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General Physics Mechanics (PHYS 121) Physics Cheat Sheet

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Sandra Watson
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Case Western Reserve University

General Physics I – Mechanics (PHYS 121)

1) Before you start, consider the acceleration. Is it zero? Then note that a = 0. Is it another constant? Then a = another
number. Make note of this, keep it in mind, and be ready to use it later.
2) Draw a separate Free-Body Diagram (FBD) for each relevant body in a problem, and represent each body as a point only
with forces acting upon it. I strongly recommend doing this. Avoid the habit of drawing FBDs on top of your sketch (a bad
habit that is nonetheless widely practiced in some physics texts and by your lab TAs but is not rewarded on homework and
tests). Instead, draw each FBD separately. This helps you because it is important to think about each body as an “isolated
point” with forces on it, ignoring everything in a problem except for the forces acting in each particular FBD. That’s why we
call it a “free-body”. Free! Get it?
3) Sketch the problem at hand carefully.
4) Identify all bodies that are relevant. Make a list of bodies if you are unsure what they are. A body is relevant if you need to
know the forces on a body or the motion of a body or both.
5) Also, before you start drawing forces on your point body, you might want to write down in the corner a little coordinate system
to make it clear which directions you are talking about If your motion is 1-D then just write a little “x” or “y” vector to indicate
clearly which way is defined as positive. If you have options for choosing coordinates, then always choose a coordinate
system so that at least one of the coordinates is lined up with the acceleration(assuming it is non-zero).
6) Include each of the forces as a vector (arrow) on your free body diagram. It is important toget the direction written down
correctly on the diagram. Don’t worry about the magnitude.I like to draw one arrow for each individual force.
7) Put only real physical forces on your FBD. Use your Force Menu for reference (see the nextpage), and only use forces that
you find on the menu. Is the net force on your list? No, itis not. So never draw it on your FBD. Is the velocity vector or the
acceleration vector onthe force menu? No, they are not even forces, so don’t draw them on your FBD. The wholepoint of the
FBD is to act as a bookkeeping device so you can be very clear about all of theforces before you try to apply Newton’s
Second Law. Don’t draw anything that is not an actual physical force on your FBD
8) . Remember the difference between contact and non-contact forces. Weight is the only non- contact force on your Force Menu,
meaning that Weight applies to any body that is located or near the surface of the earth.

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General Physics Mechanics (PHYS 121) Physics Cheat Sheet

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