2 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 questionmark. 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 canoften be used to decide the nature of each value even ifyou are not told directly in the problem what the numbervalue represents. For example: m/s must velocity or speed,newtons must a force, seconds must be time, etc.
3 Unit systems are specified as MKS* (larger metric units), CGS (smaller metric units) and English units.The MKS unit system is also called SI units.(System Internationale)Working with units generally requires us to stay in theSame 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 kilogramsin order to use it with newtons.Similarly, cm/ sec (CGS) would not be used with meters(MKS), hours would not be used with seconds.
4 Units of Commonly used Systems MKS CGS EnglishDisplacementDistanceMeters (m) centimeters (cm) feet (ft)Kilogram (kg) gram (g) slug(sg)MassTimeSeconds (s) Seconds (s) Seconds (s)velocityspeedMeter/ sec centimeter/ sec feet/ sec(m/s) (cm/s) (ft/s)accelerationMeter/ sec centimeter/ sec feet/ sec2(m/s2) (cm/s2) (ft/s2)forcenewtons (N) dynes(dn) pound (lb)
5 Units of Commonly used Systems (cont’d) MKS CGS EnglishWorkenergyKilojoules (Kj) ergs(er) foot pound (ft-lb)Kilowatt (Kw) watt(w) horsepower (hp)powerHeatenergyKilojoules joules calories(Kj) (j) (cal)ImpulseNewton x sec dyne x sec pound x sec(N x s) (dn x s) (lb x s)momentumKilogram x m/sec gram x cm/sec slug x ft/secKg x m/s g x cm/s sg x ft/sNewton x meter dyne x centimeter foot x poundN x m dn x cm ft x lbtorque
6 More Commonly Used Units anglesDegrees radians revolutionsfrequencyRevolutions per second (rps) hertzs (hz)periodSeconds / revolutionAngulardisplacementRadiansAngularvelocityRadians / secondAngularaccelerationRadians / second2
7 After identifying all the information given in the problem and Deciding on what is to be found, the next step is to select anEquation containing the unknown value.Next, see if the selected equation contains all the variables thatare given in the problem. If so, insert the number values inthe appropriate spots in the equation and solve.If the data for one of the required variables for solvingthe equation is missing search the other available equations forone that contains the missing variable and known data.This equation will allow you to find the missing variablevalue. Calculate its value and insert it into the equationcontaining the unknown and solve for the answer.
8 Solving a problem using the described method. A car moving at 20.0 meters per second brakes at3.0 meters per second2 in 0.11 minutes. What is itsstopping distance?Read the problem. What are we looking for?What are the units for the answer?Distanced = ? mIn meters!MKS units!Starting velocityVo= m/sWhat data is given? Write down and label eachvalue with a symbol and proper unitsaccelerationa = m/s2(it’s slowing)final velocityVf= 0 m/s(stops)TimeT = 0.5 min(0.11 x 60) = 6.7 s
9 (1) VAVERAGE = s/ t = (V2 + V1) / (2) VINST. = VORIGINAL + at Available Equations(1) VAVERAGE = s/ t = (V2 + V1) /(2) VINST. = VORIGINAL + at(3) dINST = V0 t + ½ at2(4) di = ½ (Vi 2 – Vo2) /aBoth equations (3) and (4) contain our unknown (d).Since we know Vi , Vo , a and t either equation willWork. Try both!Did you get 67 meters? I hope so!
10 A 200. newton object slows from 50.0 m/s to rest in Let’s try another:A 200. newton object slows from 50.0 m/s to rest in10.0 seconds. What is the braking force applied to the object?Read the problem. What are we looking for?What are the units for the answer?Forcein newtonsf = ? NWhat data is given? Write down and label eachvalue with a symbol and proper unitsfinal velocityVf= 0 m/s(to rest)TimeT =10.0 secStarting velocityVo= m/sWeightW = 200. NNot mass that wouldBe kilograms
11 (1) VAVERAGE = s/ t = (V2 + V1) / (2) Vfinal = VORIGINAL + at Available Equations(1) VAVERAGE = s/ t = (V2 + V1) /(2) Vfinal = VORIGINAL + at(3) dINST = V0 t + ½ at2(4) di = ½ (Vi 2 – Vo2) /a(5) W = m x g(6) F = m x aOnly equation (6) containsour unknown (F).We have to use it!But we need m and a!We can use equation (5) find m since weknow both w and g (9.8 m/s 2 )Next we can use equation (2) find a since we knowVfinal , V0 and t.Now we’ll insert the values found for m and aInto equation (6) and calculate the answer.
12 Did you get 20.4 kg for the mass? Did you get m/s2 for the acceleration?Remember it’s negative because it’sSlowing!Did you get -102 N for the force?This is a generalized procedure for solvingmost physics problems.Continue to use it and physics willbecome much easier.