Activity 01 — the wooden block

One block, one spring balance, and a slow steady pull. Everything section 5.2 claims can be seen on the dial of the balance.

Apparatus

A wooden block of weight 60 N, and a Newton balance (spring balance).

Figure to be added

Figure 5.2 — the wooden block on a table, a small ring screwed into it, and the Newton balance attached to the ring and pulled horizontally.

Method

Fix a small ring to the wooden block and attach the Newton balance to it.
Stand the block on a horizontal table as in figure 5.2 and pull it with a very small force. The size of the force you apply can be read off the Newton balance. At the start that force will not be enough to move the block.
Pull the block with the force increasing gradually. At one particular moment it will just begin to move. Read the force at that moment.

Observation

While the block stays still, the reading on the balance is the size of the frictional force at that instant — the two balance each other. The reading at the moment the block just begins to move is the largest frictional force those two surfaces can produce.

Motion begins because the surface of the table can no longer supply a frictional force large enough to balance the applied force — a greater frictional force than that simply does not arise.

Keep these numbers

Take the balance to read 18 N at the instant the block starts to move, and 15 N once it is sliding steadily. The next three pages, and every worked example later in the chapter, use this same block.