09 Pulling a Sliding Block

Aim¶

Showing the difference between static and kinetic friction

Subjects¶

• 1K20 (Friction)

Diagram¶

Equipment¶

• Wooden block

• Pulley

• String

• Slotted masses

Presentation¶

The slotted mass is made so heavy that the block just doesn’t move. Then you give a smash on the table and the block will start sliding and keep on sliding.

Explanation¶

When the block just doesn’t move, it means that $F_{f}$ is almost equal to $F_{f, \max }=\mu_{\text {stat }} F_{n}$ (see Figure 2).

A smash on the table means that the block is released from the table for a short moment (at least temporarily $F_{n}$ will be much smaller than $m_{A} g$ ) so A and B start moving. Since $A$ and $B$ continue to move, it means that $\mu_{k i n} F_{n}\left(=m_{B} g\right)$ is lower than $\mu_{\text {stat }} F_{n}$.

Remarks¶

• The ring for fixing the string can be moved vertically in order to maintain the pulling string horizontal when the block is moving.

• A wetted bottom of the block makes this demonstration somewhat easier.

Sources¶

• Roest, R., Inleiding Mechanica, pag. 63