Interlude B: Strings and Pulleys

Section B.2

What are the assumptions for an ideal string?

Massless, stretchless, remains taut

What are the implications of the assumptions for the ideal string?

The tension is the same at both ends of the string

Objects connected by a string must move the same distance during a time interval

Have the same change in velocity during a time interval

Have the same acceleration

How do the magnitudes of the tensions at the two ends of the string compare?

The tensions were equal

How do the accelerations of the two subsystems (left and right blocks) compare?

The accelerations were equal

What is a pulley?

The change of directions of a string and, therefore, the direction of the tension force it applies

What are the assumptions of an ideal pulley?

Massless, frictionless axle or bearings, string does not slip

What is the implications of the assumptions?

The tension is the same at both ends of the string

We will assume that all strings and pulleys are ideal. What conclusions can we make based on these assumptions?

The accelerations of objects connected by a string are equal

The tension is the same at both ends of the string

Ideal strings and pulleys transmit:

The same acceleration and the same tension throughout the connection

Sometimes a string passes over a pulley. All the pulley does is:

Change the direction that the tension force is applied

What is the name of the following device?

Atwood’s machine

How do the accelerations of the two masses in the following figure compare?

a_1x = a_2x

How do we know which direction to choose for the kinetic friction force?

There are no surroundings that would cause a block to move leftward. The block must move right, and the kinetic friction force would be leftward to impede this motion

Why would there be no kinetic friction acting on a block?

It would not be in contact with a surface that could provide a kinetic friction force