Solving Physics Problems

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A general approach to problem solving in physics.
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Solving Physics Problems

  1. 1. Solving problems in physics generally requires a few basic but essential steps. Read the question carefully and decide nature of the answer. What are we asked to find? Mass, velocity, displacement, force, etc. Write down the symbol for the answer with a question mark. For example: m = ?, v = ?, d =? or f = ? Reread the question to find out what information is given. Record each bit of information as the problem is read. For example: vo = 2.0 m/s, t = 3.0 sec, vf = 5.4 m/s, etc. Notice that the units are included with each value. Units can often be used to decide the nature of each value even if you are not told directly in the problem what the number value represents. For example: m/s must velocity or speed, newtons must a force, seconds must be time, etc.
  2. 2. Unit Systems Unit systems are specified as MKS* (larger metric units), CGS (smaller metric units) and English units. Working with units generally requires us to stay in the Same unit group for all values used in solving a problem. For example: we would not use newtons (an MKS unit) with grams (CGS). We would convert grams to kilograms in order to use it with newtons. Similarly, cm/ sec (CGS) would not be used with meters (MKS), hours would not be used with seconds. The MKS unit system is also called SI units. (System Internationale)
  3. 3. Units of Commonly used Systems MKS CGS English Displacement Distance Meters (m) centimeters (cm) feet (ft) Mass Kilogram (kg) gram (g) slug(sg) Seconds (s) Seconds (s) Seconds (s)Time Meter/ sec centimeter/ sec feet/ sec (m/s) (cm/s) (ft/s) velocity speed acceleration Meter/ sec2 centimeter/ sec2 feet/ sec2 (m/s2) (cm/s2) (ft/s2) force newtons (N) dynes(dn) pound (lb)
  4. 4. Units of Commonly used Systems (cont’d) MKS CGS English Work energy Kilojoules (Kj) ergs(er) foot pound (ft-lb) power Kilowatt (Kw) watt(w) horsepower (hp) Kilojoules joules calories (Kj) (j) (cal) Heat energy Impulse Newton x sec dyne x sec pound x sec (N x s) (dn x s) (lb x s) momentum Kilogram x m/sec gram x cm/sec slug x ft/sec Kg x m/s g x cm/s sg x ft/s Newton x meter dyne x centimeter foot x pound N x m dn x cm ft x lb torque
  5. 5. More Commonly Used Units angles frequency period Angular displacement Angular velocity Angular acceleration Degrees radians revolutions Revolutions per second (rps) hertzs (hz) Seconds / revolution Radians Radians / second Radians / second2
  6. 6. After identifying all the information given in the problem and Deciding on what is to be found, the next step is to select an Equation containing the unknown value. Next, see if the selected equation contains all the variables that are given in the problem. If so, insert the number values in the appropriate spots in the equation and solve. If the data for one of the required variables for solving the equation is missing search the other available equations for one that contains the missing variable and known data. This equation will allow you to find the missing variable value. Calculate its value and insert it into the equation containing the unknown and solve for the answer.
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