is at rest. (of an object that is at equilibrium?)

not always true

The object is moving and moving with a constant velocity. (of an object that is at equilibrium?)

not always true

All the forces acting upon the object are equal. (of an object that is at equilibrium?)

true

The object is at rest. (of an object that is at equilibrium?)

true

The object is falling and has reached its terminal velocity. (of an object that is at equilibrium?)

true

The object is moving and moving with a constant velocity. (of an object that is at equilibrium?)

true

Little Billie weighs 360 N on Earth. What is Little Billie's mass on the moon where the force of gravity is approximately 1/6-th that of Earth's? ________ Explain or show your work.

~36 kg-The mass of an object is related to weight by the equation W = m•g where g = ~10 m/s/s on Earth and one-sixth this value (~1.67) on the moon. So if Billy weighs 360 N on Earth, then his mass is approximately ~36 kg. His mass on the moon will be the same as his mass on Earth. Only his weight changes when on the moon; rather than being 360 N, it is 60 N. His weight on the moon could be found by multiplying his mass by the value of g on the moon: (36 kg) • (9.8/6 m/s/s) = ~60 N