Why are word problems in physics posing a challenge for self-taught learners?

[Nano-Passion]

Man they are getting pretty tricky, I'm surprised. I am self teaching myself and doing chapter 2 (Kinematics in One Dimension)...seemingly simple. But when I do the challenging problems (they have a star or two next to them to indicate that they are challenging) in this particular section I start encountering a lot of trouble.

They are word problems and require quite a bit of logic..

Is there a lot more of this coming, or is this chapter a bit tricky? Am I stupid? I took pre-calculus and completely aced it but I am having trouble with the challenging questions in simple kinematics?

Here let me give you an example of a word problem:

A cab driver picks up a customer and delivers her 2.00 km away, on a straight route. The driver accelerates to the speed limit and, on reaching it, beings to decelerate at once. The magnitude of the deceleration is three times the magnitude of the acceleration. Find the lengths of the acceleration and deceleration.

^seems easy at first till you try to tackle it. =/
I feel a bit intimidated because I'm really challenged by a simple concept. Is this a bad sign for my physics future?

 

[Bloodthunder]

Well, it's just language. You'll get used to it. The situation is more or less the same in all studies.

 

[Pengwuino]

It's not even so much the language, it's basically figuring out the physical situation and looking at the equations to figure out what is actually being told to you. You also need some implications. For example, you need to read into the question that the driver does stop at exactly the 2km mark and it really is a stop with a final velocity of 0.

This is definitely not a 'plug and chug' type problem. Realize that your problem now relies on 2 different accelerations. What problems like these typically involve, when you have changing accelerations, are multiple versions of the same equations, each with their own initial values and times. There are also usually relationships between the two situations:

1) You don't know the accelerations, but you do know that the second acceleration is 3 times as great.

2) If you assume the distances covered are x_1 , x_2, you don't immediately know what those two values are, but you have the constraint that x_1 + x_2 = 2km which will be needed in the problem.

So the trick is usually figuring out everything you know from the statement of the problem, what you can legitimately infer from the wording, and being careful as to what your equations are telling you. I'll give you a hint on this problem, don't bother screwing around with time. You're just not given enough information for any equation with time to be useful I believe. What you do know about this problem for sure are displacements and initial/final velocities along with some information about accelerations.

 

[sophiecentaur]

I guess you have heard of the 'suvat' equations? (The Equations of motion - you will be using appropriate ones to solve these problems already, no doubt).
If you write down a list
s (speed)
u (initial velocity)
v (final velocity)
a (acceleration)
t (time)

for each part of your problem.
You then fill in what you know already and decide on what you want to know.
Choose the appropriate equation of motion (one with only those quantities in). For instance, sometmes it may not be necessary to know either s or t.

If the problem is a difficult one then you may need to produce two simultaneous equations, for two phases of the process and solve them together. Basically, you can always (I should sincerely HOPE!) rely on the fact that there will be a solution to the problem you have been set, using what you've been given.

It sometimes takes 'true grit' and a bit of confidence rather than inspiration.

 

[AlephZero]

If you get stuck trying to understand what's going on, sometimes it helps just to "play around" with something similar and see what happens.

For example, suppose he accelerates at 1 m/s^2 for 1 second, and then decelerates at -3m/s^2 till he stops.

Clearly it is not very likely he will travel 2 km by doing that, but now you can try "playing around" with the scenario to make it fit all the facts. For example what happens if he accelerates at 1m/s^2 for t seconds ? Or what happens if he accelerates at x m/s^2 for 1 second? Can you make the distance 2 km by choosing t, or x?

The hard part of learning physics (or any science) is figuring out how to turn the "real world" situation into some equations you can solve. The question isn't particularly "tricky" and you probably aren't stupid either - you just have to practice. Reviewing "word problems" in math might help a bit.

 

[Studiot]

There is one technique that helps in most circumstances.

Draw a diagram.

 

[Nano-Passion]

My concern is, does it keep getting trickier and trickier?

Right now I'm going through all the challenging problems, I just hope it doesn't get so much trickier because I'm having trouble with these. I'll get there though, its a matter of time and effort.

There is one technique that helps in most circumstances.

Draw a diagram.

I actually do that, thanks.

The hard part of learning physics (or any science) is figuring out how to turn the "real world" situation into some equations you can solve. The question isn't particularly "tricky" and you probably aren't stupid either - you just have to practice. Reviewing "word problems" in math might help a bit.

That interests me, I just need to get better at that. Since I was a slacker in high school I always hated word problems and I guess now I'm paying for it. But I will get there. ^.^