Final answer:
A spring scale is the tool used to measure the size of a force by utilizing the spring's restoring force to provide a direct reading of force magnitude.
Step-by-step explanation:
The tool that can be used to measure the size of a force is a spring scale. When a spring within the scale is stretched by the force exerted, such as the weight of an object, it provides a measure of that force. This measurement takes advantage of Hooke's Law, which states that the force required to extend or compress a spring by some distance scales linearly with respect to that distance. The scale will often have units of force marked along its length, allowing for the direct reading of the magnitude of the force.
Two strong permanent bar magnets can exert measurable magnetic forces, but a spring scale is more suitable when dealing with standard forces like gravity on an object. The spring's restoring force (Frestore) is calibrated to indicate force in units such as newtons or pounds. A ruler may be used in conjunction with this exercise to measure distances involved in the calibration process or when visualizing forces as vectors on a diagram.