# TBR Physics Section 4 Passage 4

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#### ch00ber

##### New Member
10+ Year Member
The question is based on a torque experiment. "A group of students decides to study the physical variables that affect torque, using an apparatus that has a weighted bar connected to a spindle. The spindle turns when a mass attached to a cord is allowed to fall freely.
Experiment 1: Attached to a spindle is a light-weight nylon cord ending in a loop from which weights of varying mass can be suspended. The spindle is mounted on a shaft that has a perpendicular bar at the top. Objects may be attached with screws at any point along the bar. The students compare the rate of rotation of the upper cross-bar with the masses of the objects affixed to it and with the radius of the spindle. The mass of the nylon cable is assumed to be negligible relative to the mass of the falling weight in all trials, and the system is not assumed to be fricitonless."

Can anyone explain how this experiment works? From my understanding, the weight falls, causes the spindle to turn, therefore causing the top bar to turn. However, I don't understand where torque comes into play with the top bar.

#### dmf2682

##### Membership Revoked
Removed
10+ Year Member
The question is based on a torque experiment. "A group of students decides to study the physical variables that affect torque, using an apparatus that has a weighted bar connected to a spindle. The spindle turns when a mass attached to a cord is allowed to fall freely.
Experiment 1: Attached to a spindle is a light-weight nylon cord ending in a loop from which weights of varying mass can be suspended. The spindle is mounted on a shaft that has a perpendicular bar at the top. Objects may be attached with screws at any point along the bar. The students compare the rate of rotation of the upper cross-bar with the masses of the objects affixed to it and with the radius of the spindle. The mass of the nylon cable is assumed to be negligible relative to the mass of the falling weight in all trials, and the system is not assumed to be fricitonless."

Can anyone explain how this experiment works? From my understanding, the weight falls, causes the spindle to turn, therefore causing the top bar to turn. However, I don't understand where torque comes into play with the top bar.

Well, you've got the mass on the rope falling with force m*g, and the radius of the spindle r_spindle. So torque from the falling mass would be m*g*r_spindle. Then you have the perpendicular bar so you've got a mass and a radius there which gives you your moment of inertia (I=m*r_bar^2). Use your formula relating torque to angular velocity and moment of inertia (T=I*omega) to solve for omega.

#### Erythropoietin

##### EPO
10+ Year Member
The mass is attached to a cord which is wrapped around the spindle. So as the mass falls, it causes the cord to move as well, which in turn causes the spindle to turn.
The greater the mass you have, the larger the torque.

Imagine a spindle that is 10 meters above from the ground and consider two cases. In one case you have 5000 kg of mass. As the mass falls, it gains so much momentum that the spindle turns at a such fast rate. But now consider a second case where you have 1 kg of mass. When it starts to fall, the spindle will still turn but at a much slower rate because when 1 kg falls, it falls with a smaller momentum compared to the 5000 kg mass, causing the spindle to turn less because its torque will be small (compared to 5000 kg).
Again, this difference is due to their difference in momenta and potential energies.