Laboratory work. Measuring forces with a dynamometer
A dynamometer is an instrument for measuring force. Its main part is a spring: the greater the force, the more the spring stretches, so its length increases in proportion to the force. It has a scale; the value of one division is found by dividing the difference of two neighbouring numbers by the number of divisions between them. Before work check that the pointer is at zero. To measure friction, pull a wooden block horizontally across a desk with the dynamometer, uniformly (without stopping, at constant speed). In uniform motion the pulling force equals the friction force, so the dynamometer reading gives the friction force. A load on the block increases friction; a smoother surface reduces it.
Laboratory work: pull the block uniformly over a desk, a book and a cloth, writing the reading each time; then repeat with a load on the block. On which surface is friction greatest? Safety: check the zero mark, do not overstretch the spring and do not let it snap back.