PHYSICS EXAM 2 PRAC. EXAM QUESTIONS
Plug in the numbers to the equation displayed before?
(30D)*(200 m/s) + (50D)*(0 m/s) = (30D)*(-50 m/s) + (50 D)v'50
What is the velocity of the free ball?
-0.125 kg - m/s / -0.10 kg = 1.25 m/s
If the fish shown in question before STOPPED SWIMMING, fish B would remain where it is shown. What would happen to fish A AND C?
-Fish C would FLOAT TO THE TOP -Fish A would SINK TO THE BOTTOM
What should we estimate the dimensions of a typical balloon to be?
0.1 m by 0.1m by 0.1 m, so the VOLUME IS 10^-3 M^3
What is the magnitude of the sum of the two forces of q to A?
0.707 = 1/sq root of 2
1.5 The diffusion coefficient for oxygen molecules in a cell membrane is around 10-7 cm2/s (though there are differences based on the type of cell). On average, to the nearest order of magnitude, how much time would you expect it to take for oxygen molecules to diffuse across a membrane 4 nm (= 4 × 10-9 m) thick?
0.8 * 10^-6 seconds
What is the BC magnitude force?
1- 0.707 = 0.293 and the ranking of the forces is A > B > C
Sometimes you read the most amazing things on the internet. And sometimes they are totally bogus! A report this week stated that Samsung had to pay Apple $1billion for a court decision on a patent violation about smart phones. It claimed Samsung sent Apple "30 trucks full of five cent coins." Given that a maximum allowed truckload is about 25 tons, estimate whether that would have been enough trucks to carry that much money as nickels. Be sure to clearly state your assumptions and how you came to the numbers you estimated, since grading on this problem will be mostly based on your reasoning, not on your answer.
1.) Estimate the mass of the nickel (roll of 20), mass is 100 g / 20 = 5 GRAMS A PIECE Penny = 2.5 GRAMS 2.) 1 metric ton = 1000 kg, 20 nickels= 0.1 kg, 20 x 1000 nickels in 1 ton. Worth (0.05) x 20 x 10,000 = 10,000 dollars One truck can carry 25 x 10,000 dollars = 0.25 x 10^6 dollars or 1/4 of a million dollars 30 trucks x 1/4 x 10^6 = 7.5 x 10^6 dollars
A setup is arranged so that a ball can be placed at the end of a rigid swinging rod. When the ball is attached to the rod is RELEASED, it FALLS, pivoting downward. Falling ball can either be CLAY OR SUPERBALL, CB comes to a dead stop after hitting, SB bounces back. 1.) The time from when the falling ball is released until JUST BEFORE it hits the FREE BALL 2.) The time from just before the TWO BALLS HIT UNTIL just after they have separated slightly (so they are NO LONGER touching) 3.) The time after the TWO BALLS have just SEPARATED SLIGHTLY and continue to move INDEPENDENTLY until just before the FREE BALL hits the ground. If the SB is used, is the total momentum of the two balls conserved in any of the three things?
1.) NO 2.) YES 3.) NO
Divided into three segments: 1. The time from when the falling ball is released until just before it hits the free ball 2. The time from just before the two balls hit until just after they have separated slightly (so they are no longer touching 3. The time after the two balls have just separated slightly and continue to move independently until just before the free ball hits the ground. If the superball is used, is the total momentum of the two balls CONSERVED in any of the three time segments?
1.) NO 2.) YES 3.) NO
What are the FOUR expressions of electrical forces facts from Coulomb's law for thinking about the QUALITATIVE DEPENDENCE of the forces?
1.) They are PROPORTIONAL to the magnitude of EACH OF THE CHARGES 2.) The DIRECTION OF THE FORCE is attractive for OPPOSITE CHARGES AND REPULSIVE for the same charges 3.) Newton's THIRD LAW holds for ELECTRICAL FORCES 4.) They get WEAKER AS YOU GET FARTHER AWAY, and by the SQUARE OF THE DISTANCE.
How does twice as far mean for strength in electrical forces?
1/4 as strong
What is the speed of the 50D?
150 M/S
Using the molecular masses provided, what is helium the density of air?
4/30 the density of air
The main blood vessel carrying blood out of your heart is the AORTA. It carries BLOOD DOWN towards the legs. In your abdomen it SPLITS INTO TWO, the common iliac arteries, as shown in the figure at the right. The diameter of a typical aorta is 2 CM, while teh common iliac arteries (split into two) have a diameter of 1 CM. A typical value for the SPEED OF THE BLOOD in the aorta is Va = 30 cm/s when the heart is contracting. The speed of the BLOOD FLOWING in the illiac arteries is closest to...
60 CM/S
If any unit is changed in an equation....
ALL THE TERMS in the equation will change in the SAME WAY and the equation will still be held
Explain the NO for segment 3: The time after the two balls have just separated slightly and continue to move independently until just before the free ball hits the ground.
After the ball is STRUCK, the second ball falls OFF THE TABLE and accelerates and the FIRST BALL EITHER STAYS STILL (Clay) or accelerates BACK UPWARDS (slowing down). The external forces on the two balls are UNBALANCED so their momenta are CHANGING (not due to each other) and their MOMENTUM IS NOT CONSERVED.
Now consider a capillary inside a muscle. The muscle has done work and used up its oxygen. The density of oxygen inside the capillary is 1 million molecules/(µm)3 and in the fluid surrounding the muscle is 0.3 million molecules/(µm)3. Which way will the oxygen flow across the membrane? Explain why
Again, the FLOW OF OXYGEN will be from HIGH DENSITY to the LOW DENSITY, in this case from INSIDE the capillary into the MUSCLE. Again, it is because MORE OXYGEN MOLECULES are HITTING THE WALL of the capillary on the MORE DENSE SIDE than on the LOW DENSE side, so MORE WILL GET THROUGH.
Three fish are swimming in a FISH TANK. They all HAVE THE SAME volume, but A HAS GREATEST mass, B has LESS, Fish C has EVEN LIGHTER. The fish are swimmingly slowly through the tank. How do the BUOYANT FORCES exerted by the water on the three fish rank?
BFA = BFB = BFC
What are the average speeds of the molecules determined by?
BY THE TEMPERATURE of the system; CONCENTRATION DOES NOT affect them.
Why do we use dimensional analysis for a particular length?
Because a length of 1 cm can be represented as 0.01 meters or 0.3937 inches. They mean the same thing, even with DIFFERENT NUMBERS.
Helium balloons FLOAT due to BUOYANT force, because helium is LESS dense than air. (The atomic WEIGHT of HELIUM is 4. AIR is a mixture of gases with an average molecular WEIGHT around 30.) But wait... the balloon itself is made of a solid material that is DENSER than air, so if it's TOO heavy, it will "sink." Estimate the MAXIMUM MASS that an empty balloon can have so that it still floats when it is filled with helium (assuming that it is inflated to a typical size).(The DENSITY of air at STP is around 1.3 kg/m3).
By Archimedes' Principle, the BUOYANT force on a HELIUM BALOON IS EQUAL to the weight of the air that would TAKE UP THE SAME VOLUME AS THE BALLOON. The balloon will FLOAT ONLY IF THE MASS of the balloon itself is LESS than the mass of AIR THAT FILLS THIS VOLUME.
How can we do dimensional analysis?
By identifying the kinds of measurements (or units) involved in an equation: Length (L), Time (T), Mass (M), and charge (Q).
How do molecules move? When does net diffusion arise?
CONTINUALLY across the membrane; the net diffusion ARISES WHEN THE CONCENTRATION difference means MORE is passing from one side than from the other.
André says, "This equation (Coulomb's Law) is useful MAINLY for CALCULATING electric forces." Charles objects, saying, "I think it's good for thinking about the QUALITATIVE character of the FORCES." Who do you agree with more? Discuss in the context of this particular equation. Note: This is an essay question. Your answer will be judged not solely on its correctness, but for its depth, coherence, and clarity. What roles do coulomb's law can play?
COULOMB'S Law can play BOTH ROLES. You can use it to calculate forces as we have done in the case of the "WATER COAT" PROBLEM (force between a WATER MOLECULE AND AN ION) and it can be USEFUL FOR CALCULATING the force BETWEEN any DISTRIBUTION of charges.
What is an example of using dimensional analysis?
Checking an EQUATION that you have CREATED from SYMBOLIC MANIPULATION or SUBSTITUTION. --OR guessing an equation for something if you have a GOOD UNDERSTANDING OF HOW it might depends on the various CONSTANTS AND VARIABLES in the problem.
What is the formula that exemplifies the iB = 2iC?
DeltaV = iR Since Rb is HALF AS BIG as Rc, the current through RB WILL BE TWICE as much as the current through Rc (the BIGGER current goes through the SMALLER RESISTANCE)
) One of the tools that we have been developing in this class is dimensional analysis. Describe the approach briefly and tell why it is important. Have you ever used dimensional analysis in this class in a situation in which it was not explicitly assigned? If so, give an example. If not, explain why you have not found it useful.
Dimensional analysis is an EXPRESSION of the idea that the NUMERICAL QUANTITIES WE WORK with in physics are NOT JUST NUMBERS, but represent MEASUREMENTS.
Explain the NO for segment 1: The time from when the falling ball is released until just before it hits the free ball
During the time that the ball is on the ROD IS FALLING it is SPEEDING UP DUE to the force of GRAVITY (external to the TWO BALLS). The sitting ball DOESN'T CHANGE it's momentum, so the SUM OF THE TWO MOMENTA INCREASES.
Consider an individual molecule in the air outside the alveolus. How is it MOVING? How does it know how to get into the capillary?
Each molecule moves at RANDOM. It doesn't KNOW TO GET INTO THE CAPILLARY, just some do.
Consider an individual molecule in the capillary (after is has been released by the hemoglobin). How is it moving? How does it know how to get into the capillary?
Each molecule moves at random. It DOESN'T "know" to get into the capillary, just some do.
What is the rate of flow governed by?
Equation J = -D dn/dx It is PROPORTIONAL to concentration gradient or difference in concentrations.
1.2 At time t0, the oxygen concentration on one side of the membrane is 200 µM, and the oxygen concentration on the other side is 600 µM. At time t1, the oxygen concentration on one side of the membrane is 300 µM, and the oxygen concentration on the other side is 500 µM. What can you say about the FLOW RATE of oxygen across the membrane at each time?
Flow rate is 2 TIMES GREATER at time to then at time t1.
What is this law for iB = 2iC?
Follows OHM'S LAW - since B and C are PARALLEL, they must have the SAME VOLTAGE DROP across them.
What is iA = iB + iC?
Follows from KIRCHOFF PRINCIPLES --FLOW RULE: THE AMOUNT of current that comes into a JUNCTION (or volume) must EQUAL the amount GOING OUT. At the top T junction between the THREE RESISTORS, iA goes in, and iB and iC go OUT so iA = iB + iC
When is the first equation seen? ∆VB = ∆VC
From the fact that the TWO RESISTORS B AND C are in PARALLEL, so their beginning voltages and ending voltages MUST BE THE SAME, so DROPS ARE THE SAME.
Complete the statement, A chunk of IRON IS SITTING on a table. It is then COVERED BY A BELL JAR, which has a nozzle connected to a VACUUM PUMP. The air is extracted from the bell jar. >, < or EQUAL TO = Statement: The force the table exerts on the chunk when the chunk is SITTING IN A VACUUM is ____________ the FORCE that the TABLE EXERTED ON THE CHUNK when the chunk was sitting in the AIR.
GREATER THAN (>)
What does Fick's Law tell us?
HOW a concentration DIFFERENCE DRIVES FLOW.
Why are 1 and 3 a NO?
Have ONE BALL or the OTHER FALLING BALL/FREEBALL undergoing acceleration due to an UNBALANCED EXTERNAL FORCE (earth is outside the 2 ball system! Momentum is NOT CONSERVED.
You haven't been told the mass of either ball or even whether they are equal. What did you assume for your answer to C? Does it matter? How?
I didn't assume ANYTHING, it DOESN'T MATTER
SCENARIO 2: Joe said "It CAN'T BE. It slows down and speeds up"
If the SYSTEM IS THE BALL ALONE (the Earth is NOT PART of the system), THEN THE GRAVITY force of Earth on ball is AN EXTERNAL FORCE and this ball system is NOT ISOLATED. Therefore, momentum CANNOT BE CONSERVED (Kevin is wrong, Joe is RIGHT).
SCENARIO 1: Kevin says "The answer is YES since momentum is ALWAYS CONSERVED" Explain momentum.
If the system is BALL PLUS EARTH, than Kevin is right that momentum is CONSERVED (but WRONG that it is ALWAYS) In this system the force of gravity Earth has on ball that causes the SPEED OF THE BALL to change (as Joe notes) is INTERNAL so it DOES NOT CHANGE the momentum of the (Ball +Earth) system. NICK IS WRONG, here as there is ANOTHER OBJECT involved (Earth).
How is momentum conserved in segment 2?
In 2, FACTOR OF GRAVITY is NOT A FACTOR because it's balanced by the force of the swinging rod. This is also an ISOLATED SYSTEM so the momentum is conserved
A setup is arranged so that a ball can be placed on the end of a rigid swinging rod. When the ball attached to the rod is released, it falls, pivoting downward, until it strikes a free (not connected to anything) ball sitting on the table. The free ball then shoots of the edge of the table and falls to the ground. The falling ball can be either a clay ball (CB) or a superball (SB). The CB comes to a dead stop after hitting the free ball while the SB bounces back with the same speed that it was going when it hit the free ball.
Just read
Capillaries are the narrowest elements in the blood transport system of animals. They connect the arteries (the vessels that carry blood away from the heart) to the veins (the vessels that carry blood back to the heart). They are the place where the chemicals that the blood is carrying are taken from source regions (lungs for air, intestines for food, glands for hormones) and delivered to the cells that need them. They are pretty small - about 5 µm in diameter. One of the interesting questions about this process is: How do oxygen molecules know where to go? For example, Wikipedia says that red blood cells "take up oxygen in the lungs or gills and release it while squeezing through the body's capillaries." In order to consider this let's make a(n oversimplified) model in which oxygen diffuses into the blood stream in the lungs, is carried by fluid flow out to the cells where it diffuses out. (In reality, the oxygen is bound to hemoglobin, carried, and released, but the beginning and end processes are as described here.)
Just read
What is the equation of this? ' as the FINAL
M30V30 + M50V50 = M30V'30 + M50V'50
What would the mass of the air be using this density?
Mass of the air would be 1.3 x 10^-3 kg or 1.3 grams.
Kevin, Joe and Nick are studying for their physics exam, but they are very confused about MOMENTUM CONSERVATION. A reading question in their text asks whether the momentum of a ball thrown upward is CONSERVED after it leaves the hand. EXPLAIN RULE OF MOMENTUM FIRST.
Momentum is CONSERVED for a system of OBJECTS if that system is CLOSED (no mass ENTERS OR LEAVES the system) and ISOLATED (there are NO EXTERNAL forces on the system.
What equations do the balances imply?
N = (m1 + m2)g F0 = u*N And you SOLVE THESE FOR u to give you = F0/N = F0/ (m1 + m2)g
If the clay ball is used, do any of your answers change? For which time segment? Explain why.
NO CHANGE! (Elastic SB or inelastic CB does NOT change conservation momentum conditions. Answer to A did NOT RELY on properties of SB!
Did this equal 30 trucks?
NO, it equaled 7.5 x 10^6, it is OFF by a factor of 1000/7.5 = 133, so they need 133 x 30 = 4000 trucks, NOT THIRTY.
If the clay ball is used, do any of your answers change? For which time segment? Explain why.
NO, whether there are UNBALANCED forces is the SAME IN BOTH situations.
What are the final answers about who is right and wrong?
Nick - ALWAYS WRONG Kevin - Only PARTIALLY RIGHT --RIGHT in #1 scenario of system ball plus earth JOE - Only right in Case #2 external forces
Nick says 'Theres NO OTHER OBJECT involved after it's released so it must be CONSERVED"
Nick is STILL CONFUSED as there IS ANOTHER OBJECT involved even in this case (Earth) which is an EXTERNAL OBJECT providing an external force.
1.4 At time t2, the oxygen concentration is 400 µM on both sides of the membrane. What is happening now?
Oxygen is moving across the membrane, at an EQUAL rate in both directions.
How would the electric FIELD measured at point A change i f the magnitude of the TEST CHARGE WAS INCREASED BY A FACTOR OF two?
SAME
How would the electric field measured at point A change if the magnitude of the test charge was increased by a factor two?
SAME
For which case, CB or SB do you expect that the struck ball will travel faster after the collision? Explain.
SB will be FASTER RESULT for struck ball (Freeball). *show flashcards
Suppose the CB has a mass of 50 grams and the free ball has a mass of 100 grams. If the CB is traveling at a speed of 2.5 m/s at the end of time segment (I), can you find the speed at which the free ball goes off the table? (Neglect friction and rolling.) If you can, do so. If you can't, explain why. Formulas?
Since momentum is conserved the formula would be change in (McbVcb + MfbVfb) = 0 change in (McbVcb) = - change in (MfbVfb)
Why is this seen in the figure?
Since the source charges we are considering are all the SAME distance from A and have the same MAGNITUDE of CHARGE, the magnitude of the force they CONTRIBUTE (in blue- thin vectors) we can easily SEE that they ADD up to a net electric force (in red - heavy vectors) that has the SAME magnitude but a DIFFERENT direction.
What does the IDEAL GAS LAW say about the N molecules of helium? Discussing volume, density, and mass.
They take up the SAME VOLUME AS N MOLECULES OF AIR (at the same temp. and pressure) so the DIFFERENCE IN DENSITIES of each gas depends only on the DIFFERENCE IN THE MASS of each molecule.
How do we ESTIMATE the TIME?
Use the equation <x^2> = 2Dt and solve for t. Must convert the units, since D and x are given in different units.
Explain the YES for segment 2: The time from just before the two balls hit until just after they have separated slightly (so they are no longer touching
When the TWO BALLS HIT each other, gravity is CANCELLED by the rod force on the FIRST BALL AND THE NORMAL force from the table on the second. The ONLY UNBALANCED force on each ball is the FORCE FROM EACH other. Therefore, there are NO EXTERNAL UNBALANCED forces acting on the system of the TWO BALLS and their MOMENTUM IS CONSERVED.
If your answer was yes, find an equation for µ, the coefficient of static friction between the bottom box and the floor, expressing it in terms of the symbols F0, M1, M2, and g (the earth's gravitational field). If your answer was no, explain what additional information you would need and make your case using the basic principles of motion.
When the force is LARGE enough to just start the boxes, there must be an UNBALANCED net force. So we infer that when we exert the force F0, it BALANCES the MAXIMUM (static) friction force that can be exerted. If we draw a Free-Body Diagram for the two boxes considered as one object, the UP (normal) and DOWN (weight) forces BALANCE and the LEFT (applied) and RIGHT (friction) forces BALANCE.
A worker is pushing a couple of boxes, one sitting on top of the other as shown in the figure at the right. When he pushes with a force that's just a bit greater than F0, the two boxes start to move together... If you know the masses of the two boxes and F0, could you infer the coefficient of (static) friction between the bottom box and the floor?
YES
In the upper atmosphere, chemical reactions play a critical role in important environmental issues, such as the formation of ozone and nitrous oxides. Consider the collision of two molecules in the atmosphere. (The molecules are traveling fast enough that gravity can be ignored during the collision.) Suppose that a molecule of mass 30 Daltons is APPROACHING a molecule of 50 Daltons with a speed of 200 m/s. The 50 D molecule is at REST. After the collision, the 30 D molecule has BOUNCED BACK with a speed of 50 m/s. Using the information given in this problem, can you find the SPEED with which the 50 D molecule leaves the collision?
Yes, because MOMENTUM IS CONSERVED in this COLLISION since there are NO OTHER OBJECTS that are EXERTING FORCES that change the MOMENTUM of EITHER OF THE MOLECULES.
Which of the following must be true about the POTENTIAL DROPS in the CIRCUIT? (three)
∆VB = ∆VC ∆V0 = ∆VA + ∆VB ∆V0 = ∆VA + ∆VC
What is the difference at t0 and difference at t 1?
T0 = 400 um T1: 200 uM --Expect TWICE THE FLOW at t0 as we do at t1
Two negative charges are placed as shown in the figure at the right. A positive test charge then placed at A measures both the electric force and an electric field. If a test charge of magnitude twice as large as the original test charge were placed at point A, how would the force it feels compare to the force felt by the original test charge (test charge placed at point A)?
TWICE AS BIG
Two negative charges are placed as shown in the figure. A POSITIVE TEST charge then placed at A measures both the electric force and electric field. *one neg. charge above A, but one neg. charge How would the electric FORCE measured at point A change if the MAGNITUDE OF THE TEST CHARGE was increased by a factor of two?
TWICE as big
What does this mean about the mass of helium and the balloon?
That the helium in the balloon has a mass of about 0.2 GRAMS which means that the balloon itself can have A MASS OF NO MORE THAN 1.1 GRAMS. (1.3 g - 0.2 grams)
Suppose the NEGATIVE CHARGE above point A is replaced by a POSITIVE CHARGE as shown in the figure at the right. How would the ELECTRIC FORCE measured by the TEST CHRAGE COMPARE to the original situation prior to problem before?
The DIRECTION OF THE FORCE felt would CHANGE, but the MAGNITUDE would stay the SAME.
Why is it the same?
The ELECTRIC field measured at the position of test charge A equals the force felt by the TEST CHARGE DIVIDED by QA.
What does the momentum conservation in this case say about the INITIAL TOTAL MOMENTUM?
The Initial total momentum is EQUAL to the FINAL TOTAL MOMENTUM of the two molecules.
For which case, CB or SB do you expect that the struck ball will travel farther before hitting the ground? Explain.
The SUPERBALL - The struck ball changes its MOMENTUM OPPOSITE to that of the STRIKING BALL since momentum is CONSERVED IN THE COLLISION. Since the SB changes its momentum more than the CB, the struck ball picks up MORE MOMENTUM in the SB case and will be TRAVELLING FASTER.
How would the electric force measured by the test charge compare to the situation in problem 1.1?
The direction of the force felt would CHANGE but the MAGNITUDE would stay the SAME.
Why twice as big?
The force on the test charge at A is EQUAL TO THE SUM of the forces from each of the NEGATIVE CHARGES, WHATEVER the two force vectors are, DOUBLING the charge on the test charge will double the resulting force vectors and their sum
Plug the numbers into the formulas?
The initial velocity of the CB is 2.5 m/s and the final velocity is 0, initial velocity of the free ball is 0. McbVcb final - McbVcb initial = - (MfbVfb final - MfbVfb initial) 0 - (0.05 kg)*(2.5 m/s) = - (0.10 kg)* Vfb final - 0)
1.3 In the same scenario as question 1 (t0: 200 uM, other side 600 uM; t1: 300 uM, other side 500 uM), what can you say about the AVERAGE SPEEDS of individual oxygen molecules across the membrane at each time?
The molecules are moving at the SAME SPEEDS at both times.
How are the other equations decided? ∆V0 = ∆VA + ∆VB ∆V0 = ∆VA + ∆VC
The others follow from the loop rule. Around any loop the sum of the rises equals the sum of the drops. e) is the application of the loop rule to the left pane, f) the application to the outer (whole) pane.
Consider a capillary in an alveolus in the lung. The DENSITY of OXYGEN inside the capillary is 0.3 MILLION molecules/(µm)3 and in the air outside is about 5 MILLION molecules/(µm)3. Which way will the oxygen FLOW across the membrane? Explain why.
The oxygen will flow from THE SIDE WIHT HIGHER DENSITY (outside) to the LOWER (inside) EACH MOLECULE moves at RANDOM, but the fact that MORE ARE HITTING the membrane from ONE SIDE (the outside) means that on the AVERAGE MORE WILL PASS through from outside to inside.
What can dimensional analysis tell us?
The possible ways DIFFERENT VARIABLES can relate to each other. Every term that is added and both sides of the equation have to have the SAME NUMBER OF POWERS OF LTMQ.
1 Physics 132 spring 2016 file In the circuit shown at the right, RA is identical to RB, and their resistance is half of RC: RA = RB = ½ RC. The current through resistor A is iA, iB is the current through resistor B, and iC through resistor C. The potential drop across resistor A is ∆VA and so on. The battery provides an EMF = ∆V0 Which of the following MUST BE true of the currents in the circuit?
iA = iB + iC
What is the relationship between iB and iC?
iB = 2iC
