# Question about Physics in General

#### Duosellz

Hello there, I am a high school student new to physics. I don't quite understand physics as I do Calculus. My main question for physics revolves around is what I can do to understand the questions.

For example, I am learning about the laws of motion. My book introduces the laws and I somewhat understands such as Force = Mass * Acceleration, but as soon as I start the homework, my mind goes blank. For example, one of the questions is A 5.0-g bullet leaves the muzzle of a rifle with a speed of 320m/s. What force (assumed constant) is exerted on the bullet while it is traveling down the 0.82-m-long barrel of the rifle?

When I see that question, I cannot connect acceleration or force to the given data. This situation goes for most questions that I encounter. I can never connect the relations between the data and the things I have to solve. I am wondering how I can connect these data together. I try to understand the formulas also, but I still can't make the connection.

Any suggestions will help a lot for me right now. Thanks.

#### Andy Resnick

My students often voice similar concerns, and I explain that it's a general deficiency in problem-solving skills.

There's no 'magic technique' to solving problems, and everyone has to figure out an approach that works for them. I give the students some suggestions: underline key words in the question, draw a picture, list knowns/unknowns, etc... those are not rules, but suggestions on how the student can begin to sort the problem and break it down into smaller pieces.

Here's an analogy- most of us get dressed every morning (presumably you dress yourself . How do you decide what to wear? Whatever algorithm you use, it's a method that ensures you are dressed properly for the weather, activities, etc.

So, for the problem above, I would suggest identifying the key words, so you can think about what is going on- for example, what are the important pieces of information in the problem? What do you need to calculate 'force' from the given data? Do you need to make any assumptions?

Problem solving skills take time to develop- don't get discouraged.

#### Studiot

A good way to start is to draw a diagram.

Not a posh pretty one, just one to display the information given, such as the one at the top of my attachment.

Then write down all the knowns and the unknowns and a list of all the equations you know about this area of physics.

I have done this in the second part of the attachment.

Within the list of data (knowns and unknowns) you should find the unknown you are seeking.
You should also find some other unknowns, that you do not want to know.

Comparing the data list with the equation list you can start to pick out which equations will help obtain your wanted unknown.
You will find some equations not helpful as they obtain the wrong unknowns. You may also find that sometimes you need to calculate and intermediate quantity to reach your wanted one.

In my attachment I have shown this situation. You do not really require to calculate time so should avoid equations involving time.

Finally if you cannot find enough equations or data look again at your sketch. You may have missed something eg in my attachment the fact (not explicitly stated in the problem) that the bullet starts from rest.

Hope this helps and enjoy your future studies.

#### Attachments

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#### Obis

I am quite sure that your problem is how you think about physical quantities. That is, as soon as you read "force", you think of F, and equations that involves this F. So you associate each physical quantity, given in the problem, with the letter that represents it, and not with the actual meaning of that quantity. And the same goes with equations - you see them mostly as relationships between letters. So in your example, it goes like this:
Data given : m, v, d.
The problem asks for : F.
F is : F = ma.
We have the m, but not the a, so we don't know what to do now.

So, first of all, try to understand every physical quantity as good as possible, get the "feeling" of it. For example, the force is quite intuitively understood from our daily life - a big force pushes things hard, a small force hardly moves them at all. Try to get such feeling about every physical quantity. What's the difference between an object moving with acceleration and moving without it? What's the difference between an object having a huge momentum and a small one? This may sound trivial and stupid, but think again, what if I ask : What's the difference between a vector field with a high divergence and without it? (Don't mind that you probably don't even know what it is, it's just a more subtle question than the ones before). From my own experience I can tell, that only a small percentage of college students (at least in my university) could answer this in their own words, and not give the formal definition, that is, the plain letters, symbols.

The same goes with understanding equations, try to get the feeling of them. For example, F = ma. Let's write this in a more intuitive form : a = F/m. Why acceleration is proportional to force? Why acceleration is inversely proportional to mass? Easy, right? Then let's move forward : d = at^2/2 (d is distance traveled with constant acceleration a in time t, starting from zero velocity). Why the distance is proportional to the square of time, and not to time to the power of one or three? I would answer that like this : traveled distance in it's general form is d = average velocity * time. Average velocity in our example is (at)/2, since it goes from 0 to at continuously. So we get : d = average velocity * time = (at)/2 * t = at^2/2, as expected. So the distance is proportional to time squared because, first of all, as time passes, the average velocity raises (proportionally to t), and then we multiply this average velocity by time and get our t^2 proportionality. This is also quite easy, but what if the analysed equation is something like : something equals the volume integral of a sum of.. So you should constantly improve your ability to analyse physical quantities and relationships between them (equations). To say all of this in short : ask question "Why?" as frequently as possible to the textbook you read, to the teacher you listen to and to Life in general.

To get the force, we still need the acceleration a, and we should somehow get it from the given distance d and velocity gained while traveling that distance v. Let's model this situation in our head : a bullet is travelling some distance d, while it is travelling it feels a force F, that gives it acceleration, and because of that the speed of the bullet rises constantly. It goes out of the distance d with v. Let's define t as the time that the bullet spent in the distance d. So the bullet was travelling with constant acceleration a in time t. What speed it gained in that time? v = at, right? But we don't know the time t, however, we know that the bullet's speed was rising continuously, from 0 to at = v, so the average speed was at/2 = v/2. So we know the distance, and we know the average speed, the time is simply t = d/v[avg] = 2d/v. Put this into v = at, we get : v = a * 2d/v => a = v^2/(2d), so the force F = ma = m * v^2/(2d).
Now, this could have been solved much faster using the energy conservation principle : the kinetic energy acquired by the bullet is mv^2/2, and the work done by the force to the bullet is F * d, both of them are equal, and we immediately get F = mv^2/(2d). So we didn't even need the formal definition F = ma.

So this is how it's done : you model the situation in your head, think about every quantity that is given, think about the relationships between them, and if you understood everything properly, the general idea how you could get the required quantity should come out.

#### Duosellz

Thanks you everyone for your replies. I am starting to see small relations in my homework problems. I realized my main problem is not being able to relate the known and unknown, but I am slowly overcoming that. I feel much better now that I can do a question in a long time rather than skipping it. Thanks again!

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