PL-1) Jane is experimenting using the apparatus for this experiment. In the figu

Question

PL-1) Jane is experimenting using the apparatus for this experiment. In the figure below, we see a “snapshot” of the apparatus at one instant during its rotation. What is the direction of the centripetal force and acceleration vectors of the bob (B)?

(A) Up,

(B) Down,

(C) To the left,

(D) To the right,

(E) Into the page,

(F) Out of the page.

PL-5) With the rotation of the apparatus stopped, Jane attaches a string and mass holder (W), as shown in the figure. The mass of the mass holder (W) itself is 12.8 g. Using the data collected in PL-4 [period = 2.14 s, radius = 13.2 cm, mass of bob = 253 g], predict how much mass (in grams) Jane will need to add to the mass holder (W) in order to stretch the spring so the bob aligns with the pointer (P).

Explanation / Answer

PL-1) The direction of centripetal force and acceleration vector of bob B is ( D) to the right.

PL-5) period,T = 2.14 s,

radius,r = 13.2 cm,

mass of bob,mb = 253 g

mass of the mass holder (W) = 12.8 g.

mass (in grams) Jane will need to add to the mass holder (W) = m=?

Centripetal force on bob, F = mb*w2* r [ w= 2*pi*f = 2*pi/T]

F = mb*( 2*pi/T)2*r

F = 0.255 *(2*3.14/ 2.14)2 *0.132 = 0.289 N -----------------1

Also force on mass add to mass holder (W) ,F = (m+0.0128)*g ------------------2

from eq 1&2:

(m+0.0128)*g = 0.289

(m+0.0128)*9.8 = 0.289

m = 0.01668 kg

m= 16.68 gm

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