Solving problems in physics generally requiresa 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 canoften be used to decide the nature of each value even if

you 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.

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 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.

The MKS unit system is also called SI units.(System Internationale)

Units of Commonly used Systems

MKS CGS English

DisplacementDistance 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

accelerationMeter/ sec2 centimeter/ sec2 feet/ sec2

(m/s2) (cm/s2) (ft/s2)

force newtons (N) dynes(dn) pound (lb)

Units of Commonly used Systems (cont’d)

MKS CGS English

Workenergy Kilojoules (Kj) ergs(er) foot pound (ft-lb)

power Kilowatt (Kw) watt(w) horsepower (hp)

Kilojoules joules calories(Kj) (j) (cal)

Heat energy

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/sec

Kg x m/s g x cm/s sg x ft/s

Newton x meter dyne x centimeter foot x poundN x m dn x cm ft x lb

torque

More Commonly Used Units

angles

frequency

period

Angulardisplacement

Angularvelocity

Angularacceleration

Degrees radians revolutions

Revolutions per second (rps) hertzs (hz)

Seconds / revolution

Radians

Radians / second

Radians / second2

After identifying all the information given in the problem andDeciding 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 thatare 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.

Solving a problem using the described method.

A car moving at 20.0 meters per second brakes at 3.0 meters per second2 in 0.11 minutes. What is its

stopping distance?

Read the problem. What are we looking for? What are the units for the answer?

Distanced = ? m

In meters!MKS units!

What data is given? Write down and label eachvalue with a symbol and proper units

Starting velocityVo= 20.0 m/s

accelerationa = -3.0 m/s2

(it’s slowing)

final velocityVf= 0 m/s

(stops)

Time T = 0.5 min

(0.11 x 60) = 6.7 s

(1) VAVERAGE = s/ t = (V2 + V1) /

(2) VINST. = VORIGINAL + at

(3) dINST = V0 t + ½ at2

(4) di = ½ (Vi 2 – Vo2) /a

Available Equations

Both equations (3) and (4) contain our unknown (d).Since we know Vi , Vo , a and t either equation will

Work. Try both!

Did you get 67 meters? I hope so!

Let’s try another:A 200. newton object slows from 50.0 m/s to rest in

10.0 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?

Force in newtons

f = ? N

What data is given? Write down and label eachvalue with a symbol and proper units

Starting velocityVo= 50.0 m/s

final velocityVf= 0 m/s

(to rest)

Time T =10.0 sec

WeightW = 200. N

Not mass that wouldBe kilograms

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 a

Only equation (6) contains our unknown (F).We have to use it!

But we need m and a!

We can use equation (5) find m since we know 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.

Did you get 20.4 kg for the mass?

Did you get - 5.0 m/s2 for the acceleration?

Remember it’s negative because it’s Slowing!

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.