Book Review: June 2013

Problem Solving Guide

1 Introduction

Are you having trouble with solving dynamics problems? Do you often haven’t got a clue where to start?

Then this problem solving guide might come in handy for you.

In this problem guide, the dynamics course has been split up in a lot of small subjects. For every subject,

it is mentioned...

• How to recognize it. Now you will know what problem corresponds to what subject!

• The important equations of that subject. This should help you solve the problem.

• The frequency of occurrence. How often does this subject occur on exams? Quite handy to know,

isn’t it?

However, dynamics is a diﬃcult subject. A question hardly ever corresponds to just one subject. So in

this problem solving guide, it is also often mentioned which subjects are linked in some way.

This problem solving guide contains various bold lines. If you only read these bold lines and the equations,

you will, in fact, read a summary of this document. If you want further clariﬁcation of the bold lines,

then you should also read the normal text next to it.

I would advise you to read this problem solving guide carefully. Then try to solve dynamics exam

problems, using this problem solving guide. And then you just might be able to solve those diﬃcult

dynamics problems you were never able to solve.

1.2 General plan of approach

2.1 When to use?

At every question. The plan of approach below contains tips that can help you at every problem. So

you should deﬁnitely apply this at every problem! Study it carefully.

2.2 Plan of Approach

1. Read the question twice. You may wonder, why twice? This is because you often miss things

the ﬁrst time you read something.

2. While reading, look for keywords, and underline them. There are several keywords that

are important for dynamics problems. Just to mention a few:

• Initially at rest. This means that the initial velocity V0 is zero.

• Massless. This means that the mass m is zero. It also means that the moment of inertia I is

zero.

• In the horizontal plane. When something moves in the horizontal plane, you do not have

to consider gravity.

• In the vertical plane. In this case you do have to consider gravity. The gravitational

acceleration points downward and has magnitude g = 9.81m/s2

• g = 9.81m/s2

. This is a subtle hint. If they give the value for g, you probably will have to

use it. So in this case, do consider gravity.

• Do/do not neglect friction. Just do what it says. If you can neglect friction, it’s great! If

you can not neglect friction, you probably have to use static and dynamic friction coeﬃcients.

Besides these keywords, there are also a few keywords belonging to certain subjects. We will discuss

them in the paragraphs belonging to those subjects.

3. Underline all given values. You can even write them down on a piece of scrap paper. This is

rather important. It gives you a good overview of everything that is given.

4. Look at what is asked. This is just as important as the previous point. You can also write down

what is asked. (For example, if the velocity in point B is asked, write down VB =?) This once more

helps you get an overview.

5. Make a drawing. Dynamics is a tangible subject. It is about real-life situations. So you can

perfectly draw pictures about it! So, ALWAYS draw a picture (unless, perhaps, if one is already

drawn in the exam). In this picture you can then draw velocities/accelerations (if you are dealing

with a motion-problem) or forces/moments (if you are dealing with a forces-problem).

6. Try to ﬁgure out what subject the question is about. If you know the subject a problem

corresponds to, you’re well on your way to solve the problem. You now know what equations you

can (and often should) use. See if you can combine these equations with the variables that are given

and asked. Then you only have to work out a few equations to ﬁnd your solution.

7. Write down the solution. This is a very important step. Without it you won’t get any points.

You may think, ”Duh... of course!” However, things often go wrong in this step. Students often just

write down numbers and equations, without clarifying what they’re doing. Instead, continuously

apply the following two steps, while writing down your solution:

• First say in words what you will do.

• Then do exactly that.

For example, your solution can start with ”First we calculate the moment of inertia”, after which

you actually calculate the moment of inertia (with equations). Then you can continue with, ”Now

we insert it into Newton’s second law for rotations”, after which you apply PM = Iα. You should

build up your entire solution like this. Try to hand in something you’re proud of.

8. Check your work. This is a step which only takes a couple of seconds, but can get you a LOT

of points. Many errors are made due to negligence. After you have ﬁnished writing down your

solution, read it over one more time. While reading, check for the following things.

• Is my solution correct and logical? Quickly glance over your equations. Are there any

obvious errors in them? If so, then ﬁx them. If a person on a bike travels at 120m/s, you

probably did something wrong. (Okay, unless the bike has a rocket engine installed on it.)

• Do I need to ﬁll in numbers in the ﬁnal equation? Often you need to derive an

expression for something. (This is the case if no values are given, or just very few.) Then your

ﬁnal answer will just be that expression. But sometimes values are given. Do not forget to

insert them into your equation and calculate a ﬁnal answer!

• Do I need to write a unit behind my answer? If you do need to give a number as a ﬁnal

answer, don’t forget the unit! They will subtract points if you forget it.

• Have I answered the question? Sometimes they don’t ask for a value, but for a conclusion.

For example, they can ask if a bungee jumper will survive his jump from a bridge with height

h = 90m. If you calculate that he will travel a distance of 108m before he stops, then you can

conclude that they’d better call the memorial service.

2.3 General exam tips

What do you do when you ﬁrst see your exam? Panic? Hopefully not. Instead, this is what you should

do.

1. The ﬁrst couple of minutes, browse through the entire exam. Then start with the easy

problems. (The easy problems are usually problems 1 and 5.) You can also start with problems

belonging to subjects you’re good at. Solve those easy problems ﬁrst. It would be a waste if you

spend all your time on diﬃcult problems, while leaving the easy points behind.

By the way, you’re allowed to make the problems in any order you like. Just make sure you clearly

indicate what solution corresponds to what problem.

2. When you seem to get stuck on a question, continue with another one. If you are looking

at a question for 15 minutes and still haven’t got a clue how to solve it, continue with another

question. It would be a shame if you eventually do not have enough time left to solve problems you

were able to solve.

3. Always write something down. Sometimes you haven’t got a clue how to solve a problem.

However, just write down some equations. Then ﬁll in some values. You can score points with that!

Even calculating a moment of inertia can get you quite some points. You will be surprised by the

diﬀerence this can make.

4. The last ﬁve minutes of your exam, look through your work for minor errors. Always

do this, even if you haven’t ﬁnished a couple of questions yet. It is almost always worth while to

check your work. In fact, you can often pull up your grade by 0.5 point in those ﬁve minutes. Now

THAT is eﬃcient.

Credit: Dr Zainuddin Rashid

Book Review

Pages

Fluid Mechanics: Fundamentals and Applications, 2nd Edition Yunus A. Cengel, John M. Cimbala

Calculation Formula

Thermodynamics - An Engineering Approach By Yunus A. Cengel, Michael A. Boles

Advance Study:Tip and Trick (Dynamics)

Engineering Mechanics Dynamics By Bedford and Fowler

My Name,Haikal

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