Notes for your UW Homework Tutorial: Forces
A: FBD – Checklist
1. Do your forces have arrows?
2. Do the lengths of your forces roughly represent the relative magnitude? For example, if your net
force in the y-direction is zero, do all upward vectors add up to the same as all the downward
vectors? Roughly is good enough.
3. I recommend that – for each FBD – you include a coordinate system and write an equation for
Newton’s second law next to the FBD. It makes your point clear and is excellent practice.
4. Do all forces have subscripts? Are they all correct?
5. Friction forces have three subscripts, one to say whether it is kinetic of static, two for ‘on’ and ‘by’.
Example: 𝑓⃗𝑠𝐴𝐵 is the static friction on A by B.
1a
1b
2a
2b i
2b ii
2b
iii
2c
3a
3b
3c
4a
4b i
4b ii
4c
5a
5b
5c i
5c ii
6a
6b
see A (FBD Checklist)
Carefully read this question. The answer could look something like this:
forces with
forces without
When a string is present this force …
string:
string
⃗𝑁⃗𝐴𝐵
⃗⃗𝐴𝐵
has same direction but greater magnitude
𝑁
⃗⃗𝐴𝐶
⃗⃗𝐴𝐶
has same direction and magnitude
𝑁
𝑁
etc
etc
etc.
List as many reasons as possible.
see A (FBD Checklist)
see 1b
Note that the question refers to the scenario WITH the hand pushing down.
⃗⃗𝐴𝐵 ), can you write down the reaction force?
Hint: if I give you a force (e.g. 𝑁
see A (FBD Checklist)
Hint: start with link 4, try it out on scratch paper first, watch out for N3 pairs
don’t forget this
I will show you a good way to write this efficiently when I return the draft.
Make sure NOT to write:
⃗⃗𝐴𝐵 > 𝑁
⃗⃗𝐴𝐶 You can only rank the magnitudes of vectors, you cannot rank the vectors
𝑁
(greater than is not defined)
see A (FBD Checklist)
For example: if there is a tension force on S12 where does that force appear in the two FBDs
for links 1 and 2 on the previous page? What is it called there?
Simply list each force for S12 and match it up with the corresponding forces. Example:
⃗⃗12𝑆 ∶ 𝑇
⃗⃗1𝑆
𝑇
Are there any forces in the FBDs for links 1 and 2 that are not in 4b i. Answer is yes but which?
see A (FBD Checklist)
see A (FBD Checklist)
see 2c
What is the net force in each case?
Cryptic hint: be careful that you know what you are comparing.
see A (FBD Checklist)
see 2c
Notes for your UW Homework Tutorial: Forces