Physics Mid Term

Object A has a mass of 1 kg and a speed of 2 m/s. Object B has a mass of 2 kg and a speed of 1 m/s. Objects A and B have the same kinetic energy.

j. FALSE - When it comes to kinetic energy, speed is doubly important (recall v2). So in this case, object A would have more kinetic energy. Doing the calculation yields 2 J for object A and 1 J for object B.

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

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 Fgrav) = m•g

Position-time graphs cannot be used to represent the motion of objects with accelerated motion.

FALSE - Position-time graphs represent accelerated motion by curved lines.

Moving objects cannot have potential energy

a. FALSE - Potential energy has nothing to do with speed; an object could be moving at an elevated position. It is this elevation above zero level which gives an object potential energy.

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

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.

An accelerated object's motion will be represented by a curved line on a velocity-time graph.

c. FALSE - An object which has an acceleration will be represented by an line that has a slope. It may or may not curve, but it must have a slope other than zero.

A ball is thrown upwards and is rising towards its peak. As it rises upwards, it is NOT considered to be in a state of free fall.

c. FALSE - Any object - whether rising, falling or moving horizontally and vertically simultaneously - can be in a state of free fall if the only force acting upon it is the force of gravity. Such objects are known as projectiles and often begin their motion while rising upwards.

If an object is on the ground, then it does not have any kinetic energy.

c. FALSE - If an object is on the ground, then it does not have potential energy (relative to the ground).

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

c. 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.

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

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.

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

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.

The mass of an object can be measured in pounds.

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.

Kinetic energy is a scalar quantity.

g. TRUE - Kinetic energy does not have a direction associated with it; it is a scalar quantity.

A very massive object will free fall at the same rate of acceleration as a less massive object.

g. TRUE - The acceleration of free-falling objects (referred to as the acceleration of gravity) is independent of mass. On Earth, the value is 9.8 m/s/s (the direction is down). All objects - very massive and less massive - experience this acceleration value.

A contact force results from the physical contact between two objects.

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.

If an object changes its direction, then the line on the velocity-time graph will have a changing slope.

h. FALSE - An object which changes its direction will be represented by a line on a v-t graph that crosses over the time-axis from the + velocity region into the - velocity region.

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

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.

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.

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

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

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).

An object has a kinetic energy of 40 J. If its mass were twice as much, then its kinetic energy would be 80 J.

h. TRUE - Kinetic energy is directly related to the mass of an object.

The unit of measurement for potential energy is the Joule.

h. TRUE - The Joule (abbrev. J) is the standard metric unit of energy - all forms of energy.

The area on a velocity -time graph is representative of the change in position of the object.

b. TRUE - This is equally important. The area is the displacement.

The standard metric unit of mass is the kilogram.

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

Inertia is a force.

a. False - Inertia is not a force.

An object that is moving upwards and slowing down has an upwards acceleration.

FALSE If an object is slowing down, then the acceleration vector is directed opposite the direction of the motion; in this case the acceleration is directed downwards.

An object with a negative acceleration will be represented on a position-time graph by a line which curves downwards.

FALSE - (Once more, there is confusing wording here since we might not all agree on what "curving downwards" means.) A line that slopes downwards and has a curve (perhaps you call that "curving downwards " as I do) has a negative velocity (due to its negative slope). If the curve is "concave up" (you might say leveling off to a horizontal as time progresses) then the object is slowing down and the acceleration is positive.

An object with a positive acceleration will be represented on a position-time graph by a line which curves upwards.

FALSE - (This is confusing wording here since we might not all agree on what "curving up" means.) A line that slopes upward and has a curve (perhaps you call that "curving up" as I do) has a positive velocity (due to its positive slope). If the curve is "concave down" (you might say leveling off to a horizontal as time progresses) then the object is slowing down and the acceleration is negative.

If an object is at rest, then the position-time graph will be a horizontal line located on the time-axis.

FALSE - Not necessarily true. If the object is at rest, then the line on a p-t graph will indeed be horizontal. However, it will not necessarily be located upon the time axis.

The slope on a position-time graph is representative of the acceleration of the object.

FALSE - The slope of a position-time graph is the velocity of the object. Some things in this unit are critical things to remember and internalize; this is one of them.

Accelerating objects must be changing their speed

False Accelerating objects could be changing their speed; but it is also possible that an accelerating object is only changing its direction while maintaining a constant speed. The race car drivers at Indy might fit into this category (at least for certain periods of the race).

Both speed and velocity refer to how fast an object is moving

False Speed refers to how fast an object is moving; but velocity refers to the rate at which one's motion puts an object away from its original position. A person can move very fast (and thus have a large speed); but if every other step leads in opposite directions, then that person would not have a large velocity.

The direction of the velocity vector is dependent upon two factors: the direction the object is moving and whether the object is speeding yup or slowing down

False The direction of the velocity vector depends only upon the direction that the object is moving. A westward moving object has a westward velocity.

A vector is a large quantity and a scalar is a small quantity

False Vector = magnitude and direction Scalar = no direction

A scalar quantity depends only upon the initial and final values of the quantity; this is not the case for vector quantities.

False Vectors are the types of quantities which depend only upon initial and final state of the object. For instance, the vector quantity displacement depends only upon the starting and final location.

An object that is accelerating will eventually (if given enough time) be moving fast.

False If the accelerating object is slowing down, then it will eventually stop and not reach a fast speed. And if that doesn't convince you, then consider an object that is accelerating by moving in a circle at constant speed forever; it will accelerate the entire time but never being going any faster than at the beginning.

Weight refers to a force experienced by an object.

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.

Accelerated objects are represented on position-time graphs by curved lines.

TRUE - Accelerating objects (if the acceleration is attributable to a speed change) are represented by lines with changing slope - i.e., curved lines.

The value of g on Earth is approximately 9.8 m/s2.

TRUE

A straight, diagonal line on a position-time graph is representative of an object with a constant velocity.

TRUE - A straight diagonal line is a line of constant slope. And if the slope is constant, then so is the velocity.

An object with a negative velocity will be represented on a position-time graph by a line with a negative slope.

TRUE - Since slope on a p-t graph represents the velocity, a negative slope will represent a negative velocity.

An object with a positive velocity will be represented on a position-time graph by a line with a positive slope.

TRUE - Since slope on a p-t graph represents the velocity, a positive slope will represent a positive velocity.

10. Which one of the following is NOT consistent with a car which is accelerating?

c. A car is moving with a high speed.

Potential energy is the energy stored in an object due to its position

True

The average velocity of an object on a round-trip journey would be zero

True

The velocity of an object refers to the rate at which the object's position changes

True

Velocity is a vector quantity and speed is a scalar quantity

True

The direction of the acceleration vector is dependent upon two factors; the direction the object is moving and whether the object is speeding up or slowing down

True This is the case and something important to remember. Consider its application in the last three parts of this question.

Accelerating objects must be changing their velocity

True Accelerating object MUST be changing their velocity -either the magnitude or the direction of the velocity.

An object which is slowing down has an acceleration.

True Accelerating objects are either slowing down, speeding up or changing directions.

Acceleration is a vector quantity

True Acceleration is direction-conscious

Acceleration is the rate at which the velocity changes.

True This is the very definition of acceleration. Know this one - its the beginning point of all our thoughts about acceleration.

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

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.)

The slope on a velocity-time graph is representative of the acceleration of the object.

a. TRUE - Now this is important! It is the beginning point of much of our discussion of velocity-time graphs. The slope equals the acceleration.

An object that is free-falling is acted upon by the force of gravity alone.

a. TRUE - Yes! This is the definition of free fall.

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

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).

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

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.

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

a. True - This is a great definition of force.

A falling skydiver which has reached terminal velocity is considered to be in a state of free fall.

b. FALSE - Skydivers which are falling at terminal velocity are acted upon by large amounts of air resistance. They are experiencing more forces than the force of gravity. As such, they are NOT free-falling.

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.

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.

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.

b. 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.

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

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.

If an object is at rest, then it does not have any kinetic energy.

b. TRUE - Kinetic energy depends upon speed. If there is no speed (the object is at rest), then there is no kinetic energy.

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.

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.

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

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 1/6-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.

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

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

8. If an object has an acceleration of 0 m/s2, then one can be sure that the object is not ____.

c. changing velocity

11. A fullback is running down the football field in a straight line. He starts at the 0-yard line at 0 seconds. At 1 second, he is on the 10-yard line; at 2 seconds, he is on the 20-yard line; at 3 seconds, he is on the 30-yard line; and at 4 seconds, he is on the 40-yard line. This is evidence that

c. he is moving with a constant speed (on average).

Objects with positive acceleration will be represented by upwardly-curved lines on a velocity-time graph.

d. FALSE - An object with positive acceleration will have an positive or upward slope on a v-t graph. It does not have to be a curved line. A curved line indicates an object that is accelerating at a changing rate of acceleration.

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

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.

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

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.

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.

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

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

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.

An object in free fall experiences an acceleration which is independent of the mass of the object.

d. TRUE - The unique feature of free-falling objects is that the mass of the object does not effect the trajectory characteristics. The acceleration, velocity, displacement, etc. is independent of the mass of the object.

All objects have inertia.

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.)

9. If car A passes car B, then car A must be ____.

d. moving faster than car B, but not necessarily accelerating.

A ball is thrown upwards, rises to its peak and eventually falls back to the original height. As the ball rises, its acceleration is upwards; as it falls, its acceleration is downwards.

e. FALSE - The acceleration of all free-falling objects is directed downwards. A rising object slows down due to the downward gravity force. An upward-moving object which is slowing down is said to have a downwards acceleration.

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

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.

The weight of an object can be measured in kilograms.

g. 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.

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

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 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.

Forces always cause objects to move.

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 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.

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

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.

Two objects of the same mass can weigh differently.

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.

If an object is at rest, then the velocity-time graph will be a line with zero slope.

e. TRUE - An object that is at rest has a 0 velocity and maintains that zero velocity. The permanence of its velocity (not the fact that it is zero) gives the object a zero acceleration. and as such, the line on a v-t graph would have a slope of 0 (i.e., be horizontal).

If the mass of an elevated object is doubled, then its gravitational potential energy will be doubled as well

e. TRUE - The equation states that PEgrav = m•g•h; if the h is doubled, then the PE will be doubled as well.

A more massive object has more inertia than a less massive object.

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.

A line with zero slope on a velocity-time graph will be representative of an object which is at rest.

f. FALSE - A line with zero slope is representative of an object with an acceleration of 0. It could be at rest or it could be moving at a constant velocity.

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

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.

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

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.

Fast-moving objects have more inertia than slow-moving objects.

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.

To gain weight, one must put on more mass.

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.

Gravitational potential energy is lost as objects free-fall to the ground

f. TRUE - As objects free-fall, the height (h) decreases; subsequently, the PE decreases.

A ball is thrown upwards, rises to its peak and eventually falls back to the original height. The speed at which it is launched equals the speed at which it lands. (Assume negligible air resistance.)

f. TRUE - If the object is truly in free-fall, then the speed of the object will be the same at all heights - whether its on the upward portion of its trajectory or the downwards portion of its trajectory. For more information, see the Projectiles page at The Physics Classroom.

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

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.

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

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.

A line with a negative slope on a velocity-time graph is representative of an object with negative velocity.

g. FALSE - A negative slope indicates a negative acceleration. The object could be moving in the positive direction and slowing down (a negative acceleration).

A free-falling object experiences a balance of forces.

g. 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.

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.

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 unbalanced force - the force of air resistance.

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

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.

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

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

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

h. 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.

An object which is slowing down is represented by a line on a velocity-time graph which is moving in the downward direction.

i. FALSE - An object which is slowing down has a velocity which is approaching 0 m/s. And as such, on a v-t graph, the line must be approaching the v=0 m/s axis.

The symbol g stands for the force of gravity.

i. FALSE - Nope. A careful physics teacher will never call g the force of gravity. g is known as the acceleration of gravity. It might be best to call it the acceleration caused by gravity. When it comes to the force of gravity, we have yet another symbol for that - Fgrav. But that's a topic to be discussed in a later unit.

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.

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.

Spring and tension forces are examples of field forces.

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.

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

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.

Unbalanced forces cause objects to move.

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.

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.

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.

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

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.

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

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.

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

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