I am currently teaching two discussion sections for an introductory physics class. I will use this page to collect helpful tips and resources for students seeking extra help and advice on how to succeed.
Use units. Ironically, I learned this from my high
school chemistry teacher, who was a big stickler on units, but using them
consistently and knowing them well will get you a long way in physics
- Always use units for at each point when
plugging in values in the equation and as you calculate (add, multiple)
numbers, you should also be doing that with the units.
- If you find
you are adding two units that are not the same (i.e. m/s (a velocity) and
m/s^2 (an acceleration), you know this is impossible and can look for a
mistake you made (in this example, it is likely that the second term was
missing a time variable – since multiplying by units for time, ‘s’, would
give you velocity units)).
- When you multiply units, you might end up with
a big fraction, but all units should cancel nicely to give you the units
of whatever you find in the end. Here, it’ll help to know how “bigger”
units break into “smaller” units (e.g. units for force, N = kg*m/s^2, this
makes sense if you consider that F=m*a, the units for Newton are just the
units for mass and acceleration multiplied).
Don’t let lots of information overwhelm
you (like this webpage). In physics, having
lots of "knowns" is a good thing! We might have to use more than one
equation, but in a class, you can be sure you have everything you need!
- Follow the procedure outlined in the section below by making a list of things you know,
including things we assume to be true, like gravity, and things you are
asked to solve for – break things into x and y components at this step so
that it makes it easier for you!
Realize that physics is about applying
and less about memorizing. I
always had a rough time in life science classes because I am not good at
memorizing lots of definitions, words, etc. – or maybe I just don’t want
to. In any case, physics doesn’t ask you to memorize things – this is why
the equations are given. You might memorize some of them down the line the
more you use them – this is awesome – but you don’t need to! (Following
the steps in the section below will help you choose the equation if you aren’t sure about
Eliminate easy and frequent errors. As you go about solving problems, ask yourself questions, for example, did you consider negative signs for opposite
directions? Doing more problems might tell you where you tend to make the
same errors, like maybe you get sin and cos confused.
Maintaining a positive attitude. Physics might seem a little weird, complicated, different,
at the start, especially if this is your first class, but in the end it’s
a very logical, albeit math-based (depends on if you like math or not)
How to Approach a Physics Problem
By "type of problem" I mean e.g. projectile motion, relative motion, balancing forces, rotational motion, energy, etc. To identify the problem, look for the quantities described by phrases like “force”, “velocity with respect to”, and take a look at the picture, if there is one. More complicated problems might combine two or more "basic" types of problems.2. What is given by the problem? What are we looking for?
Then look at only the equations that pertain to that particular type of problem (equation sheets tend to organize the equations so that this is a bit easier for you, and if you make your own sheet, organize it). For example, in the case of a relative motion problem, that doesn’t ask about forces, energy, and nothing is rotating, you could exclude the latter three groups of equations
Look at the text and figure (if provided) for anything that is given i.e. the direction and magnitude of a force, and initial velocity, etc. Sometimes information is provided without numbers, e.g. “the block starts from rest” -> initial velocity = 0.3. What equation can/should I use?
Also consider things assumed that might not be written (the most common example is that acceleration in the y-direction is gravity!). I really recommend writing these down on a sheet of paper yourself, not just highlighting all over the page, this helps keep you focused and organized.
Finally, write down what the problem is looking for. For example, the final velocity, v_final = ?
Look at what information you have and what information you need - try to find an equation that includes both of these, without anything extra.4. Plug in and solve.
If you can’t find a “perfect” fit, perhaps choose an equation with two unknowns, you might need to write a second equation to solve for one unknown and then eventually figure out the final answer. Likewise, you may need three, or more, but generally start from the/an equation that has (ideally) only the variable you are looking for and lots of variables you already know.
- Use units! (using units is more tedious, but is very valuable and allows you to check your answer – for example if you think you solved for time, but your final units are m/s, or 1/s^2, you did something wrong)
- Use negative signs depending on how you define your system (i.e. up is positive, down is negative) – be consistent!
- Break up x- and y-directions from the very beginning (this means that when you write your known variables you should already denote the directions (for example you might know the initial velocity in the y-direction but not the initial velocity in the x-direction). Only combine at the end (if needed!).
- Ask yourself if your answer makes sense at the end – a boat only taking 0.1 seconds to cross a wide river is unrealistic, and likely wrong.
What Should You be Thinking?
During my classes, I find myself asking this question a lot. I realize that the answer isn't always evident, and created this non-exhaustive (I will add things as they come to mind or come up in class/conversation) list.
To clarify, I am referring to inferences made on any provided information or results. The inferences listed may be useful in a variety of problems, but not all inferences can be associated with all problems. Therefore, it is still important to think through each problem yourself and use this tool only as a guide
| "at rest"||(initial) velocity = 0 (therefore, acceleration is also zero)|
equilibrium (sum of forces = 0 and no change in energy)
static friction (as opposed to kinetic)
|"rough surface"||consider friction (if moving, kinetic; otherwise, static)|
|"smooth surface"||no friction necessary|
|"constant velocity"||acceleration = 0 (simplifies kinematic equations)|
no change in force (implied from above equation and Newton's 2nd Law)
| "elastic" collision|| conservation of momentum and kinetic energy|
| "inelastic" collision|| only momentum is conserved|
| "free fall"||acceleration = gravity (be consistent with your use of the negative sign)|
only concerns y-component (x-component added for parabolic motion)
| "at an angle"|| break apart vector (velocity, force, etc.) into x- and y-components using trigonometry|