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All Physics 28 Tips
| Question | Answer |
|---|---|
| Frame of Reference | point in space with respect with which the motion of objects can be measured. |
| Inertial Frame of Reference | one which is ether at rest or is moving at a start line at constant speed (constant velocity). In all inertial frames of reference, the laws of physics are the same and hold in the same way. |
| Non-inertial | frame of reference that is undergoing accelerated motion |
| Postulates of the Special Theory | Two postulates formulated by Einstein -> 1) all inertial frames of reference are equivalent (no frame is favored over another) 2) observers measure the same value for speed of light in a vacuum (c is the same regardless of relativity) |
| Ether | proposed medium that allows electromagnetic waves to propagate |
| Michelson and Morley Experiment | tried to detect Earth's motion through a light-carrying medium (ether) but instead proved that no such medium exists and light's speed is the same in every direction (regardless of relativity or reference frame) |
| Michelson and Morley Experiment Outcome | scientists assumed that inferometer rotation would turn the constructive interference into destructive, but the results were always null and rotation did not make any changes |
| Proper time (to) | Measured by an observer who happens to be present at the beginning and at the end of the event (usually the astronaut) |
| Dilated time (t) | Measured by an observer who is located in a different inertial frame of reference and therefore NOT going to be present at the beginning or the end of the event (usually the earth observer). |
| Twin Paradox | A twin takes off in a spaceship while the other twin stays behind, both claiming that the other will age (but if all frames are the same, then the spacecraft twin can also be considered to be the rest). |
| Twin Paradox Answer | There is only one inertial frame — the Earth — and the spacecraft is non-inertial because it experiences acceleration at the start, end, and turnaround. The spacecraft twin will age less. |
| How to solve for a problem where an object is moving parallel/perpendicular | One side is going to have a length contraction while the other remains unchanged. The contraction is usually the horizontal while the vertical stays the same. |
| When you need to solve for the diagonal of something in movement | Use Pythag with the pre-existing side and the length contraction |
| Keeping Track of Contracted/Dilated/Proper | contracted distance will usually refer to the astronaut inside the spacecraft because they're moving in respect while proper distance will be the earth-bound observer at rest. Dilated time = earth because not there for full duration, proper is astronaut. |
| Tip about Lightyears | they are often regarded as a unit of distance — not time! |
| Proper Time Vs. Dilated Time Equations | There will be many problems in which you'll have to solve for the contracted length first before moving onto time. Remember that proper time can be evaluated using the distance equation while time dilation uses the dilation equaton. |
| Relativistic Addition of Velocities | Two objects moving with respect to each other with a relative velocity (v). |
| What does u equal | Velocity of a third object with respect to the non-moving object. |
| What does u' equal | Velocity of a third object with respect to the moving object. |
| Tip about Relativistic Addition of Velocities | Try to draw your cute little diagram before you take any additional steps! |
| Rest Energy | An object at rest must carry an energy, all of it in the form of mass. Once the object starts moving, the extra energy will be carried in the form of kinetic. |
| Mass Energy Equivalence | mass can be converted to energy and energy can be converted to mass |
| Relativistic KE (not on formula sheet) | KE = (m - mo)c^2 |
| Relativistic Momentum | In order to preserve the conversation of linear |
| What to do if they give you potential difference but nothing else for finding kinetic energy? | Use the energy work theorem, WFe = change in KE, cancel out either KEi or KEf based on the question, set remainder equation to -qdeltaV. Remember that q equals charge, V equals potential difference. |
| If they ask for the electron's speed after passing through the potential difference | Remember that you have to find the relativistic mass first and can do this using E = mc^2. Then put that into the relativistic mass equation. You can set that up by doing mo/m and squaring both sides. |
| Does 1/2mv^2 equal KE work with relativistic mass? | No, just use KE itself. |
| How to convert m/s to c units | Divide the value by 3 * 10^8 m/s |
| How to convert c units to m/s | Multiply the value by 3 * 10^8 m/s |
| If they ask for average speed... | You can figure out what the proper/relativistic time + contracted/proper length. You can set L = vto from the x = vt equation, sub in the rest of the L equation in place of L, single out v, and solve. |
| If you're ever stuck | Try finding the contracted/proper length + contracted/proper time |
| The value you get for v/c... | Will be your final speed answer. Just add c to the end of the number. |
| When you have a relative speed equation involving two spacecrafts, you should | Draw an earth-bound observer (your at-rest observer), assign one of the spacecrafts observer in motion, and make the remaining craft object 3. |
| Does the negative sign matter in your final relative speed answer? | No, you would just take the number attached to the c. But negative signs WOULD matter during the calculations. |
| Does the proper time interval depend on the speed of the observer? | No, because the observer should be at rest. |
| When you start with contracted length, then proper time like "how long will it take for so and so to cross from..." | You usually have to solve for contracted length, then input that into x = vt. Remember that the velocity is still squared! |
| In momentum problems, you can occasionally... | Put E/Eo. |
| Area of a triangle is... | 1/2bh |
| When one spaceship overtakes another... | The relative velocity between them would be negative. |
| To convert MeV to Ev... | Multiply by 10^6 |
| The mass of the proton, electron, or neutron provided in the formula sheet is | Equal to the resting mass |
| When the question asks for something like how far does the blank need to move for KE to equal resting energy, remember... | You can set both KE and Eo equal to moc^2, solving that way. If KE equals a portion of E, the same thing can happen. |
| Difference between how fast you must travel for this distance and how you must travel to shrink distance | The former requires you to set distance and length contraction equations equal to each other and the latter just uses length contraction. |
| When you set distance and length contraction equal to each other | The c in the calculations equals 1 |
| Tip for horizontal motion | The only thing contracting is motion along the hypothenuse, not the hypothenuse itself. Laddies ARE hypothenuses, so you can use pythag without it specifically asking you to solve for the diagonal. |
| If it asks for the motion parallel to one of a square's diagonals | Remember to multiply the side by sqrt of 2 before inputting it into the length contraction equation. Then use pythag. |
Created by:
smurtab