learning object #2
TRANSCRIPT
LO: Simple Pendulum
You are swinging a coin suspended from a string of length 0.5m from side to side
(on a 2-‐dimensional plane). You are in a lab and measure with some instruments
that the net tangential force at a certain point in time is 20N and the tension force in
the string at that point in time is 10N. Before you do anything else, Joss “The Boss”
Ives (your Physics 101 professor) jumps in to help you measure the mass of the
coin, but manages to tip over a water-‐filled graduated cylinder and break the scale.
He quickly apologizes, but you don’t worry because you know how to find the mass
using the whiz-‐kid physics powers Joss taught you in your lectures (which you
happen to attend with the greatest enthusiasm!).
Part a) Find the mass of the coin that was suspended from the string. Part b) Out of curiosity, you also want to find the displacement at that point. In other words, find s (hint: arc distance). Part c) Find the frequency and the angular frequency of oscillation of the pendulum.
Length = 0.5m
s = ?
m = ?
T = 10N
Fnet,t = 20N
Solution: Given Information: FT = 10N Fnet,t = 20N Relevant Equations: T = mgcos θ Fnet,t = -‐mgsin θ s = Lθ f = ω/2π = (1/2π)*sqrt(g/L) Part a)
|𝐹!"#,!𝑇 | = |
−mgsin𝜃mgcos𝜃 | = |−
sin𝜃cos𝜃 | = tan𝜃
𝜃 = tan!!𝐹!"#,!𝑇
𝐹!"#,! = −𝑚𝑔 sin𝜃
𝑚 = −𝐹!"#,!𝑔 sin𝜃
𝜃 = tan!!20𝑁10𝑁 ≅ 1.11°
𝑚 =20𝑁
(9.8)(sin 1.11°) ≅ 2.29𝑘𝑔
Part b)
𝑠 = 𝐿𝜃 𝑠 = 0.5𝑚 1.11° ≅ 0.56𝑚
Part c)
𝑓 = 12𝜋
𝑔𝐿 =
12𝜋
9.80.5𝑚 ≅ 0.71 𝐻𝑧
𝜔 = 2𝜋𝑓 ≅ 4.46𝑟𝑎𝑑𝑠