Physics Conceptual Questions (Exam #1 Practice)
If B is added to A, under what conditions does the resultant vector have a magnitude equal to A + B?
A and B are in the same direction.
The height of a horse is sometimes given in units of "hands." Why is this a poor standard of length?
Palm length varies from person to person, so this unit of measurement isn't very good.
A ball rolls in a straight line along the horizontal direction. Using motion diagrams (or multiflash photographs), describe the velocity and acceleration of the ball for each of the following situations.
Starting at the right-most image, the images will be getting farther apart as one moves toward the left.
A ball rolls in a straight line along the horizontal direction. Using motion diagrams (or multiflash photographs), describe the velocity and acceleration of the ball for each of the following situations.
Successive images on the film will be separated by a constant distance if the ball has constant velocity
A ball is thrown vertically upward. (b) What is the acceleration of the ball just before it hits the ground?
The acceleration of the ball remains constant in magnitude and direction throughout the ball's free flight, from the instant it leaves the hand until the instant just before it strikes the ground. The acceleration is directed downward and has a magnitude equal to the free-fall acceleration g.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (+) Acceleration (+)
The car is moving to the east and increasing in speed.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (+) Acceleration (0)
The car is moving to the east at constant speed.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (+) Acceleration (-)
The car is moving to the east but slowing in speed.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (-) Acceleration (-)
The car is moving to the west and speeding up.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (-) Acceleration (0)
The car is moving to the west at constant speed.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (-) Acceleration (+)
The car is moving to the west but slowing in speed.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (0) Acceleration (+)
The car starts from rest and begins to speed up toward the east.
Describe what the particle is doing in each case, and give a real-life example for an automobile on an east-west one-dimensional axis, with east considered the positive direction Velocity: (0) Acceleration (-)
The car starts from rest and begins to speed up toward the west.
Notice how everything follows from the two kinematic equations. Once they are written down and the constants correctly identified as in Equations (1) and (2), the rest is relatively easy. If the ball were thrown downward, the initial velocity would have been negative. How would the answer to part (b), the maximum height, change if the person throwing the ball jumped upward at the instant he released the ball?
The maximum height would increase; if his starting point increased, his max height would also increase.
True or False? (c) An object with constant nonzero acceleration can never stop and remain at rest.
True
(Yes or No) Must two quantities have the same dimensions if you are adding them?
Yes
(Yes or No) Must two quantities have the same dimensions if you are subtracting them?
Yes
(Yes or No) Consider the following equations of motion: v =v0 + at Δx =v0t + 1/2at^2 v^2 =v0^2 + 2aΔx (b) Can they be used when the acceleration is zero?
Yes, it just cancels out.
Estimate the order of magnitude of the length, in meters, of each of the following. (c) a basketball court
between 10^1 m and 10^2 m
A skydiver jumps out of a hovering helicopter. A few seconds later, another skydiver jumps out, so they both fall along the same vertical line relative to the helicopter. Assume both skydivers fall with the same acceleration. (a) How does the vertical distance between them change?
it increases Explanation: The only force acting on them(air resistance aside) will be gravity. G=9.8 m/s = 10 m/s Diver 1 will fall for 3 seconds(30m/s) as Diver 2 starts falling(0m/s). After 1 second D1 will have fallen for 4 seconds and D2 will have fallen for 1 second.
Which of the lengths or time intervals given in Table 1 and Table 2 could you verify, using only equipment found in a typical dormitory room? (Select all that apply.) (b) time interval one day the duration of a nuclear collision the age of the earth correct the time between normal heartbeats
one day the time between normal heartbeats (the only two you could use with a stopwatch or clock)
An object moves along the x-axis, its position measured at each instant of time. The data are organized into an accurate graph of x vs. t. Which of the following quantities cannot be obtained from this graph? the speed of the particle at any instant the displacement during some time interval the acceleration at any instant the velocity at any instant the average velocity during some time interval
the acceleration at any instant Explanation: acceleration is found from a v/t graph, not an x/t
A ball rolls in a straight line along the horizontal direction. Using motion diagrams (or multiflash photographs), describe the velocity and acceleration of the ball for each of the following situations.
the images will be getting closer together as one moves toward the left.
Which of the lengths or time intervals given in Table 1 and Table 2 could you verify, using only equipment found in a typical dormitory room? (Select all that apply.) (a) length the diameter of a hydrogen atom the length of one light year correct the length of a football field correct the length of a housefly
the length of a football field the length of a housefly (H atoms are too small and you can't really measure a light year with stuff in your bedroom so this one was kinda dumb)
A racing car starts from rest and reaches a final speed v in a time t. If the acceleration of the car is constant during this time, which of the following statements must be true? (Select all that apply.) (a) The average speed of the car is v/2. (b) The car travels a distance vt. (c) The velocity of the car remains constant. (d) The acceleration of the car is v/t. (e) none of these
(a) The average speed of the car is v/2. (b)The car travels a distance vt. Explantion: refer to EoM
(a) Can the instantaneous velocity of an object at an instant of time ever be greater in magnitude than the average velocity over a time interval containing that instant? (b) Can it ever be less?
(a) Yes; The instantaneous velocity of an object can differ from the average velocity of an object over a period as long as the object undergoes some form of acceleration. example: a car accelerating from 40 - 50 mph over 10 seconds will have averaged 45 mph (assuming constant acceleration) but it will have only actually been doing 45 mph for an instant during the acceleration. (b) Yes
A juggler throws a bowling pin straight up in the air. After the pin leaves his hand and while it is in the air, which statement is true? (a)The velocity of the pin is always in the same direction as its acceleration. (b) The acceleration of the pin is zero. (c) The velocity of the pin is opposite its acceleration on the way up. (d) The velocity of the pin is in the same direction as its acceleration on the way up. (e)The velocity of the pin is never in the same direction as its acceleration.
(c) The velocity of the pin is opposite its acceleration on the way up. Explanation: Gravity is always acting vertically downwards. If you take all quantities moving in the upward direction as positive, then acceleration due to gravity would be taken as negative and vice versa.
A tennis player on serve tosses a ball straight up. While the ball is in free fall, its acceleration does which of the following? (a) decreases then increases (b) decreases (c)remains constant (d)increases (e) increases then decreases
(c) remains constant Explanation: A freely falling object is any object moving freely under the influence of gravity ALONE, regardless of its initial motion. Gravity is constant
A ball is thrown straight up in the air. For which situations are both the instantaneous velocity and the acceleration zero? (a) at the top of the flight path (b) halfway up and halfway down (c) on the way up (d) on the way down (e) none of these
(e) none of these Explanation: the acceleration of the ball is not zero at any point because of gravity.
The figure below shows the unusual path of a confused football player. After receiving a kickoff at his own goal, he runs down field to within inches of a touchdown, then reverses direction and races back until he's tackled at the exact location where he first caught the ball. During this run, what is each of the following? (c) his average velocity in the x direction
0 yds/sec Explanation: a positive and negative value would cancel out (since he reverses direction)
The figure below shows the unusual path of a confused football player. After receiving a kickoff at his own goal, he runs down field to within inches of a touchdown, then reverses direction and races back until he's tackled at the exact location where he first caught the ball. During this run, what is each of the following? (b) his displacement
0 yds; 100yds - 100 yds= 0 yds
The figure below shows strobe photographs taken of a disk moving from left to right under different conditions. The time interval between images is constant. Taking the direction to the right to be positive, describe the motion of the disk in each case. (a) light moves left (b) lights are equidistant (c) light moves right 1) For which case is the acceleration positive? 2) For which case is the acceleration negative? 3) For which case is the velocity constant?
1) C 2) A 3) B
Find the order of magnitude of your age in seconds.
10^ 9 s explanation: 25 years x 365 days/ 1 year = 9,125 days 1 day = 24 hours x 3,600 second/hour = 86,400 seconds ans: (86400*9125)
An object with a mass of 1 kg weighs approximately 2 lb. Use this information to estimate the mass of the following objects. (b) your physics textbook
10^0 kg
Estimate the order of magnitude of the length, in meters, of each of the following. (b) a pool cue
10^0 m (this is one meter)
Estimate the order of magnitude of the length, in meters, of each of the following. (d) an elephant
10^1 m
Estimate the order of magnitude of the length, in meters, of each of the following. (e) a city block
10^2 m
Estimate the number of atoms in 1 cm^3 of a solid. (Note that the diameter of an atom is about 10−10 m.)
10^24 atoms
An object with a mass of 1 kg weighs approximately 2 lb. Use this information to estimate the mass of the following objects. (c) a pickup truck
10^3 kg
(b) Estimate the number of human heart beats in an average lifetime.
10^9
An object with a mass of 1 kg weighs approximately 2 lb. Use this information to estimate the mass of the following objects. (a) a baseball
10^−1 kg
Estimate the order of magnitude of the length, in meters, of each of the following. (a) a mouse
10^−1 m (this is about 10 cm and mice are small)
(a) Estimate the number of times your heart beats in a month.
10e6
The figure below shows the unusual path of a confused football player. After receiving a kickoff at his own goal, he runs down field to within inches of a touchdown, then reverses direction and races back until he's tackled at the exact location where he first caught the ball. During this run, what is each of the following? (a) the total distance he travels
200 yds; once up a 100 yd field and once back down it
Suppose two quantities, A and B, have different dimensions. Determine which of the following arithmetic operations could be physically meaningful. (Select all that apply.) A − B A + B A/B AB B − A
AB A/B Explanation: If two quantities have different dimensionality, there are certain things you can and cannot do. Addition and subtraction (which is negative addition) - no. So A+B, A-B, etc are forbidden. As an example, take distance and time. You can't add 10 seconds to 3 metres. Multiplication and division are fine. Using our example, you can get metres per second (which is speed) and technically you can get metre-seconds, although I can't think what that might be used for. Or consider energy and time. Joules per second is power and Joule-second is the unit for Planck's constant.
A ball rolls in a straight line along the horizontal direction. Using motion diagrams (or multiflash photographs), describe the velocity and acceleration of the ball for each of the following situations.
As one moves from left to right, the balls will first get farther apart in each successive image, then closer together when the ball begins to slow down.
A ball is thrown vertically upward. (a) What are its velocity and acceleration when it reaches its maximum altitude?
At the maximum height, the ball is momentarily at rest. (That is, it has zero velocity.) The acceleration remains constant, with magnitude equal to the free-fall acceleration g and directed downward. Thus, even though the velocity is momentarily zero, it continues to change, and the ball will begin to gain speed in the downward direction.
What types of natural phenomena could serve as time standards?
Atomic clocks are based on the electromagnetic waves that atoms emit. Also, pulsars are highly regular astronomical clocks.
True or False? (a) A car must always have an acceleration in the same direction as its velocity.
False Explanation: when you brake, you can still continue the direction you are driving while you're acceleration slows the opposite direction
Newton's law of universal gravitation is represented by F =GMm/r^2 where F is the gravitational force, M and m are masses, and r is a length. Force has the SI units kg · m/s2. What are the SI units of the proportionality constant G?
G = m^3/kg·s^2 Derivation and substitution: F =GMm/r^2 kg·m/s^2 = (G)((kg^2)/(m^2)) G = m^3/kg·s^2
Why is the metric system of units considered superior to most other systems of units?
In the metric system, units differ by powers of ten, so it's very easy and accurate to convert from one unit to another.
(b) A displacement is related to time as x = A sin(2πft), where A and f are constants. Find the dimensions of A. (Hint: A trigonometric function appearing in an equation must be dimensionless.)
L Explanation: x = A sin(2πft) ----> A = x/sin(2πft) The hint tells you that sin(2πft) is dimensionless so you can disregard that part. A = x A=L
(a) Suppose that the displacement of an object is related to time according to the expression x = Bt^2. What are the dimensions of B?
L/T^2 Explanation: A simple way to get the dimensions is just to rearrange the equation. So rearrange: x = Bt^2 ----> B= x/t^2 Now you are told that x is displacement (L) and t is time (T) so sub these in. B= L/T^2 Therefore the dimensions are L/T^2. (note that when you square the value, the dimensions are also squared)
(Yes or No) Must two quantities have the same dimensions if you are dividing them?
No
(Yes or No) Must two quantities have the same dimensions if you are multiplying them?
No
(Yes or No) Consider the following equations of motion: v =v0 + at Δx =v0t + 1/2at^2 v^2 =v0^2 + 2aΔx (a) Can the equations above be used in a situation where the acceleration varies with time?
No, acceleration must be constant in PHYS 1603 Equations of Motion. **does not apply to force
(a) If an equation is dimensionally correct, does this mean that the equation must be true?
No; A dimensionally correct equation isn't necessarily true. For example, the equation 2 dogs = 5 dogs is dimensionally correct, but isn't true.
As seen in this example, average speed can be calculated regardless of any variation in speed over the given time interval. Does a doubling of an object's average speed always double the magnitude of its displacement in a given amount of time?
No; An object's average speed will only double the magnitude of its displacement in a given amount of time if it is moving in a straight line without changing direction.
Two cars are moving in the same direction in parallel lanes along a highway. At some instant, the velocity of car A exceeds the velocity of car B. Does that mean that the acceleration of A is greater than that of B at that instant?
No; Car B may be traveling at a lower velocity but have a greater acceleration at that instant.
True or False? (b) It's possible for a slowing car to have a positive acceleration.
True Explanation: (+) and (-) only indicate direction
How can an estimate be of value even when it is off by an order of magnitude? Explain and give an example.
While this may seem vague, order of magnitude calculations are actually quite a powerful tool because they allow you to compare values to see how well they measure up to each other, even without access to precise information about those values. Your order of magnitude estimate can help you check your results. For example, you may not know exactly how much energy is put into manufacturing a paper plate or a car, but you probably do have a reasonable guess at arriving at an order of magnitude estimate. Using this estimate, you can quickly conclude that making one paper plate is insignificant when compared to making a car.
(Yes or No) Must two quantities have the same dimensions if you are equating them?
Yes
(b) If an equation is not dimensionally correct, does this mean that the equation can't be true?
Yes; If an equation is not dimensionally correct, it cannot be true.
If the velocity of a particle is nonzero, can the particle's acceleration be zero?
Yes; If the velocity of the particle is nonzero, the particle is in motion. If the acceleration is zero, the velocity of the particle is unchanging or is constant.
If the velocity of a particle is zero, can the particle's acceleration be nonzero?
Yes; The particle may stop at some instant, but still have an acceleration, as when a ball thrown straight up reaches its maximum height.
If a car is traveling eastward, can its acceleration be westward?
Yes; if this occurs the acceleration of the car is opposite the velocity, meaning it is slowing down.
REMARKS The trooper, traveling about twice as fast as the car, must swerve or apply his brakes strongly to avoid a collision! This problem can also be solved graphically by plotting position versus time for each vehicle on the same graph. The intersection of the two graphs corresponds to the time and position at which the trooper overtakes the car. QUESTION The graphical solution corresponds to finding the intersection of what two types of curves in the tx-plane? (Select all that apply.) a straight line sloped upward a straight horizontal line a straight line sloped downward a downward-shaped curve whose slope decreases and then increases as x increases an upward-shaped curve whose slope increases as the displacement x increases
an upward-shaped curve whose slope increases as the displacement x increases a straight line sloped upward
The three graphs in the figure below represent the position vs. time for objects moving along the x axis. Which, if any, of these graphs is not physically possible?
any graph that goes left towards time because you cannot have 2 points on one x value (you can't go back in time)
A skydiver jumps out of a hovering helicopter. A few seconds later, another skydiver jumps out, so they both fall along the same vertical line relative to the helicopter. Assume both skydivers fall with the same acceleration. (b) How does the difference in their velocities change?
it stays the same D1=g4=10m/s(4s)= 40m/s D2=g1=10m/s(1s)=10m/s