C O N C E P T UA L P H Y S I C S
December 17, 2020
By submitting this exam, you are subject to the code of academic integrity at
Lehman College. Show your work everywhere. Not doing so will only allow you
to get partial credit. If you use an equation that we haven’t seen in class or is not
in the review sheet, you have to show how to obtain it from what we actually saw
in class. Otherwise, you will get no credit for it. This exam has to be submitted to
Blackboard (Assignments > Final exam) by Thursday, Dec. 17 at 11:59 pm.
problem 1 – falsifiability
Choose three of the five statements below1 and, for each of them, answer
these two questions:
(i) do you think the statement is true or false? why?
(ii) is the statement falsifiable? justify your answer.
• No human will ever live for more than 1000 years.
• After today, it will never snow again.
• Momentum is conserved in every physical process.
• There will be additional pandemics in the future.
• The Universe will eventually come to an end.
If you answer to more than three, I will only consider the first three appearing on your response,
regardless of the quality of the answer.
problem 2 – momentum conservation
A child who is ice skating has lost control over her skates and can’t stop. She
moves at a constant velocity towards an adult who is not moving. When the
child arrives at where the adult is, he grabs her and holds her between his
arms. As this happens, he recoils back from the impulse given by the child.
vchild = 7 m/s
mchild = 32 kg madult = 95 kg
Answer the following questions, showing all your work:
(i) what is the momentum of the child before she arrives at where the
(ii) what is the momentum of the adult before the child arrives at him?
(iii) what is the total momentum before they collide?
(iv) what is the total momentum after they collide?
(v) what is their velocity after they collide?
problem 3 – the nucleus
Some scientists have measured the electric charge and the mass of a nucleus
to try to understand how many protons and how many neutrons are inside.
They have obtained the results shown in the table:
total charge q = 1.282 × 10−18 C
total mass m = 2.989 × 10−26 kg
For this problem, you may need the following numbers:
proton charge QP = 1.602 × 10−19 C
neutron mass mn = 1.675 × 10−27 kg
proton mass mp = 1.673 × 10−27 kg
Answer the following questions, showing all your work. Don’t round your
results in whole numbers without explaining why.
(i) How many protons are there inside this nucleus?
(ii) How many neutrons are there inside this nucleus?2
(iii) Using the answers to your previous questions, write down the values
of the following numbers that serve to identify nuclei:
2 For this part it might be useful to consider that the mass of a neutron and the mass of a proton
are the same.
(iv) what is the full name of the atom? Explain how you arrived at your
answer to get full credit. (The answer to this question should be something like
oxygen-16 or carbon-15.)
(v) in questions (i) and (ii), did you obtain whole numbers? Explain why
and/or why not, for each part.
problem 4 nuclear chain reactions
Consider the following nuclear reaction:
235U −→ X +
93Nb + 3n,
where ”n” is a neutron, 235U is a uranium-235 nucleus, and 93Nb is a niobium93 nucleus.
(i) by using proton and neutron conservation, identify which nucleus is X.
This means that you have to give the values of ZX, NX, and AX, and its
(ii) In the table below you can see the rest energies of all the parts involved
in the previous reaction. Is there more rest energy before or after the
Does the reaction occur?
name symbol rest energy (J)
neutron n 1.507 × 10−10
uranium-235 235U 3.513 × 10−8
unknown X 2.092 × 10−8
niobium 93Nb 1.388 × 10−8
3×neutrons 3n 4.522 × 10−10. For more information on Physics check on this: https://en.wikipedia.org/wiki/Physics
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