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A problem set for the physics 8.012: mechanics i course at mit, fall term 2008. It includes collaboration policies, reading assignments, and various problems to be solved. Topics covered include radial and tangential accelerations, two trains and a bee, and dimensional analysis. Students are encouraged to discuss problems with each other but must write up their solutions individually.
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Physics 8.012: Physics (Mechanics) I Fall Term 2008
PROBLEM SET 1
Collaboration policy: You are encouraged to freely discuss homework problems with other 8. students and teaching staff. However, you must write up your solutions completely on your own—do not simply copy solutions from other students. You are forbidden from consulting solutions from previous years or from the web. Violations of this policy may result in disciplinary action.
Reading: Kleppner & Kolenkow, Chapters 1 and 2
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(a) (10 points) Find an expression for the distance dn covered by the bee after its nth encounter with a train. Define d 0 as the distance traveled during the first flight from Aville towards the train near Bville, d 1 the distance traveled by the bee during the first trip from the Bville train to the Aville train, etc. Sum the resulting series to get the final answer. (b) (5 points) Devise another way to obtain the same answer using very little calculation.
You can often derive the solution of a problem by considering the dimensions of the rele vant variables, including all related fundamental constants, and match the dimensions of the quantity you want to determine. This is called dimensional analysis, and it is a powerful ap proximation technique as well as a method of determining how quantities scale with different variables. The basic idea is to write the unknown quantity (X) as a factor of all your relevant variables (Vn) and constants (Cn):
X = V 1 aV 2 bV 3 cC 1 dC 2 e^ ... (1)
and then solve for the powers a, b, c, d, e, ... so that the dimensions on both sides of the equation work out.
(a) (3 points) Derive an expression for the vibration frequency of a star of mass M and radius R, if that vibration is caused by gravitational instabilities. (b) (4 points) Derive an expression for the drag force on a ball of radius R and mass M moving with velocity v through a medium with mass density ρ. (c) (4 points) Derive an expression for the terminal velocity of a falling ball of radius R and mass M close to the surface of the Earth, when it experiences a drag force of the form F = bv^2. Can you find an alternate way of deriving this velocity? (d) (4 points) Derive an expression for the frequency of a pendulum of mass M , hanging from a rope of length L near the surface of the Earth, released from rest at an initial angle θ 0. Warning! θ 0 is dimensionless. Is it possible to constrain how the frequency depends on this variable?