Aim: • To make use of spring extension to measure forces.

Apparatus and Materials:
a 18-cm laboratory steel spring
standard slotted weights (up to 500g)
Scotch tape
thread
small lump of plasticise
a 30-cm rule with a hole at the upper end
a small wooden block (3 cm by 5 cm by 4 cm)
an optical pin

Procedures and Observations:
1.    Using a retort stand, clamp a nail firmly so that the 30-cm rule can be hung on it as shown.

2.    Hang the 18-cm steel spring in front of the ruler on the nail. With the help of a small lump of plasticise or blue tag attach an optical pin as a pointer to the lower end of the spring.
3.   You are to construct a force meter out of this spring using the standard slotted weights as reference. Note and record the pointer reading against the ruler without any added weight in the table below.
4.   Now attach a 50 g mass to the lower end of the spring. The spring will stretch under gravitational force. Note and record the pointer reading in the table.

6.      Plot a graph of the force readings against the pointer readings of the ruler. This will be your calibration graph for your spring to be used as a force meter.

7.    With the help of strings and scotch tape, attach a wooden block to the lower end of the spring.

9. Slowly increase the pulling force on the block. Note the pointer reading against the ruler as you pull. Record the reading when the block just begins to slide. Using this reading and your graph, can you tell what is the frictional force which prevents the block from sliding?
10. You may now investigate how the frictional force varies with the weight of the block by putting some slotted weights on top of the block as shown. Record your observations in the table below.