The relative strength of two forces can be compared by observing the difference in how they move a common object.
Force and Motion
In prior grades students learned to use appropriate words to describe the position and motion of objects and the effects of forces on objects. In grades 2-3 students learn that forces work not only to push and pull objects, but also affect objects when they are dropped or thrown. Whenever the motion of an object changes, there is a force involved. Greater forces on a given object result in greater changes of motion. In addition to being able to describe how forces change the motion of objects, students are expected to measure the position of objects using measuring instruments such as rulers. Students can also measure time to the nearest minute. Emphasis should be on comparisons of forces and motions rather than on calculation so that students develop conceptual understanding of how forces make things move.