1.1 Characteristic of the idea

Harlen (Harlen, ed., 2015) characterizes this idea as follows: A force acting on an object is not seen directly but is detected by its effect on the object’s motion or shape. If an object is not moving the forces acting on it are equal in size and opposite in direction, balancing each other. Since gravity affects all objects on Earth there is always another force opposing gravity when an object is at rest. Unbalanced forces cause change in movement in the direction of the net force. When opposing forces acting on an object are not in the same line they cause the object to turn or twist. This effect is used in some simple machines.

1.2 Suggested level of preconception development for primary level

In the younger school age (7–11 year olds), which corresponds to primary science education, (according to Harlen‘s concept of developing Big Ideas of Science), pupils should already understand that forces can push, pull or twist objects, making them change their motion or shape. Forces act in particular directions. Equal forces acting in opposite directions in the same line cancel each other and are described as being in balance. The movement of objects is changed if the forces acting on them are not in balance. The speed of a moving object is a measure of how far it would travel in a certain time. How quickly an object’s motion is changed depends on the force acting and the object’s mass. The greater the mass of an object, the longer it takes to speed it up or slow it down, a property of mass described as inertia.