Situations where several forces act
Teaching Guidance for 14-16
The effects of several forces
Wrong Track: This force tries to push the box in this direction and these forces try to push the box in that direction. So the box is trying to move in two directions at once.
Right Lines: Things like boxes are not like you or me. They don't try to move. Here's a better way to describe it, less likely to lead us into unhelpful thinking. Both forces act on the box. The resultant force is the sum of these two forces. The resultant force changes the motion.
Find the resultant force first
Thinking about the learning
Expressions such as
the surfaces are trying to move past each other are rather common when talking about frictional forces. That's not the only place: the danger arises every time there is more than one force acting on an object.
Thinking about the teaching
Avoid these difficulties by relating the changes in motion to the resultant force acting. Particular difficulties with frictional forces can be mitigated by using the distinction between grip, slip and drag (introduced in the SPT: Forces topic and again in the Physics Narrative on this episode).