# Solving Physics Problems

advertisement ```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.
Unit Systems
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 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.
Units of Commonly used Systems
MKS
Displacement
Meters (m)
Distance
feet (ft)
gram (g)
slug(sg)
Seconds (s)
Seconds (s)
Meter/ sec
(m/s)
centimeter/ sec
(cm/s)
feet/ sec
(ft/s)
Meter/ sec2
(m/s2)
centimeter/ sec2
(cm/s2)
Kilogram (kg)
Time
Seconds (s)
acceleration
force
English
centimeters (cm)
Mass
velocity
speed
CGS
newtons (N)
dynes(dn)
feet/ sec2
(ft/s2)
pound (lb)
Units of Commonly used Systems (cont’d)
MKS
Work
energy
Kilojoules (Kj)
power
Kilowatt (Kw)
Heat
energy
Kilojoules
(Kj)
Impulse
Newton x sec
(N x s)
momentum
torque
CGS
English
ergs(er) foot pound (ft-lb)
watt(w)
horsepower (hp)
joules
(j)
dyne x sec
(dn x s)
calories
(cal)
pound x sec
(lb x s)
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
Nxm
dn x cm
ft x lb
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
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.
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?
Distance
d=?m
In meters!
MKS units!
What data is given? Write down and label each
value with a symbol and proper units
acceleration
a = -3.0 m/s2
(it’s slowing)
final velocity
Vf= 0 m/s
(stops)
Starting velocity
Vo= 20.0 m/s
Time
T = 0.5 min
(0.11 x 60) = 6.7 s
Available Equations
(1) VAVERAGE = s/ t = (V2 + V1) /
(2) VINST. = VORIGINAL + at
(3) dINST = V0 t +
&frac12; at2
(4) di = &frac12; (Vi 2 – Vo2) /a
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
Starting velocity
Vo= 50.0 m/s
What data is given? Write down and label each
value with a symbol and proper units
final velocity
Vf= 0 m/s
(to rest)
Weight
W = 200. N
Not mass that would
Be kilograms
Time
T =10.0 sec
Available Equations
(1) VAVERAGE = s/ t = (V2 + V1) /
(2) Vfinal = VORIGINAL + at
(3) dINST = V0 t +
&frac12; at2
(4) di = &frac12; (Vi 2 – Vo2) /a
Only equation (6) contains
our unknown (F).
We have to use it!
But we need m and a!
(5) W = m x g
(6) F = m x 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 know
Vfinal , V0 and t.
Now we’ll insert the values found for m and a
Into 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 solving
most physics problems.
Continue to use it and physics will
become much easier.
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